LIMITATIONS OF 2D BRACHYTHERAPY R Holla*/S V Jamema** • Amrita Institute of Medical Sciences, Kochi • ** Tata Memorial Hospital, Mumbai

2D Brachytherapy - What we can see? • Great view of the applicator and the

source positions • Can locate some cardinal points on

some OARs which act as surrogates – bladder neck- by inserting foley

catheter with contrast – Vaginal mucosa through radio-

opaque gauze – Rectum when radio-opaque

marker is inserted or by rectal separator inserted in the vagina.

• Point A based dosimetry system • Reasonably uniform across the

centers • Time tested

2D Brachytherapy- What we cannot see • Target / disease at the

Cervix and parametrium • Uterus • Rectum – Anterior wall • Bladder –posterior wall • Sigmoid • Small bowel

History • Dosimetry systems

• Stockholm • Paris • Fletcher • Manchester

Original definition of Point “ A” • 2 cm lateral to the uterine canal and 2 cm from the

mucous membrane of the lateral superior fornix of the vagina in the plane of the uterus

Modified Point ‘A’

Although point ‘A’ was defined in relation to important anatomical structures, these cannot be

revealed on a radiograph.

• So point ‘A’ definition was modified in 1953 and is sometimes denoted as Ao or As (o stands for external os).

Point Av • Over the years, point A has been defined in many ways. • Point Av ( v stands for vagina) was proposed as 2 cm

lateral to the mid point of the cervical collar and 2 cm above the top of the colpostats(Potish 1987), measured at their intersection with the tandem mid point on the lateral radiograph.

ABS Point ‘A’ • Retained original Manchester

system point A denoted as Ao Or Af . • For tandem and ovoids, localization

of point A can be carried out using radiographs as follows: draw a line connecting the middle of the sources in the vaginal ovoids on the AP radiograph and move 2cm (plus radius of the ovoid), superiorly along the tandem from the intersection of this line with the intrauterine source line and then 2 cm lateral on either side of the tandem.

ICRU 38 & Point A

• ICRU 38 discouraged the use of point A and B because the exact meaning and their definitions have not always been interpreted in the same way in different centers and even in the same center over a period of time.

• Encourages the use of target volume for dose prescription and reporting along with the reference volume for 60Gy absorbed dose prescription.

• This report is being revised and may include some dose points similar to the classical systems.

2D - Treatment planning

Standard loading pattern

Tandem (6cm)

Ovoid (1.5,2.0cm)

1 4 3 5 5 6 7 7** 10 13* 16 20

4cm

Source

Empty Dwell Pos Flang

e

Tandem

Tandem

Ovoid

* 4cm tandem **ovoid dia 2.5cm

OARs – ICRU rectum and bladder point

Correlation of ICRU reference point and D2cc

• Rectum: ICRU rectal reference point correlates with the D2cc dose of the organ rectum

• Bladder: ICRU bladder reference point, does not correlate

well with bladder complications (ICRU 38 bladder point underestimates the bladder dose)

Upper rectal and sigmoid points on 2D radiographs

• 27 Patients treated with CT image based dosimetry

• Upper rectal and sigmoid points were marked on CT images

• Searched for a reproducible point with respect to applicator and other points

• No point was found that was reproducible that can act as a surrogate for upper rectal and sigmoid

• No agreement was found among the researchers yet on sigmoid point for 2D brachytherapy

• Different methods of definition provide different values for the calculated dose rate to point A.

• Therefore, if the prescribed dose to point A is used to

calculate the total time of application, different values of time will be obtained for different methods used to assign the prescription point.

• Relates to position of sources and not to specific anatomic

structure.

• It is very sensitive to position of ovoid sources relative to tandem which should not be deciding factor in deciding on implant duration.

• Depending on size of cervix point A may be inside or outside of tumor.

Limitation of Point A

R.Potter et.al. Acta Radiologica. 47:2008

2D and 3D Optimisation

Point - A is here to stay…..

• Recommended while reporting treatment regimens. • Allows comparison between different approaches. • Acts as a link to non- 3D image-based approaches.

• Serves as a quality assurance parameter along with TRAK. • Standard loading point-A normalized plan acts as a perfect

starting point for complicated IC+IS plans.

Limitation of 2D Planning/benefit of image based

Comparison of DVH parameters (in 57 pt’s) TanderUp et al, RO 2010

Point A Standard loading

HR-CTV, OARs MRI optimised

2D 3D

HR-CTV D90 and D100 HR-CTV, EQD2 dose (Gy)

50

75

100

125• Mean value almost not

affected by optimisation

• Variation decreases significantly by optimisation

D100 std 72 Gy

Prescribed dose 85 Gy

D100 opt 74 Gy

D90 std 92 Gy

D90 opt 91 Gy

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D90 HR CTV, EQD2 Gy

D2cc

Sig

moi

d, E

QD2

Gy

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D90 HR CTV, EQD2 Gy

D2cc

Bla

dder

, EQ

D2 G

y

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50 60 70 80 90 100 110 120 130 140

D90 HR CTV, EQD2 Gy

D2cc

Rec

tum

, EQ

D2 G

y

10/57 pts

K Tanderup et al, Radiother Oncol 2010

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D90 HR CTV, EQD2 Gy

D2cc

Sig

moi

d, E

QD2

Gy

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D90 HR CTV, EQD2 Gy

D2cc

Bla

dder

, EQ

D2 G

y

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D90 HR CTV, EQD2 Gy

D2cc

Rec

tum

, EQ

D2 G

y

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D90 HR CTV, EQD2 Gy

D2c

c R

ectu

m, E

QD

2 G

y

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D90 HR CTV, EQD2 Gy

D2cc

Bla

dder

, EQ

D2 G

y

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90

100

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50 60 70 80 90 100 110 120 130 140

D90 HR CTV, EQD2 Gy

D2cc

Sig

moi

d, E

QD2

Gy

44/57 pts

K Tanderup et al, Radiother Oncol 2010

Volume is important!

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140

0 10 20 30 40 50 60 70 80 90 100 110

Volume HR CTV, cm3

HR

-CTV

D90

sta

ndar

d

Standard

Median volume: 32cc

Violation of OAR constraint K Tanderup et al, Radiother Oncol 2010

Volume is important!

50

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0 10 20 30 40 50 60 70 80 90 100 110

Volume HR CTV, cm3

HR-C

TV D

90 o

ptim

ized

Optimised

Violation of OAR constraint Application of needles

11% needles 64% needles

Benefit of optimisation (57 pt’s, Aarhus)

Small tumours < 32cc

Large tumours > 32cc

STANDARD OPTIMISED STANDARD OPTIMISED

Target covered 93% 93% 14% 71%

OAR respected 24% 90% 64% 93%

Fraction of patients respecting DVH constraints

K Tanderup et al, Radiother Oncol 2010

2D Brachytherapy – large volume tumors are treatable?!

HR-CTV vol < 20cc

HR-CTV vol ~ 30cc

HR-CTV vol > 40-50cc 5mm

5mm

10mm

Change of applicators!!!

2D brachytherpy works Optimization mandatory

2D brachytherapy works good target coverage Caution: dose to OARs

Summary & Take home message • Point A dose is NOT a surrogate for tumor dose. • For small tumors, 2D planning delivers high dose to the

tumor while exceeding the dose to the OARs • For large tumors, 2D Planning under-dose the tumor • ICRU rectal reference point correlates with the D2cc

dose, while no correlation was found with bladder • No agreement on sigmoid point for 2D brachytherapy

Image guidence

Radiographs

CT Plan with Point ‘ A’ Prescription

Thank You