Comparison of dosimetric and radiobiological effects of various IMRT techniques regarding to joint volume between target tissue and organs at risk in prostate cancer patients

Intensity-modulated radiotherapy (IMRT) is one of the most usable methods in prostate radiotherapy that is used with different techniques. The aim of this study was to evaluate and compare the dosimetric and radiobiological effects of prostate IMRT techniques regarding to joint volume between the target tissue and organs at risk as a patients anatomical parameter.
This research was a cross-sectional, analytical, and quantitative study that was carried out from April 2016 to June 2018 at the radiotherapy and oncology center of Shoheday-e-Tajrish Hospital and Medical Physics Department of Tarbiat Modarres University Tehran, Iran. Four various prostate IMRT techniques (9, 7 and 5 fields and automatic) were planned on 63 prostate cancer patients CT scans. Radiobiological effects were calculated using Relative Seriality model for the organs at risk (bladder and rectum) and target tissue. Results of mentioned prostate IMRT techniques were compared based on the patient’s anatomical parameter. 
For the patients with joint volumes ranged from 0 to 15%, statistical differences were not observed among various IMRT techniques. The tumor control probability and complication free tumor control probability values decreased as a function of joint volume. The normal tissue complication probability value increased as a function of joint volume. The 9 and 7 fields IMRT techniques had not any significant differences (P=0.06) in all of the joint volume ranges. In patients with the joint volumes higher than 30%, the 9 and 7 fields techniques showed significantly better radiobiological values in comparison with 5 fields and automatic techniques (P=0.009).
In the patients with lower percentage of joint volume, all the mentioned prostate IMRT techniques showed same radiobiological effects; however, in the patients with higher joint volume percentages (> 30%), the 9 and 7 fields techniques have better results. It is proposed to use the 7 fields technique instead of the 9 fields technique, especially in prostate cancer cases with high uncertainty in patients’ setup.