Clinical thermoluminescence dosimetry: how do expectations results compare?*
T o m a s Kron , Meg Schneider , Alan M u r r a y and H e d y M a m e g h a n
Department of Radiation Oncology, The Prince of Wales Hospital, Randwick, NSW, Australia
(Received 2 December 1991, revision received 1 September 1992, accepted 16 September 1992)
and
Key words: Thermoluminescence dosimetry; TLD; In vivo dosimetry; Radiotherapy treatment verification
Summary
Thermoluminescence dosimetry (TLD) for radiotherapy treatment verification is performed in the Prince of Wales Hospital in Sydney for a wide range of applications: (A) to determine the dose in difficult treatment geometries, (B) to record the dose to critical organs, and (C) to monitor special treatments such as total body irradiation (TBI). TLD measurements were performed with the aim to investigate cases where dose prediction is difficult and not as part of a routine verification procedure. We re- viewed 1058 reports of TLD performed during the treatment of 502 patients between 1986 and 1991 to evaluate how the TLD results compare with the dose determined by the treatment plan. Reasons for possible discrepancies should be identified. In 19% of all investigated cases a discrepancy of more than 10% was found between expected and measured doses. The discrepancies could be divided into three groups: (1) errors made in the TLD determination or evaluation, such as placement errors of the TLD chips (21% of all discrepancies); (2) mistakes made during the patient set-up, such as insufficient shielding or inadequate patient immobilisation (30~o); (3) inadequate treatment planning and dose calculation procedure, such as wrong inverse square law corrections or errors due to limitations of the two-dimensional treatment planning system used (41~0 of all). In 8% of all discrepancies the reason remained unclear. A number of changes to treatment plans and modalities (e.g. changed scrotal shield, modified bolus) were introduced due to TLD results. The increasing number of TLD requests per year attests to the value of TLD as a treatment verification method in clinical practice.
Introduction
With increasing complexity of treatment techniques, quality assurance becomes an important task in radio- therapy. In vivo dosimetry is an essential part of this quality assurance [5]. In vivo dosimetry on patients undergoing radiotherapy can help to minimise uncer- tainties in dose delivery [9] and it has been demon- strated recently that many errors occurring during treatment preparation and delivery could be avoided, if patient dosimetry was performed [36].
In vivo dosimetry is usually limited to the patients surface but can occasionally be performed as intracav-
ity measurement [ 19,24]. Various dosimetric techniques are available for in vivo dosimetry. Ion chambers are not widely used because of the high voltage applied and the cables attached to the chamber. They are occasion- ally discussed in connection with X-ray total body ir- radiations (TBI) [ 1 ]. However, most in vivo dosimetry utilises either semiconductors or thermoluminescence dosimeters [ 3 ].
Following the extensive study of G. Rikner on semi- conductor dosimetry [46--48], entrance dose measure- ments with semiconductor diodes are routinely per- formed in a number of Scandinavian centres [19,40]. These measurements with the diode under appropriate
Address for correspondence andpresent address: Dr. Tomas Kron, Inawarra Cancer Care Centre, Wollongong Regional Hospital, PO Box 1798, Wollongong, NSW 2500, Australia. * Parts of the paper were presented at the Advanced Radiation Therapy Symposium, Munich, April 1991 and at the 42nd Annual Meeting of The Royal Australasian College of Radiologists, Adelaide, September 1991.
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build-up allow a routine check of set-up and machine performance [19]. They make use of the advantages of semiconductors such as ease of handling and the dose determination in real time.
In contrast to that TL dosimeters provide a delayed dose reading. Therefore, direct intervention during the treatment using an interlock system is not possible. However, in general the dose reading can be obtained before the next attendance of the patient in fractionated radiotherapy, allowing changes to be made before de- livery of the next fraction.
The main advantages of LiF TL dosimeters are: their small physical size, their relatively low effective atomic number of 8.31 [27], no directional dependence of the dose reading, and their stand alone character. These make TLD a useful tool for in vivo dosimetry on patients in particular if complicated set-ups are to be investigated.
