60 Gy isodose volumes in carcinoma of the cervix and their relation to thegeometry of uterine tube and ovoids

The International Commission on Radiation Units and Measurements recommends the use of 60 Gy isodose volumes for reporting doses in the intracavity tr eatment of carcinoma of the uterine cervix. This study was aimed at determi ning the variation in isodose volumes while using different sizes of intrau terine tubes and ovoids, with different applicator geometries. It was based on the treatment plans of 175 patients with cervical cancer, treated with low dose rate intracavitary brachytherapy with or without additional extern al beam radiotherapy. The volumes encompassed by the 60 Gy isodose curves w ere calculated using the Nucletron planning system. Applicator positions in 15 patients who were treated to the same point A dose, using 6 cm intraute rine tube and medium ovoids, were recorded. This was to discover how variat ions in applicator geometry influences isodose volumes. The 60 Gy isodose v olumes increased with increasing point A dose; For a constant point A dose prescription, reference isodose volume increased with ovoid applicator size used, but showed no consistent variation with the length of intrauterine t ube. There were individual variations in the isodose volumes within a stand ard set-up (same sized intrauterine tube and ovoids and same point A dose), due to variations in applicator geometry. Displacement of the ovoids chang ed the volumes encompassed by the reference isodose. There are significant variations in the volumes encompassed by the 60 Gy isodose during intracavi tary treatment using a standard set-up, while treatment using applicators o f different sizes can give equivalent values of 60 Gy isodose volume. 60 Gy isodose volumes may hence be useful in dosimetric comparisons but have a l imited role in predicting clinical response.