A Dosimetric Comparison of the Field in Field Method versus Electronic Compensation in the Treatment of Left Sided, Low Stage Breast Cancer. Authors: Ryan Salem, BS, RT(T), Christina Ong BS, Sean Fergusson BS, RT(T), Rodger Williams, BS, CMD, RT(R)(T) Medical Dosimetry Program at the University of Wisconsin – La Crosse, WI Abstract Introduction: The objective of the case study was to perform a dosimetric comparison between the 3D conformal field in field (FIF) and electronic compensation, or irregular surface compensator (ISC), techniques in whole breast radiation therapy for low stage breast cancer. The metrics of target coverage, healthy tissue constraints, and maximum dose regions were analyzed to determine the more effective treatment method. Case Description: Ten patients were retrospectively selected from a single cancer center who were diagnosed with low stage, left sided breast cancer ranging from ductal carcinoma in situ (DCIS) to Stage II T2N0M0. All patients received a lumpectomy procedure prior to radiation treatment planning. The patients’ planning target volume (PTV) and gross tumor volume (GTV), representative of the breast tissue and lumpectomy site, respectively, were contoured by a single physician according to Radiation Therapy Oncology Group (RTOG) 1005 guidelines for all the CT datasets. Treatment plans were created by a single dosimetrist for each respective treatment method following RTOG 1005 recommendations and a series of dose delivery metrics.
Transcript
A Dosimetric Comparison of the Field in Field Method versus Electronic Compensation in the Treatment of Left Sided, Low Stage Breast Cancer.
Authors: Ryan Salem, BS, RT(T), Christina Ong BS, Sean Fergusson BS, RT(T), Rodger Williams, BS, CMD, RT(R)(T)
Medical Dosimetry Program at the University of Wisconsin – La Crosse, WI
Abstract
Introduction: The objective of the case study was to perform a dosimetric comparison between
the 3D conformal field in field (FIF) and electronic compensation, or irregular surface
compensator (ISC), techniques in whole breast radiation therapy for low stage breast cancer.
The metrics of target coverage, healthy tissue constraints, and maximum dose regions were
analyzed to determine the more effective treatment method.
Case Description: Ten patients were retrospectively selected from a single cancer center who
were diagnosed with low stage, left sided breast cancer ranging from ductal carcinoma in situ
(DCIS) to Stage II T2N0M0. All patients received a lumpectomy procedure prior to radiation
treatment planning. The patients’ planning target volume (PTV) and gross tumor volume (GTV),
representative of the breast tissue and lumpectomy site, respectively, were contoured by a single
physician according to Radiation Therapy Oncology Group (RTOG) 1005 guidelines for all the
CT datasets. Treatment plans were created by a single dosimetrist for each respective treatment
method following RTOG 1005 recommendations and a series of dose delivery metrics.
Conclusion: **
Key Words: Field in field, electronic compensation, irregular surface compensator, low stage
breast cancer, conformal **
Introduction:
In women, breast cancer has the highest incidence of all cancers in the United States at
125 cases per 100,000 women. It is also the second deadliest cancer in women behind lung and
bronchus cancer.1 Whole breast radiotherapy is a common way to treat low stage breast cancer.
Both FIF and ISC techniques are modern methods used in breast radiotherapy, as they use
advanced technology to improve dose conformity while reducing dose to healthy tissues
throughout treatment.2 Research has not yet shown a direct comparison between the 3D FIF and
ISC techniques with modern planning technology to the whole breast. Multiple studies have
found treatment differences when the PTV is determined by a GTV expansion, but not with a
physician contoured PTV representative of the whole breast on the affected side.
The FIF technique is a widely preferred method of delivering tangential whole breast
radiation therapy by utilization of multi leaf collimation (MLC) field shaping. Open tangential
fields are created by use of MLC to carry most of the dose. To further increase target dose, the
FIF technique incorporates subfields with MLC to block high dose regions, resulting in increased
dose conformity. Many studies suggest that the FIF technique results in better dose homogeneity,
reducing hot regions, and limiting dose to healthy tissue.3 Irregular surface compensation is an
intensity modulated radiation therapy (IMRT), tangential breast technique that utilizes forward
planning to deliver a homogenous dose to irregular surfaces by use of dynamic MLC. The MLC
movement is determined by the treatment planning system’s (TPS) calculated intensity of the
treatment beam to areas receiving dose.4 The modification of fluence is also done by the MLC
leaves which improves dose homogeneity when delivering treatment to an irregular surface, such
as a breast. The ISC has been found to reduce hot regions of dose and acute toxicity in women,
particularly those with larger breast and inferior tumors.5
By increasing dose homogeneity and reducing dose to healthy tissue, many post-
treatment complications from breast radiotherapy can be reduced. Common post-treatment
complications include tumor recurrence, chest wall recurrence, contralateral breast tumors, heart
toxicity, and radiation pneumonitis.6 Guidelines in RTOG 1005 were followed for both target
delineation and OAR dose constraints. A single physician drew all target volumes in this study.7
Many studies that have demonstrated comparisons in common breast radiotherapy
techniques are ridden with variables that can lead to inaccurate findings regarding the most
effective treatment method. Single, low population studies included breasts on either side of the
body, a wide range breast sizes with PTVs not drawn consistently, and tumors of variable stages.
