Orthogonal film-based treatment planning is the most commonly adopted standard practice of treatment planning for cancer of the uterine cervix using high dose rate brachytherapy (HDR). This study aims at examining the variation in rectal and bladder doses when the same set of orthogonal films was given to different observers. Five physicists were given 35 pairs of orthogonal films obtained from patients who had undergone HDR brachytherapy. They were given the same instructions and asked to plan the case assuming the tumor was centrally placed, using the treatment-planning system, PLATO BPS V13.2. A statistically significant difference was observed in the average rectal (F = 3.407, P = 0.01) and bladder (F = 3.284, P = 0.013) doses and the volumes enclosed by the 100% isodose curve (P < 0.01) obtained by each observer. These variations may be attributed to the differences in the reconstruction of applicators, the selection of source positions in ovoids and the intrauterine (IU) tube, and the differences in the selection of points especially for the rectum, from lateral radiographs. These variations in planning seen within a department can be avoided if a particular source pattern is followed in the intrauterine tube, unless a specific situation demands a change. Variations in the selection of rectal points can be ruled out if the posterior vaginal surface is clearly seen.
Keywords: High dose rate brachytherapy; interobserver variation; uterine cervix.