Since its first use for in vivo dosimetry during ra- diotherapy [7], TLD has become an important tech- nique for clinical dosimetry [22,38]. Due to their small dimensions TL dosimeters can be used for a wide range of applications, ranging from lens dose measurements [18,52] to TBI dosimetry [1,33]. Placed without build-up on the patient's skin they can provide infor- marion on the dose received by the skin. This is of importance when unwanted skin reactions shall be avoided (or at least predicted) as well as in cases when there is a risk of tumour recurrence in the skin. There- fore, TL skin dose determination has been applied in a wide range of photon and electron applications such as head and neck treatments [26] and total body skin irradiations (TBSI) [ 10,28].
However, most radiotherapy departments limit themselves to a small number of TLD applications be- cause the TLD apparatus is expensive and the provi- sion of a TLD service is labour intensive [44].
In 1975 TL dosimetry was introduced at the De- partment of Radiation Ontology at The Prince of Wales Hospital in Sydney. In addition to portal imaging to check the patients positioning, TL dosimetry plays an important role in the verification of complicated treat- ment procedures. During the investigation period from 1986 to 1991, TLD measurements were requested for 100 patients on average each year. About 200 TLD measurements were performed per year in a wide va- riety of applications [32].
Since this is the highest number of direct dose mon- itoring on radiotherapy patients in Australasia [ 12], it was the aim of the present study to review the TLD service and to compare retrospectively the dose mea-
sured with TL dosimetry with the dose expected from the patients treatment plan. Where a discrepancy be- tween expected and measured dose could be identified, an attempt was made to determine the cause of the discrepancy. The benefit of this study should be to recoguise critical steps in clinical TL dosimetry, treat- ment set-up and the treatment planning procedure, and to find strategies to avoid such problems in the future.
Materials and methods
Patient treatment and planning During the investigation period, January 1986-July 1991, about 1600 patients were treated per year with external beam radiotherapy. Irradiation was delivered on four linear accelerators (VARIAN Clinac 4, Clinac 6, Clinac 10/18 and Clinac 1800), one orthovoltage (Siemens Stabilipan) and one superficial (Phillips RT 100) treatment unit. During the investigation period, approximately one in five patients was treated with electrons.
The planning of most treatments was performed on a GE RT-Plan computerised planning system. The computer uses two-dimensional algorithms for X-ray [39] and electron [21] dose calculations. Fast and full scatter correction methods are available for electron dose calculation. An algorithm based on the work of Cunnigham et al. [6] is used to calculate the dose in irregularly shaped X-ray beams. The planning com- puter interpolates the dose in the build-up region with a 3rd-order polynomial between measured surface dose and the dose at the depth of the maximum dose.
Only few treatment types such as electron TBSI were planned from collected physics data only [28]. Approximately 30~o of the treatment plans were cal- culated using computed tomography (CT).
If the treatment geometry was complicated or if crit- ical structures were close to the treatment field, TLD measurements were requested for a particular patient on one of the first treatment days. If the measured dose differed from the expected dose, the TLD measurement was repeated. In cases where a discrepancy persisted, the TLD measurement technique, the treatment set-up and plan was reviewed and changed if required.
TLD measurements Clinical TLD measurements were made using LiF TLD chips (TLD 100 square chips, 3.1 x 3.1 x 0.9 mm 3, Hat- shaw*, and round chips, diameter 4.5 mm, thickness
* Harshaw/Filtrol, 6801 Cochran Road, Solon, Ohio 44139, USA.
TABLE I
Characteristics of the two TLD measurement systems employed in the study.
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TLD reader (both Harshaw, Ohio, USA) 2000D/2080 2000A/2000B
Period of use Mode of usage Type of heating Maximum readout temperature (° C) Readout cycle time (s) Day-to-day reproducibility of the TLD fight Signal of a single chip (2 S.D.) Reproducibility of the dose measurement after
evaluating the chips using standards (single chip; 2 S.D.) Accuracy of the dose measurement in a cubic
phantom (2 S.D.)
