16 Cervix Carcinoma

Cervix Cancer 321

These are in particular the height, width and thickness of the target and the respective dimensions of the pear-shaped dose distribution of the treated volume. As a minimum (if the height is not accessible), the maximum width and thickness of the target and the maximum width and thickness of the pear-shaped dose distribution must be calculated, as well as at the level of point A (width and thickness). The treated volume may be different from or identical to the reference volume which is introduced for treatment comparison (see 14.16 and chapter 6.8 on ICRU recommendations). For recording and reporting, the dimensions and the volume should be given for the treated volume (ICRU 38), for the 60 Gy reference volume (ICRU 38), and also for the isodose going through point A (width and thickness also at the level of point A); the dose rate should be given at point A and for the 60 Gy encompassing isodose (see in detail chapter 6.8 on reporting). If other dose rates than classical LDR are used (50 cGy/hour), an isoeffective dose can be indicated e.g. for the “point A dose” and for the “point A volume” and for the “60 Gy reference volume” based on weighting factors from biological modelling of clinical experience (see chapter 6.8 on reporting and chapter 4 on radiobiology). Recently, a full 3D treatment planning approach has been introduced based on CT and MRI, respectively (9,32,44,113,114). After the sectional images have been directly entered into the treatment planning computer via a network, a delineation of GTV/PTV and critical organs is performed and dose volume relations are assessed. Dose distributions are displayed as DVH for the GTV/PTV and for organs at risk. Dose adaptation is then additionally based on these parameters, not only looking at dose points (radiography), but also at dose volume relations, for the rectum and bladder (CT and MRI) and for the GTV and PTV (MRI). Limitations are being set up for dose volume relations for the rectum, where a certain dose is limited to a certain rectal tissue volume e.g. of 2 cm 3 /5 cm 3 . For the PTV at the time of brachytherapy (as seen on MRI with the applicator in place) the prescribed dose can be precisely adapted so that the PTV is enclosed by the prescribed isodose (as much as possible). If this is not possible to a sufficient degree (e.g. because of extensive lateral parametrial or posterior pararectal extension) additional interstitial brachytherapy or an external radiotherapy boost may be considered. 8.2 Modern Manchester method: Standard dose rate to point A in standard implants (small, medium, large) (59, 61,62,116). Various defined standard programs are available for the different possible arrangements of applicators allowing for a standard dose rate to point A – the cardinal feature of the Manchester system. The loading patterns for the Cesium pellets (maximum strength 40 mCi) mimic the loading patterns of the classical radium Manchester system: 20 mg, 15+10 mg, 15+10+10 mg radium- equivalent content in short, medium and long intrauterine tubes, respectively; 22.5 mg, 20 mg, 17.5 mg for large, medium and small ovoids, respectively, on each side. This was achieved by using linear patterns of active and inactive pellets each 2.5 mm diameter in the Selectron system, and replacing 5 mg of Radium with one active pellet. The dose rate at point A is kept constant no matter which applicator is used but has been changed from 53 cGy per hour in the classical system to 140 to 180 cGy per hour to point A in the modern system depending on the decay of the sources.