Influence of patient positioning on dose-volume histogram and normal tissue complication probability for small bowel and bladder in patients receiving pelvic irradiation: A prospective study using a 3D planning system and a radiobiological model

Purpose: A prospective study was undertaken to evaluate the influence of pa tient positioning (prone position using a belly board vs. supine position) on the dose-volume histograms (DVHs) of organs of risk, and to analyze its possible clinical relevance using radiobiological models. Methods and Materials: From November 1996 to August 1997 a computed tomogra phy (CT) scan was done in the prone position using a belly board and in sup ine position in 20 consecutive patients receiving postoperative pelvic irra diation because of rectal cancer. Using a three-dimensional (3D) planning s ystem (Helax, TMS(R)) the DVH for small bowel, bladder, a standard planning target volume (PTV) of postoperative irradiation of rectal cancer, the int ersection of volume of PTV and small bowel (PTV boolean AND V-SB), respecti vely, of PTV and bladder (PTV boolean AND V-B) were defined in each axial C T slice. The normal tissue complication probability (NTCP) was determined b y the radiobiological model of Lyman and Kutcher using the tolerance data o f Emami. For evaluation of late toxicity alpha/beta ratio was 2.5; for eval uation of acute toxicity, it was 10. Total dose,vas 50.4 Gy (1.8 Gy/fractio n) (ICRU 50). Results: Using the prone position compared to the supine position, the medi an volume of PTV boolean AND V-B was reduced by 1815 cm(3) (62%). Median do se (related to the reference dose) to the bladder was 44.5% (22.4 Gy) in pr one and 66.05% (33.3 Gy) in supine position (p < 0.001). Median V-B within the 90% (45.4 Gy), 80% (40.3 Gy), 60% (30.2 Gy), and 40% (20.2 Gy) isodose was significantly lower in the prone position when compared to the supine p osition. Using the radiobiological models, however, there was no difference of NTCP between prone position or supine position. In the prone position, median volume of PTV boolean AND V-SB was reduced by 32.5 cm(3) (54%). The median dose to small bo,vel,vas 30.85% (15.4 Gy) in the prone position and 47.35% (23.9Gy) in the supine position (p < 0.001). Significant differences between prone and supine position were found for median V-SB within the 90 % (45.4 Gy), 80% (40.3 Gy), 60% (30.2 Gy), and 40% (20.2 Gy) isodose. Accor ding to the method of Lyman, median NTCP of small bowel was significant low er in prone than in supine position. Conclusion: The prone position with a standard belly board should be the st andard positioning technique for patients receiving adjuvant postoperative radiation therapy following surgery of rectal cancer. Both irradiated volum e and total dose to the organs of risk can be reduced significantly. As a c onsequence of this, radiation induced toxicity will be minimized. (C) 1999 Elsevier Science Inc.