From May 1991 Before May 1991 Automated Manual Contact less hot N2 Contact planchet 320 260 20 40 +_ 8% _+ 20%
0.9 mm, Alnor*).,30 chips were combined to one set with sensitivity values relating the dose response of a single chip to the mean dose response of the whole set. A uniformity and performance check of each set of TLDs was done bimonthly in a polystyrene phantom.
Each TLD measurement point on a patient con- sisted of at least two chips in a pack covered by a thin plastic foil. The chips were evaluated using the reading of two standards of four chips each. These were ex- posed immediately after the treatment in standard ge- ometry to a dose similar to the one expected for the measurement on the patient. Since the radiation qual- ity was the same for treatment and standard irradiation, this procedure overcomes problems with energy re- sponse [4], supralinearity [11,22] and fading [29] of the TLD signal.
TLD measurements up to May 1991 have been per- formed with a manual TLD reader (Harshaw 2000A/ 2000B). In the last 3 months of the investigation period an automated TLD reader (Harshaw 2000D/2080) was used to evaluate the TLD chips. The TLDs were an- nealed between each irradiation (400 °C for 1 h fol- lowed by 100 °C for 3 h; the cooling down rate was about 20 °C per min).
Table I lists the characteristics of the two TLD mea- suring systems. The results for reproducibility and ac- curacy were obtained in a polystyrene slab phantom as part of the ongoing quality assurance program. They are expressed as 2 S.D. of the mean of a delivered dose of 100 cGy and represent the worst performance dur- ing the investigation period. The relative large fluctua- tion of the raw TLD signal from day to day in both systems (7th row in Table I) is partially due to repro- ducibility problems in the heating cycle of an in-house build annealing oven (compare e.g. [8]). This problem
affects each set as a whole and can therefore be cor- rected for by evaluating the chips used on a patient with TLD chips from the same set exposed in standard con- ditions.
The difference between the two readers (reproduc- ibility 8~o vs. 2 0 ~ ) can be attributed to the different heating mechanisms applied (compare Table I) and to the age of the photomultiplier in the manual reader. By evaluating the chips with a standard out of the same set, which had the same annealing history, the reproduc- ibility of the dose measurement itself (8th row in Table I) can be greatly improved.
A more detailed description of the TLD service is given elsewhere [32].
TLD applications To assess how the dose measured with TLDs on pa- tients compares with the dose which was expected, 1058 TLD reports from the treatment of 502 patients between January 1986 and July 1991 were investigated retrospectively. TLD applications could be divided into three application groups: (A) ditticult geometries, (B) critical structures, and (C) dose monitoring in special treatments. Table II lists the number of patients inves- tigated with TL dosimetry in the three different groups divided in X-ray and electron therapy patients. A num- ber of TLD requests with more than one measurement aim qualified for more than one group. These cases are listed separately.
Table III shows several treatment types with their respective application groups.
Difficult geometn'es. TL dosimetry was requested for patients where the dose was difficult to predict from the plan. A typical example is the dose received by the skin
* Alnor Oy, PO Box 506, SF-20101 Turku, Finland.
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TABLE II
Number of patients investigated with TLDs in three different appli- cation groups.
TLD application Group X-rays Electrons
Evaluate dose in "difficult" cases A 58 62 Record dose to critical organs
(e.g. lens, scrotum) B 272 15 Monitor dose delivered in special
treatments (e.g. TBI, TBSI) C 29 13 A and B 19 28 A a n d C 0 6
378 124 Sum
in a variety of treatments such as breast [15,45] and head and neck [26] radiotherapy. The skin dose as the dose delivered to the organ skin is not necessarily equivalent to the entrance dose which is usually mea- sured under build-up to ensure electron equilibrium. Amongst others, the dose to the skin depends on the use of shadow trays, blocks, positioning devices and the obliquity of the incident beam. All these factors are difficult to account for and most planning computers give only an estimate of the dose by presenting isodoses
TABLE Il ia
TLD applications in X-ray treatments.
grouped closely near the surface. To determine the skin dose, several TLD packs were placed with the thin window foil facing the beam on the skin covering the area of interest.