Various treatment machines, manufacturers, MLC sizes, and treatment energies are also mixed
into the same studies, posing high variability in treatment planning results. Additionally,
different dose algorithms and different TPSs have also been used in studies with a single patient
population. The accuracy of correction factors and calculation methods can also lead to result
inaccuracies. A primary objective of this study was to limit variables within treatment planning
to ensure the accuracy of the results.
The objective of this study was to perform a comparison between the FIF and ISC
techniques in tangential whole breast radiotherapy. By minimizing variables in the patient
selection, target delineation, and planning process, the study provided a true comparison of two
modern planning techniques. By using the same patient population and dose delivery metrics to
the same contoured targets and OARs, the two planning methods can be compared regarding
dose homogeneity, target coverage, maximum dose regions, and OAR doses for whole breast
tangents. If one treatment method proves significantly better than the other, further research on a
larger patient population will be needed to confirm the findings.
Case Description:
Patient Selection
Ten patients were selected retrospectively who were diagnosed with low stage breast
cancer ranging from DCIS to T2N0M0 in the left breast only. Only patients with a lumpectomy
procedure were selected so that the physician could delineate the GTV. Left sided patients were
selected so that breast tissue, heart, lung, and contralateral breast doses could all be analyzed and
compared at the completion of treatment planning. The “bridge” separation, or distance between
the medial and lateral borders was taken into consideration to avoid extremes. Although variable
breast sizes and lumpectomy site locations were used, extremes were avoided so that research
could be representative for the majority of breast patients.
All patients selected underwent a free breathing CT simulation from a Phillips Brilliance
large bore CT scanner with 3mm slices. Planning for each patient was completed utilizing a 3D,
free breathing CT scan. All patients selected for the study were simulated in the same, or similar
treatment position to further avoid variability in the research. All patients were simulated in the
head first-supine position and flat with both arms up holding a T-grip in the central region. The
treatment position included a vacuum bean bag used under the arms and head while having the
patient’s head tilted away from the effected side and the vacuum bag being limited under the
thorax. An unindexed knee cushion was used for leg and back support. A ring was used around
the feet for all patients to limit leg movement during treatment and simulation (Figures 1-4). Free
breathing was used to further emphasize the importance of target coverage and OAR avoidance,
as well as to limit variability.
Target Delineation
To allow for a quantitative analysis of treatment plan quality, both breast tissue of the
affected breast (PTV) and the lumpectomy site (GTV) were contoured according to RTOG 1005
by a single physician following the 3D CT simulation using the Varian Eclipse TPS version 13.6.
This version makes use of the Analytical Anisotropic Algorithm (AAA) for photon energies. The
targets used in this study align with target definitions in RTOG 1005 part 6.4.2 for “Lumpectomy
GTV” and “Breast PTV Eval”.7 The lumpectomy GTV is contoured using the excision cavity
volume, lumpectomy scar, seroma, and/or extend of surgical clips. All patients in the study had a
clearly identifiable lumpectomy bed. The breast PTV was drawn to include the palpable breast
tissue, apparent glandular breast tissue, and the lumpectomy site. This contour was limited to
5mm from the skin and extends no deeper than the anterior surface of the ribs.7 Both treatment
volumes and the contralateral breast can be seen in Figure 5.
Contoured OARs include the contralateral breast, ipsilateral lung, contralateral lung, and
heart, again in accordance with RTOG 1005. The contralateral breast included visible glandular
tissue delineated from the CT while following the RTOG Breast Atlas. All lung volumes were
contoured with auto-segmentation and manual editing and verification. The heart was contoured
from where the pulmonary trunk branches into the left and right pulmonary arteries and extended
to its most inferior region in the diaphragm or lower.7
Treatment Planning
The prescription and fractionation schedule in this study were consistent with Arm 1 of
RTOG 1005, Standard Whole Breast Irradiation with Sequential Boost, delivering 50 Gray (Gy)
in 25 fractions of 2Gy, 5 days a week for the primary tangential plan to the whole breast.
Because boost treatment modalities and available options at different sites are variable, only the
primary plans of 50Gy were analyzed in this study to strictly compare FIF and ISC methods of
treatment.
Both FIF and ISC techniques were planned for treatment on a Varian TrueBeam linear
accelerator with the same MLC. Both treatment machines had an equal number of 0.5cm and
1.0cm MLC leaves used in collimation. Field in field had access to 6 megavoltage (MV), 10MV,
and 15MV beam energies available for planning. Irregular surface compensation had 6MV and
15MV beam energies available. 6MV beams were required in both planning modalities, and
higher energies were used to achieve planning outcomes. All planning was completed using
Varian Eclipse version 13.6 TPS, utilizing AAA.
