Endometrial Cancer_GEC ESTRO Handbook of Brachytherapy

Endometrial Cancer

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II: Clinical Practice Version 1 - 25/04/2016

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

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.

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

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