16 Cervix Carcinoma

336 Cervix Cancer

with the applicator in place the appropriate position of the applicator in the uterine cavity, to estimate gross tumour volume and its relation to the implant and to critical organs. Since 1997, it has been possible to enter sectional images directly into the treatment planning system. Delineation of GTV and PTV and critical organs is performed and dose volume relations are assessed. Dose distribution (e.g. 7 Gy volume) is displayed as a DVH for the GTV/PTV and for organs at risk. Dose adaptation is additionally based on these parameters, not only looking at dose points (radiography) but also at dose volume relations, for the rectum and the bladder (CT and MRI) and for the GTV/PTV (MRI). Limits have been defined for dose volume relations for the rectum, where a mean dose per fraction of 5 Gy (total dose 20 Gy) is limited to a rectal tissue volume of 2 cm 3 . In the recent Vienna experience, grade III/IV complications were seen after ~7 Gy per fraction (isoeffective total dose >85-90 Gy) if the rectal tissue volume was more than 2-4 cm 3 (31). For the PTV as delineated on MRI with the applicator in place the prescribed dose of 7 Gy is adapted so that as much as possible of the PTV is enclosed. In the majority of cases, >80% of the PTV receives the prescribed dose. If a major part of the PTV is not covered by the prescribed dose (e.g. extensive lateral and/or posterior- lateral pararectal extension) additional interstitial brachytherapy is considered.

Fig 14.14: Typical MRI based treatment planning based on ring applicator and stepping source technology (Vienna method)

Fig 14.14.A: MRI study with the applicator in place (patient of Fig 14.4). PTV of brachytherapy is delineated in the transverse plane 5 mm above the level of the top of the ring (pointed line). The prescribed dose based on the standard plan does not encompass a major part of the PTV in the posterior lateral direction (A1). Increase of the dwell times in the ring on the left side and one additional position in the posterior part leads to an almost complete coverage of the PTV at the limits of rectal tolerance with the 100% isodose (85 Gy) “touching” the rectal wall, but not going beyond the limitations in terms of dose volume relations (A2) (44), as the rectal wall volume included in the 100% isodose is below 1 cm 3 . In principle, dose calculation and treatment planning are performed as outlined above for each single fraction of HDR brachytherapy, which is extremely challenging in terms of resources. For practical reasons in our experience we use the following procedure: substantial differences in the implant, in the GTV and in the topography are checked for each subsequent application (2 nd - 6 th ). If the subsequent application is almost identical to the preceding one, sectional imaging with the applicator in place is not repeated. Individual dosimetry is then based only on the radiographs (with all reference points), and starts with the treatment plan which was selected the time before. Adaptations are performed, if necessary. In our early experience (93 - 97), CT with the applicator in place was carried out in 389 (181 patients) out of 694 applications (189 patients). Whereas at the