Beam angles were determined by medical dosimetrists to best suit coverage needs based
on external patient contours, RTOG recommendations, and the visual analysis of a reviewing
physician. The PTV size, GTV location, and energy availability all helped determine gantry
angles of the treatment beams. Collimator angles for the primary treatment beams were
consistent with 0 or 90 degrees but could be modified in the FIFs or ISC. Primary collimator
angles were kept this way to ensure that the radiation field was consistent between both
treatment modalities.
To accurately compare both treatment methods, all plans were normalized to achieve 3
coverage specifics regarding the PTV and GTV. To push both treatment methods capabilities,
coverage requirements exceeded those of RTOG guidelines. The maximum hot spot for any plan
could not go over 115%. Additionally, both 95% of the PTV was required to be covered by 95%
isodose, and 100% of the GTV was to be covered by 99% isodose. The dosimetrist is to choose a
normalization method that achieves all three of these requirements, even if OAR dose constraints
fail. There was no modification of target coverage doses in this study. This is to help distinguish
the capabilities of each treatment modality regarding target coverage and OAR avoidance.
Delivered doses to OARs were to not exceed the regulations per the RTOG 1005
protocol. The contralateral breast was to not exceed 310cGy anywhere, and no more than 5% of
it was to exceed 186cGy. Per RTOG, the volume of the ipsilateral lung receiving 20Gy can be
no more than 15% (V20 < 15%). Additionally, for the ipsilateral lung, the V10 must be less than
35% (V10 < 35%) and the V5 must be less than 50% (V5 < 50%). For the contralateral lung,
5Gy could not exceed 10% of the contoured organ (V5 < 10%). No more than 5% of the whole
heart was to exceed 20Gy (V20 < 5%). Also, no more than 30% of the whole heart was to exceed
10Gy (V10 < 30%). The heart also had a mean dose limit of 400cGy.7 It is of moral
responsibility of dosimetrists to ensure that OAR not only meet the regulations, but also are
made as low as possible while meeting other metrics. The medical dosimetrists in this study
ensured that OAR doses were as low as they could be given the target dose requirements.
Plan Analysis and Evaluation
Conclusion
With so many variables in previous studies comparing different methods of whole breast
radiation therapy, this study emphasized a comparison without variability. By using the same
treatment machines, planning systems, and patient datasets, treatment plans were extremely
limited in variability. With the use of a single protocol and physician to outline PTVs, GTVs,
and OARs, the analysis of dose delivery was much more accurate than previous research. The
only notable variables in this study include having different dosimetrists perform each method of
planning and FIF having access to 10MV beams. The research also represents a majority of the
patient population due to the utilization of patients with different breast volumes, body sizes, and
lumpectomy cavity locations.
-Here is the rest of our outline that will be expanded upon as planning is completed-
V. Plan Analysis and Evaluation a. Every patient had a sentence or short paragraph simply describing the planning done for both FIF and E-comp b. Plans were evaluated for overall dose conformality, hot spots, and OAR dose constraints. c. The average doses to the targets in both FIF and E-comp were described. Charts showing dose statistics for targets were shown with averages and different metrics for each method here. d. Average hot spot and hot spot location in each plan following the same visuals as above. e. Each OAR dose constraint, value, and whether the specs passed or failed for each treatment technique for each plan was shown the same way as above. f. Both planning methods were compared in terms of target doses, dose conformality, hot spots, and OAR doses. (This paragraph and discussion will be long) g. Describe if one method of planning was better than the other and to what degree. If the information shows one method better than the other, statistics were used to determine if the findings are significant. h. Explain and determine whether visual analysis identified trends or variables in treatment planning. Ex: Did FIF work better on smaller PTV volumes... Did E-comp work better on smaller GTV volumes... Did e-comp provide better GTV coverage when the GTV was further away from the center of the breast?
VI. Conclusion a. Of all identified variables in the introduction and from previous studies, the study only included a few small variables like the FIF dosimetrist being able to use 10MV and the fact that different dosimetrists did the planning. b. One planning method was or was not better than the other regarding hotspots or dose constraints when normalization methods were consistent. c. Described that the patient population was limited, but the population was selected to represent a vast number of different tissue volumes and lumpectomy sites (GTV) in different regions of the breast. d. The research was consistent (or not) with prior research. We found that when variables are limited, and normalization methods are consistent between planning techniques, our research shows that FIF or E-comp could be a more effective treatment planning technique for free breathing, left-sided breast patients across a variety of patients. e. Was heart dose a factor? If not, we predicted that this study would also be reflected on the right side... Was lung dose a factor, if so, did we think the study should be done with deep inspiration breath hold (DIBH) to further research. f. Since our findings showed one method to be more proficient in the other to some regard (whether significant or not), we call upon further research to be done on a larger population.
References
1. Breast Cancer Statistics. Centers for Disease Control and Prevention Web site.
https://www.cdc.gov/cancer/breast/statistics/index.htm. Updated June 12, 2018. Accessed
July 23rd, 2018
2. Koivumaki T, Fogliata A, Zeverino M, et al. Dosimetric evaluation of modern radiation
therapy techniques for left breast in deep-inspiration breath-