The effective depth of measurement in the chips used for skin dose determination is about 1 mm. This does not seem to be an ideal representation of dose to the skin where the dose should be assessed at 0.07 mm depth [25,41]. However, it can be seen as a reasonable approximation in many cases such as most electron treatments and in the case of breast tangentials, be- cause the dose build-up in electron fields [51] and ob- lique incident X-ray beams [43] is less steep than in a perpendicular incident high energy X-ray beam.
The most serious acute and late effects in skin orig- inate in 0.3-0.5 mm depth [24]. The dose measured with TLD chips of 0.9 mm thickness gives a reasonable estimate of the dose at this depth in most therapeutic megavoltage beams.
For many ofthe patients listed in Table IIIb the dose to the skin or a superficial tumour was determined under bolus. In these cases the dimensions of the TLD chips are small compared to the dose variations which can be expected if the bolus is fitted properly.
Treatment type No. of No. of TLD Measurements Typcial X-ray Application patients measurements per patient energy (MV) group
TB 1 29 174 Brain, head and neck (TLD aim to determine lens dose only) 223 308 Head and neck (others) 29 56 Breast 32 41 Pelvis 8 8 Treatments where TLD aim was to determine scrotal dose only 49 56 Others 8 2 I
378 664 Sum
6.0 4 C 1.4 4 and 6 B 1.9 4 and 6 A, B 1.3 4 A 1.0 10 A 1.1 6 B 2.6 Various A, B
1.7
TABLE l l lb
TLD applications in electron treatments.
Treatment type No. of No. of TLD Measurements Typical electron Application patients measurements per patient energy (MeV) group
TBSI 7 84 12.0 6* C THSI 6 68 11.3 6* C Match of fields 6 45 7.5 9 A, C Head and neck (TLD aim to determine lens dose only) 15 20 1.3 Variable B Hed and neck, face, skin dose 77 155 2.0 6 and 9 A, B Upper body 6 7 1.2 Mainly 9 A Lower body 7 15 2.1 Variable A
Sum 124 394 3.2
* Energy of the 6 MeV electron beam moderated down to 4 MeV.
Group A includes a number of breast treatments (compare Table IIIa) where the dose was determined in areas where unwanted skin reactions could be ex- pected or on scars and drainholes which required ad- ditional boosting. A wide variety of head and neck treatments with electrons and photons are part of group A as can be seen in Table III. Measurements include the assessment of the dose to the skin in open fields as well as dose measurements under build and the deter- mination of dose in body cavities.
Critical structures. The most common TLD request was to record the dose to a critical organ such as the eye lens and the scrotum. Including TLD applications sharing group A and B (see Table II), 334 of the 502 studied patients were of application group B.
For lens dose measurements the dose was deter- mined separately in AP/PA fields and lateral fields. In AP/PA fields the TL dosimeters were placed on the eye lid under 7 mm of wax to mimic the position of the lens [50]. In lateral fields one pack of TL dosimeters was placed each in inner and outer canthi of the eye [30]. The lower and higher readings were taken as the min- imal and maximal possible lens doses respectively.
The scrotal dose [13] was determined with four packs of TLD chips placed around the scrotum. The TL dosimeters were placed inside the scrotal shield if shielding was used.
Dose monitoring. For certain treatment techniques the delivered dose was monitored over the whole course of treatment. This included X-ray TBIs [1], electron TBSIs [2], and matching of electron fields [ 16] in dif- ficult geometries for skin treatments where rows of TLD chips were used across the junction line. They were located under appropriate build up. The latter applica- tion falls also under group A: complicated geometries.
X-ray TBIs were treated at 320 cm FSD with a 4 MV X-ray beam in two lateral fields. Between 6 and 11 packs of TLD chips were placed at selected locations during each treatment. 6 of the positions (forehead, lateral neck, axilla, midline chest, medial thigh and me- dial ankle) were kept for each treatment. The TL do- simeters at air-tissue interfaces were covered with 1 cm wax build-up.
For the TBSI the patient was standing on a rotat- ing platform at 320 cm distance to the electron source. The 6 MeV electron beam of the linear accelerator was moderated down to 4 MeV by means of Perspex and polystyrene filtration [28]. TLD packs were placed without build up on 22 different positions on the pa- tient's skin. The TL dosimeters were put on slightly different locations each day to avoid permanent shield-
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ing of skin. It was the aim of the TLD measurements to monitor the dose delivered on each treatment day as well as to define areas which would need boosting (e.g. inner thighs, top of head).
