The variance of bladder and rectal doses calculated from orthogonal and simple stereo films in cervix high-dose-rate brachytherapy

PURPOSE: To examine the variance in dose to the bladder and rectum during high-dose-rate brachytherapy for cervical cancer for two imaging techniques. An investigation into the patient skin dose from both techniques was also undertaken. METHODS AND MATERIALS: We examined how the variance in dose to critical structures (bladder and rectum) was affected by the degradation of image quality introduced by increasing the lateral separation in a phantom-based study. We also examined the interobserver variation and the reproducibility of C-arm alignment on the variance of the calculated values of the rectal and bladder doses. The relationship between lateral separation and skin entrance dose from collecting images was also investigated. All of these factors were investigated for the standard orthogonal imaging technique and the simple stereo method. RESULTS: It was found that imaging technique and lateral separation had little effect on the variation in the calculated doses to the bladder and rectum. However, it was found that at separations greater than 50 cm, the skin dose from a lateral x-ray increased sharply. CONCLUSIONS: The degradation of image quality for lateral images used to reconstruct cervix applicators does not result in loss of precision in the dose estimates to organs at risk when using the orthogonal technique. However, the skin dose from the lateral image increases markedly with increasing patient separation, making the simple stereo technique a better option for large patients.