16 Cervix Carcinoma

Cervix Cancer 325

Figure 14.11: Modified Fletcher method

Fig 14.11.A: Typical Dose distribution showing the 60 Gy volume (60 Gy EBT– 40 Gy BT = 20 Gy) with a standard Fletcher applicator with cylindrical colpostats (20 mm diameter) in a patient with a tumour 5 cm in width at diagnosis and 3.5 cm wide at the time of brachytherapy. Transverse planes are given at the plane of the colpostats and at point A. TRAK was 6.6 cGy at 1 meter. The dimensions of the 60 Gy reference volume are drawn. The HWT product is 560 cm 3 , which corresponds roughly (divided by 2) to a computer calculated volume of 280 cm 3 . To aid understanding the isodose line is given going through point A, although this has never been used in the modified Fletcher system. The dose in point A corresponds to 42 Gy in 72 hours. The product of these dimensions is 252 cm 3 , which corresponds roughly (divided by 2) to a computer calculated volume of 126 cm 3 . As 40 Gy were given by EBT the total dose given to this volume is 82 Gy (modified from isodose curves as provided by Barillot, with permission). Knowledge of the dose distribution in a particular patient is based upon the anatomic, tumoural, and implant parameters as identified on the two orthogonal films taken at the end of the implant. Computerized dosimetry based on these radiographs gives the dose to different points and volumes for cervix, bladder, rectum, nodes, pelvic walls. These parameters are recorded and reported according to ICRU 38 recommendations. The dimensions and the amount of the 60 Gy reference volume are reported as height, width, and thickness, with small letters (h, w, t), if brachytherapy is to be reported (independent of treatment schedule with or without EBT) and with capital letters (H, W, T), if a combination treatment with external beam radiotherapy is to be reported. The reference volume in cm 3 is reported simply as the product of these three dimensions, which means an overestimate of this pear-shape volume by a factor of about 2. Dose and dose rate to critical organs are decided on the basis of risk of complications. This risk is estimated from the calculated HWT volume and the reference dose to critical organs. These