Total head skin irradiations (THSI) were treated at an extended distance of 156 cm in a similar way irra- diating the head of the patient only. For THSI treat- ments only 11 TLD packs were used on each treatment day.
TBSIs and THSIs were planned from collected physics data only not using the planning computer.
Correlation between measured and expected dose The correlation between measured and expected dose was regarded as satisfactory, if the difference between them was less than 10~o of the measured dose. This deviation is larger than the precision for dose delivery in radiotherapy of 5 ~o which is generally regarded as a minimum requirement [9]. However, it was not possi- ble to trace down the 5 ~o difference between expected and measured doses retrospectively from the TLD re- suits quoted in the reports and the documentation of the treatment planning. In addition, the choice of a difference of 10~o between expected and measured dose makes it highly unlikely that more than one discrepancy amongst the 500 investigated cases is caused by statis- tical fluctuations of the TLD measurement itself. This can be calculated from an accuracy of + 5~'o (2 S.D.) of the detection system as documented in Table I [42].
In TLD applications of group A and C, where typ- ically a dose of 200 cGy was expected, a difference of 10 ~ between measured and calculated doses proved to be a suitable criterion. For the TLD measurements of group B (critical structures) an additional criterion was used because 10,% differences are of no clinical signif- icance for very low expected doses and should therefore not be counted as discrepancy.
The minimum cataractogenic dose is of the order of 8 Gy [ 18,50]. Therefore, it was assumed that in a typ- ical fractionation scheme of 30-35 fractions a differ- ence in lens dose of 10 cGy per treatment day would be significant. Consequently, cases of group B where the aim of the measurement was to determine and record the lens dose (compare Table III) were only counted as "discrepancy" if the difference between ex- pected and measured dose was 10~o and at least 10 cGy.
Total scrotal doses of less than 1 Gy do affect the sperm count [13]. Therefore, the criterion used for scrotal dose measurements in group B (compare Table III) was 10~/o and at least 5 cGy difference between expected and measured doses.
A difference of 5 cGy is at the limit of the precision
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of the results quoted in the TLD reports investigated in this study.
All discrepancies were investigated to identify the reasons. We noted remarks made in the TLD reports (such as "chip lost", "patient moved during treatment") and checked the raw TLD data (e.g. a standard was used for the evaluation which did not account for the supralinearity of the chips). Also, notes in the treatment sheet (such as "block edge moved", "new number of monitor units") and the isodose treatment plans were investigated. From these observations the discrepan- cies could be classified into three different categories:
(1) discrepancies due to errors made in the TLD de- termination or evaluation
(2) discrepancies due to mistakes made during the pa- tient set-up and
(3) discrepancies due to errors in the treatment plan- ning and dose calculation procedure.
The classification of a TLD measurement into more than one category was allowed. A more detailed list of reasons for the discrepancies in each group can be seen in Tables Va (TLD problems), Vb (set-up problems) and Vc (planning problems).
Results
Between January 1986 and July 1991, 6~o of all treated patients had TLD dose verification during some part of their radiotherapy treatment. Table III lists the number of patients having TLD measurements for different TLD applications divided into X-ray (Table Ilia) and electron therapy (Table IIIb). Patients undergoing ra- diotherapy with electrons were 1.5 times more likely to have a TLD measurement requested than X-ray pa- tients.
Also listed in Table III are the number of TLD measurements performed for a particular treatment ap- plication and the average number of measurements per patient. The average number of TLD measurements on all the 502 patients investigated in this study was 2.1.
In addition to patient and TLD numbers, Table III gives the X-ray and electron energy typically used for the treatments listed and lists the TLD application group for each treatment type. As can be expected, the number of TLD measurements per patient is highest for treatments of group C (monitoring of treatments).
In 96 out of 502 patients (15~o of X-ray patients, 33% of electron patients; 19~o overall)the TLD mea- surement did not confirm the dose determined from the
TABLE IVa
Number of discrepancies between expected and measured dose for X-ray treatments.
