Dosimetric comparison of two planning target volume margin recipes for prostate radiotherapy

Background and Purpose: Planning target volume (PTV) margins are applied around the clinical target volume (CTV) to cover organ motion and patient set-up related random and systematic uncertainties that occur during the course of fractionated radiation treatment. In this study two PTV margin recipes were compared retrospectively on the basis of a prospectively acquired data to investigate the effectiveness of positioning protocol for the prostate cancer patients.
Material/Methods: In total 12 patients with prostate cancer were registered for a prospective imaging and positioning study that was approved by the local ethics committee (EK272072014). All patients received 74 Gy(RBE) in 37 treatment fractions. Overall, 379 daily in-room cCTs with the patient aligned in treatment position were available for analysis. All cCTs of a patient were registered with the planning CT (pCT) in 6 degrees of freedom using bony landmarks. The prostate, bladder and rectum were contoured on all CTs and transferred from the cCTs to the pCT. The union of overlaid prostate contours of all cCTs resulted in the composite prostate contour. First, PTV margins were determined such that 95% of composite prostate was covered for 90% of the patient population by an anisotropic expansion of the pCT prostate contour (Fig. 1). Interfractional prostate motion and related systematic and random uncertainties were assessed. Second, PTV margins were calculated using van Herk’s [1] formula. Finally, volumetric modulated arc therapy plans were generated and recalculated on the cCTs using both PTV margins recipes, and coverage (D98%) and hotspots (D2%) in the CTV as well as dose volume constraints for bladder (V65Gy, V70Gy and V75Gy) and rectum (V50Gy, V70Gy) were compared (Fig. 2).
Results: PTV margins along left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were 4.3, 5.3 and 4.8 mm using the first approach, and 1.4, 4.8 and 3.5 mm for the second approach, respectively. Target coverage was fulfilled for all but one cCT using the first approach, but it was not fulfilled for 6/379 cCTs for the second approach. Dose-volume constraints for bladder and rectum were fulfilled for the second margin approach but for the first margin approach V70Gy for the rectum was not met sometimes (Fig. 2).
Conclusions: Using the composite prostate coverage approach, robust PTV margins were derived since they also took rotation and deformation into account and thereby ensuring CTV coverage over the course of treatment.