Dosimetric impact of image-guided translational isocenter correction for 3-d conformal radiotherapy of the prostate

Background and Purpose: Interfractional prostate motion during radiotherapy due to variation in rectal distension can have negative consequences. The authors investigated the dosimetric consequences of a linear translational position correction based on image guidance when a three-dimensional conformal treatment technique was used. Material and Methods: Planning CTs of seven patients with empty and distended rectum were analyzed. A reference plan for the planning target volume (PTV) and the boost were calculated on the CT dataset with the empty rectum with a standard four-field technique. The treatment plan was transferred to the CT with the distended rectum for an uncorrected setup (referenced to bony anatomy) and a corrected setup after position correction of the isocenter. The dosimetric consequences were analyzed. Results: Organ motion decreased the coverage of the prostate by the 95% isodose during simulated single treatment fractions by up to -21.0 percentage points (%-p; boost plan) and by up to -14.9%-p for the seminal vesicles (PTV plan). The mean rectum dose increased by up to 18.3%-p (PTV plan). Linear translational correction (mean 6.4 +/- 3.4 mm, maximum 10.8 mm) increased the coverage of the prostate by the 95% isodose by up to 12.7%-p (boost plan), while the mean rectum dose was reduced by up to -8.9%-p (PTV plan). For the complete treatment a reduction of complication probability of the rectum of approximately 5%-p was calculated. Conclusion: The use of an image guidance system with linear translational correction can improve radiation treatment accuracy for prostate cancer, if geometric changes are within certain limits.