Purpose: To assess the dosimetry to organs at risk (OARs) in lithotomy position with a planned time-dose pattern obtained from supine position.

Methods: The sample consists of thirty patients with carcinoma of the uterine cervix, Stage II and III. Patients often feel discomfort in supine position (S position) when compared to lithotomy position (M position) due to relaxation of pelvic floor muscles after the insertion of applicator (tandem and ovoids) or before delivery of the treatment. Each patient was imaged with orthogonal X- ray radiographs simultaneously in two positions, i.e. S position and M position. Dwell time and dwell position pattern obtained from the optimized plan in S position was used to generate plan in M position. Following dose reference points (point A, pelvic wall points, bladder points, rectal, anorectum (AR point) and rectosigmoid (RS point) points) were identified for analysis in S and M positions. The dosimetric data for reference points generated by the Brachyvision TPS was analyzed.

Results: Pelvic wall points registered lower doses in M position when compared to S position. Mean doses for right pelvic wall point (RPW) and left pelvic wall point (LPW) were reduced by -10.02 % and -11.5% in M position, respectively. International Commission on Radiation Units and Measurements (ICRU) bladder point also registered lower doses in M position with a mean dose of -6.8%. Rectal point showed dose reduction by mean of -6.4%. AR and RS points showed an increased dose in M position by a mean of 16.5% and 10%, respectively.

Conclusion: Current dosimetry procedure serves as a model with time-dose pattern planned for S position, but delivered in M position, without dose optimization. Prioritization of comfort and position can be considered in conjunction with optimization of dose.