A treatment planning study of prone vs. supine positions for locally advanced rectal carcinoma : Comparison of 3‑dimensional conformal radiotherapy, tomotherapy, volumetric modulated arc therapy, and intensity-modulated radiotherapy.

Purpose: To ascertain the optimal radiation technique and radiation position for the neoadjuvant radiotherapy of patients with rectal cancer.

Materials And Methods: Treatment plans with similar dose objectives were generated for 20 selected patients. Dosimetric comparison was performed between prone and supine positions and between different radiation techniques. Dosimetric indices for the target volume and organs at risk (OAR) as well as normal tissue complication probability (NTCP) of late small bowel toxicity were analyzed.

Results: The helical tomotherapy (HT) in the prone position provided the optimal dose homogeneity in the target volume with the value of 0. Superior conformity values were obtained for Sliding Window (SW), Rapid Arc (RA) and HT compared to three-dimensional conformal radiotherapy (3D-CRT) techniques. All of the techniques showed dose reduction to OAR in the high-dose area in prone position versus supine position. Pairwise comparison revealed significantly higher small bowel protection by RA in the prone position in the high-dose area (V75, V45Gy). Similarly, superior bladder sparing was found for 3D-CRT in the prone position at higher doses (V50, V75). More healthy tissue in the radiation volume was involved by application of 3D-CRT with no relevant difference between positions. The mean values of NTCP for the small bowel did not show clinically meaningful variation between the techniques.

Conclusion: All techniques provided superior sparing of OAR in the prone position. At higher radiation doses, treatment in prone position resulted in significant OAR protection, especially concerning small bowel sparing by RA and bladder sparing by 3D CRT.