Treatment type No. of discrepancies No. of patients % TLD related Set-up related Plan related Not clear
Reasons for discrepancies between expected and measured dose caused by the TLD measurement itself; the radiation type is given if only X-rays or electrons were used in a certain category; the alloca- tion of a discrepancy to more than one application group was allowed.
Reason for discrepancy Total No. Radiation Application group
A B C
TLDs misplaced TLDs mixed up Fields not separated for
lens dose Wrong standards used
(supralinearity) Size of TLDs too big Not clear
Sum
4 2 2 0 2 1 1 0
2 X-rays 1 2 0
5 5 2 0 4 3 2 1 5 3 2 1
22 15 11 2
treatment plan according to the criteria defined in Ma- terials and methods. Table IV lists the number of dis- crepancies for the different treatment types shown in Table III. Table IVa lists the discrepancies which oc- curred in X-ray treatments and Table IVb those in electron treatments. Also given in Table IV is the cat- egory for the discrepancy encountered as defined in materials and methods. Discrepancies were allowed to qualify for more than one category.
Tables Va-c show the different reasons for allocat- ing cases into one of the three categories. The last sub- group in each table is labelled "not clear". Even though the actual reason for these discrepancies could not be determined, it was possible to allocate them to one of the three main categories 1, 2 and 3, because of unspe- cific remarks in TLD reports and treatment sheets such as "problems with TLDs", "set-up changed", or "plan altered".
The total number of discrepancies which occurred in TLD measurements of complicated geometries (60 in group A) is higher than those in determination of the dose to critical structures (44 in group B) and during the monitoring of special treatments (15 in group C). The total number of discrepancies in the three application groups is given in more detail in Tables Va-c.
Measurement problems Mistakes made during the TLD measurement itself ac- counted for 21 °/o of all discrepancies. They are listed in Table Va. The most common problem encountered was the supralinearity of LiF (5 cases) where the standards did not cover the range of doses measured on the pa- tient. This is followed by the inappropriate placement of the chips (4 cases). The physical size of the used TLD chips was found to be too big for one lens dose measurement in a retinoblastoma treatment, in one case where electron fields were matched and in two cases where the scatter from a contact lead shield was to be determined.
Set-up problems A number ofdifferent set-up errors were discovered due to TLD measurements. They accounted for 30 ~o of all discrepancies and are listed in Table Vb. These in- cluded the use of bolus of the wrong thickness or ill fitting bolus (9 cases) and problems with the abutment of electron fields (7 cases). In addition to that, improper shielding and field alignment (9 cases) and inadequate immobilisation of patients (4 cases) were discovered as cause of discrepancies.
Planning problems The most common reasons (41~o of all cases) for a discrepancy between measured and expected dose were found to be inaccuracies and problems occurring dur-
TABLE Vb
Reasons for discrepancies between expected and measured dose caused by the treatment set-up; the radiation type is given if only X-rays or electrons were used in a certain category; the allocation of a discrepancy to more than one application group was allowed.
Reason for discrepancy Total No. Radiation Application group
A B C
Patient moves Incorrect abutment of fields Scrotal/lens shield not properly placed Blocks/field edge not properly aligned Problems with bolus Wrong number of mu given Not clear
Reasons for discrepancies between expected and measured dose caused by the treatment planning; the radiation type is given if only X-rays or electrons were used in a certain category; the allocation of a discrepancy to more than one application group was allowed.
Reason for discrepancy Total No. Radiation Application group
A B C
Plan of bolus incorrect (wrong density) Problems with shielding (transmission) Scatter from 3rd dimension Critical organ not in calculation plane Skin dose in difficult geometry Oblique incidence of beam Abutment of fields on curved surface Output in small electron fields Irregular shaped field output Shielding/tissue interface Not clear
ing the treatment planning procedure. Table Vc lists the variety of reasons for these discrepancies. Most of them occurred only a small number of times since action was taken to avoid the particular problems in the future.
