Limitation of Pencil Beam Convolution (PBC) Algorithm for Photon Dose Calculations in Inhomogeneous Medium

Abstract

Purpose: The main purpose of this study is to investigate the accuracy of pencil beam convolution (PBC) algorithm when high-density inhomogeneity is involved along the photon beam path. This study will help the PBC users understand the limitation of PBC during the treatment planning of real cancer treatment plans, especially when tumor is located beyond high-density tissue such as bone. Methods: Inhomogeneous phantom (30 cm x 30 cm, 17 cm deep) with a 5 cm thick solid water as the top layer followed by 5 cm thick PVC and 7 cm solid water was manufactured for depth dose calculations and measurements. Data were obtained beyond PVC medium for three field sizes: 5 x 5 cm2, 10 x 10 cm2, and 20 x 20 cm2. Dose calculations were performed using PBC and measurements were done using chamber. Measured and calculated data were compared against each other. Results: PBC produced dose prediction errors beyond high density medium by 3.7% to 7.3% for field size 5 x 5 cm2, by 4.8% to 6.9% for field size 10 x 10 cm2, and by 5.9% to 7.3% for field size 20 x 20 cm2. The results of this study, however, showed no clear dependency on the field size. Similarly, difference between the PBC and measurements did not show a clear trend when results at various points were compared with each other. Conclusion: PBC can overestimate the dose by up to 7.3% beyond high-density medium. High density materials such metallic immobilization devices must be avoided in the beam path during the patient treatment.