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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II: Clinical Practice
Version 1 - 25/04/2016
Endometrial Cancer
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Dose optimisation should be based on individual delineations of
the GTV and the CTV which will encompass the entire uterus.
Organs at risk are the bladder, rectum and sigmoid as defined for
cervical cancer. [27][28][29][34][37]. DVH-parameters should
be used for dose prescription, e.g. the D90 the dose to 90% of the
volume. For the OAR the minimum dose to the most exposed
2cm3 (D2cm3) should be evaluated and reported. A typical
example with Norman Simon capsules is shown in figure 15.6.
The ability to optimise the plan with sufficiently high CTV D90
and OAR dose below the constraints is closely related to the size
of the uterus, the location of the OAR and the type of applicator
used.
10.
DOSE, DOSE RATE AND FRACTIONATION
10.1 Postoperative vaginal brachytherapy
With HDR, a wide variation of schedules can be found in the
literature with a broad range of doses when converted to 2 Gy
fractions (EQD2), with additional variation introduced by
differing prescription points, typically either surface dose or
at 5mm depth. Most studies are institutional series including a
majority of low-risk patients. A frequently used schedule is 21
Gy in 3 fractions of 7 Gy prescribed at 5 mm from the cylinder
surface as was used in PORTEC-2. This schedule aims for to de-
liver a dose required for potential microscopic disease
There is considerable uncertainty with regard to the correct α/β
ratio for endometrial cancer. In the past a value of 10 has been
used however biologically it is an adenocarcinoma which is more
likely to have characteristics similar to breast and prostate can-
cer. These tumours have been shown to have a much lower α/β
and whilst the extreme estimates for prostate cancer of around
1.5 are perhaps not appropriate a figure around 4.5, similar to
breast cancer is most likely.. Using an α/β of 4.5 for tumour the
EQD2 of 21Gy in 3 fractions is 37.2Gy at 5 mm and presuming
approximately 150% at the surface the EQD2 will be 55.8 Gy.
Compared to external beam the brachytherapy is given in a
shorter time span, a factor not included in the EQD2 calcula-
tion. With α/β of 3 for OAR this is 42.0 Gy at 5 mm and 63 Gy
at the surface, explaining the increased rate of mild to moderate
Figure 15.9 Proposed dose prescription volumes for image guided brachytherapy. Using CT (a) it is not possible to define the tumour accurately and the CTV is the entire uterus; with MR (b) accurate defini-
tion of the GTV enables subvolumes of GTV, HRCTV which is the GTV with a margin including adjacent uterine walls and the IR CTV which is the entire uterus (equivalent to the CTV when using CT)
Reproduced from reference 29
Figure 15.9 (c) MR scans demonstrating planning volumes as defined in (b) above