The output in small or irregularly shaped electron fields (10 cases) posed a particular problem if treatment was delivered at extended FSD. However, usually these problems could be avoided by using individual output factors measured with an ionisation chamber.
In 8 ~o of all discrepancies the category could not be identified retrospectively.
Discussion
The precision and accuracy of the employed TLD mea- surements as shown in Table I is in the same range as in other clinical TLD systems [ 14,49]. However, far better reproducibilities have been reported recently [ 11 ] and the purchase of a new TLD reader with contactless nitrogen heating seems to be a step towards an im- proved TLD accuracy as can be seen in Table I. The purchase of a better annealing oven is also planned to improve the precision of the measurements.
During the investigation period more than one third of all TLD measurements were made in electron fields even though only one in five treatments was performed with electrons. This reflects one of the main applica- tions of TL dosimeters as surface or skin dosimeters. The use of build-up on the TLD chips broadens the range of applications. Only in 4 of the 502 patients reviewed, TL dosimeters were used in body cavities such as nose and mouth.
The absolute number of discrepancies found is rel- atively high compared with those reported in other
studies with TL dosimeters [17,37,49] and semicon- ductors [20,34,35,40] where smaller differences were found between expected and measured dose. This is due to a bias of the data in the present study where a large number of complicated geometries were investi- gated. The relative number of discrepancies in group A (complicated geometries) with 35 ~o of the cases is high. Many of these occurred in treatment situations where the prediction of the dose is known to be inaccurate. Examples for these are the dose to the skin in the build-up region and electron calculations for irregular shaped patient surfaces (e.g. perineum). As can be ex- pected, the number of discrepancies in group B (criti- cal structures) is significantly lower amounting to 13 ~o of all cases.
Most of the 31 ~o of discrepancies in group C (dose monitoring) occurred in situations where small daily variations in set-up can cause considerable variations in dose (e.g. abutment of electron fields [16]).
Few other reports are published on TLD for clini- cal in in vivo dosimetry. Mansfield and Suntharalingam investigated only a limited number of patients for one application type each [37] and Hamers et al. reported on a small number of entrance and exit dose measure- ments in the conservative treatment of the breast [ 17].
In the extensive study of Rudrn [49] TLD was used routinely on every patient for a wide range of applica- tions. Entrance dose measurements were done at least four times per treatment to verify the central axis out- put and patient set-up. The exit dose was determined on patients treated for carcinoma of the oesophagus at various points in the caudal-cranial direction. In these patients Rudrn also describes intracavity TLD mea- surements which were made up to two times during the
treatment. Also repeated lens and scrotal were per- formed on patients where fields were close to these organs to give an estimate of the dose received by these critical structures.
In Rudtn's study the relative number of discrepan- cies as defined above is about 5 in 100 [49]. This has to be compared with the 19~o discrepancies which were found in the present study. However, the group of pa- tients investigated in the present study represents only a subgroup of all patients (about 6~o of all) which had mostly treatments where the dose was difficult to pre- dict. In addition to that, most dose measurements were performed not in the relatively well defined dose region at central axis of the field. Therefore, one would expect a higher number of discrepancies to be found in the present study than in studies where in vivo dosimetry was performed routinely on all patients. The later, al- though desirable, is extremely labour intensive as can be seen from the total number of nearly 30 000 TLD measurements in one year (1974) as described by Rudtn [49].
Other TLD studies compare the dose measured in phantoms to the dose predicted from treatment plan- ning. In an investigation of the complicated geometry of tangential breast irradiation, KnOOs et al. [31 ] found up to 20 ~ difference between measured and calculated dose in the axillary region of a body-shaped phantom.
Most in vivo dosimetry with semiconductors is aimed at measuring the entrance dose [34,40]. This provides a fast and convenient check of patient set-up, beam parameters and machine performance [ 19]. Mea- suring in the centre of the irradiation fields of 68 head and neck patients Leunens et al. were able to identify a systematic difference of 2 .2~ between measured and expected dose [34]. This error could be traced down to inaccuracies in the dose calculation of the treatment planning system and to an inaccuracy of the absorbed dose determination with an ion chamber at the depth of the maximum dose. Al~er correction for this system- atic error the authors found a deviation of 5 % or more between measured and expected dose in 3~o of all 554 investigated treatment set-ups [34].
