16 Cervix Carcinoma

324 Cervix Cancer

8.3 Classical and modified Fletcher method 8.3.1 Standard loading patterns (mg.h) in standard implants (small, medium, large) (Classical Fletcher method) (22,35). Standard loadings limit the linear intensity of radium or cesium in the intrauterine tandem and the dose rate at the lateral surface of the vagina. The amount of radiation is expressed as a product of activity and treatment time in mg.h (radium-equivalent) and is limited by anatomical considerations and by the volume of disease and selected treatment schedule. Variations in the loadings selected for the tandem and the colpostats are also based on anatomical and pathological considerations. X-rays are taken in the operating room to verify the position of the applicator is appropriate and orthogonal films are used to calculate the dose to various pelvic points and structures. Doses are calculated to points representing the paracervical areas, regional nodes (“Fletcher trapezoid”), vaginal mucosa, bladder, rectum, and other structures. With this system, the length of the intrauterine tandem (determined by the length of the uterine canal), the size of the vaginal ovoids (determined by the capacity of the vaginal vault), and the position of the system in the pelvis significantly influence the dose distribution. To maximize the depth dose (to the endocervix and parametrial tissues), the whole length of the intrauterine canal is usually loaded and the largest colpostats that can be used without inferior displacement of the applicator are fitted in the vagina. The classical loading pattern for a 6.5-7.5 cm intrauterine canal is 15 + 10 + 10 mg radium-equivalent for small residual disease and 15 + 15 + 10 mg for bulkier endocervical lesions; small (2 cm diameter) ovoids are usually loaded with 10-15 mg while medium ovoids are loaded with 15 - 20 mg and large ovoids with 20 - 25 mg. If there is irregular anatomy (narrow vault, short uterine canal), the loadings are adapted. The loading pattern also may be adapted to the individual pathology (e.g., lesions involving mainly one cervical lip, invasion of one fornix, parametrial shortening, growth along the vaginal wall, or massive central disease expanding the cervix and/or isthmus). Dose distributions are given for a plane perpendicular to the sources (paracoronal) passing through the center of the colpostats and the proximal and distal intrauterine source. If the dose at the rectum is related to a reference dose in point A, which is regarded as inadequate for this system, the rectal dose is usually below the reference dose due to the typical colpostat position (perpendicular to the vaginal axis) and to the shielding inside the colpostats (posterior and anterior). Although this system does not depend upon an evaluation of the dose to paracentral reference points, correlative studies have demonstrated that the standard loadings, source positions, and applicator geometry recommended in the Fletcher-Suit-Delclos system usually results in a dose rate of approximately 45 - 55 cGy/hr at Point A. 8.3.2 Modified Fletcher method (Dijon): Individual volume adaptation (60 Gy reference volume) based on standard implants (small, medium, large) (4,22,54). In principle dosimetry follows the guidelines of the classical Fletcher system as outlined above. The dose is nowadays prescribed in terms of TRAK which depends on the tumor volume and its location. An integral part of this system is to adapt the volume to take account of dose volume relationships of organs at risk (rectum, bladder) as well as of the target. A volume adaptation (modification of the TRAK ) is therefore performed for each individual patient.