16 Cervix Carcinoma

Cervix Cancer 331

Nowadays, systematic MRI performed during brachytherapy, immediately after the insertion, just before source loading, allows an accurate definition of GTV and CTV and appropriate modification of treatment. 8.5 Vienna method: Individualised dose and volume adaptation (80 - 90Gy) (assisted by radiography, MRI, and standard loading programs) based on standard ring applicators (small, medium, large) The Vienna method utilises a HDR iridium-192 stepping source. After implantation of the applicator, AP and lateral projection images are obtained. Reference points are drawn on these images to calculate the dose at certain points in relation to the applicator and the GTV, and PTV using the treatment planning system. These reference points are the ICRU 38 points and Manchester point A. Fig 14.13: Vienna Method

Fig 14.13.A: Anterior posterior and lateral radiographs showing the ring applicator in place, the rectal probe and ICRU-reference points in bladder (balloon) and rectum, at the the pelvic wall and in the lymphatic trapezoid for dose calculation and treatment planning.

For the ring applicator, defined standard configurations with specific loading patterns for each applicator (dwell positions and times) have been generated and are thus available in the “library” of the treatment planning system. These standard dose distributions are used for starting dose calculations (Fig 13B-D). Point A is the reference point in these standard programs where the dose is prescribed. Limitations are set for the dose to the critical organs: less than 70% of the dose at point A to the ICRU rectum reference point (5 Gy per fraction) and less than 80%, if possible, to the ICRU bladder reference point. As the position of the rectum is radiographically known relative to the applicator and the GTV, an adaptation is often possible by adapting dwell times and/or dwell positions in one posterior part of the ring These limitations apply for each brachytherapy fraction. In the treatment schedule for small tumours, the upper limit for brachytherapy dose is nominally 24 - 30 Gy (6x4 - 5 Gy), and in large tumours 16 - 20 Gy (4x4 - 5 Gy). Applying the linear quadratic model (alpha beta value 3) and taking into account the respective treatment schedule from external beam therapy (four field box technique:25 x 1 Gy for limited disease with shielding the brachytherapy volume in the ap/pa direction and 25 x 2 Gy for extended disease), the total physical dose is 49 - 55 Gy and 61 -