More information than with entrance dose measure- ments only can be obtained with in vivo dosimetry if entrance and exit dose determination can be combined [17,35]. These measurements allow the determination of the target absorbed dose and check the planning algorithm and patient data, such as contours and tis- sue inhomogeneities [35]. In 230 measurements on 34 patients with neck and oral cavity malignancies, Leun- ens et al. found underdosages of 5 % and more at mid- line in 1/5 of the measured treatment set-ups [35].
In the two studies of Leunens et al. [34,35] the rea-
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son for all large deviations (> 5 %) could be determined. An attempt was made in the present study to identify also the reasons for discrepancies. Since this had to be done retrospectively only 92~o of all discrepancies could be allocated to one of the three different catego- ries for discrepancies defined in Materials and meth- ods.
Measurement problems One aim of the present study was to determine prob- lems in the TLD dosimetry service itself. In about 4% of all patients who underwent in vivo dosimetry, mis- takes were made in the measurement procedure itself. The errors have been picked up and the measurement was repeated with usually satisfying results. The only problem in Table Va which could not be overcome in the repeated TLD measurement is the size of the TLDs. Consequently, a study on the use of TLD chips in different sizes has been initiated.
Set-up problems In vivo dosimetry with TL dosimeters discovered a wide range of set-up related treatment problems. Most of them could be corrected before the delivery of the following treatment fraction and the improvement was verified in a repeated TLD measurement. Only in some field-matching situations with electron fields (3rd row in Table Vb) the treatment was monitored over the whole treatment course. The gap between the fields was ad- justed on a daily basis in reference to the treatment plan to achieve the best overall dose distribution on the skin and in the underlying tissue.
Planning problems Some of the planning problems shown in Table Vc could have been avoided using a 3-dimensional plan- ning system. Typical examples are ignored scatter from the 3rd dimension or cases where the critical organ was not located in the plane of calculation. However, some of the problems such as the surface dose in complicated geometries depend on a wide range of factors such as electron contamination from trays and blocks, which axe difficult to account for even in very sophisticated planning systems. Therefore, there will still be a place for in vivo dosimetry to explore the delivered dose on an individual basis.
A number of treatment modifications were intro- duced due to TLD measurements. Examples for the impact of the in vivo dosimetry in clinical practice at the Prince of Wales Hospital in Sydney axe:
- Treatment modalities, such as bolus thickness and shielding, have been changed for individual patients.
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- A bigger scrotal shield had been designed. - The angle of lateral fields in head and neck cases has
been slightly adjusted to minimise lens dose at the contralateral side.
- Spacing of fields for the treatment of superficial le- sions has been adjusted.
- The output factors for small electron fields in ex- tended FSD have been reviewed. It was recom- mended to measure the output factor for each indi- vidual field prior to the treatment using an ion chamber.
- It was recommended to request TLD measurements in small and irregularly shaped electron fields.
- The dose in areas which should receive a radiation boost after or concurrent to normal treatment has been determined from TLD results. Example is the dose to the inner thighs which do not receive the prescribed dose in TBSIs [ 16].
- The dose has been measured and adjusted in treat- ments which cannot be planned adequately with the present treatment planning system (e.g. TBSI).
C o n c l u s i o n
The results of in vivo dosimetry on patients as well as TLD measurements in phantoms make TLD an im- portant clinical dosimetry tool. Results obtained in di- rect dose measurements on patients help to improve the treatment of the patient involved, and have an impact on future treatments. TLD proved to be particular use- ful in difficult geometries where best use could be made of the advantages of TL dosimeters such as their stand- alone character and their small physical size. This value outweighs the costs of the service. The recognition of TLD as a useful clinical verification and dosimetry tool is reflected in the increasing number of TLD requests in The Prince of Wales Hospital in Sydney [32].
A c k n o w l e d g e m e n t s
We would like to thank Ann Waiters and Elizabeth Chapman for their technical assistance and Chris Kar- olis for suggesting the idea for the study.
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