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 cm 2 , 10 x 10 cm 2 , and 20 x 20 cm 2. 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 cm 2 , by 4.8% to 6.9% for field size 10 x 10 cm 2 , and by 5.9% to 7.3% for field size 20 x 20 cm 2. 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.

<p>Dosimetric comparison of different algorithms in stereotactic body radiation therapy (SBRT) plan for non-small cell lung cancer (NSCLC)</p>

“…The application of different algorithms in the radiotherapy physics plan has always been the research direction of scholars 3,4,7–13. The AXB algorithm is similar to the Monte Carlo algorithm, the physical action of the ray in the medium was clearly simulated, and the linear Boltzmann linear transmission problem was solved in the discrete and multidimensional space 6.…”

“…The PBC algorithm is a pencil beam algorithm, which calculates dose distribution through a very narrow beam, each pencil takes into account the attenuation factor of different beams, even the method of equivalent tissue air ratio is used for non-uniformity correction. But the lateral non-uniformity correction is not considered in the PBC algorithm 7…”

Investigation of the effects on dose calculations of correction-based algorithms in different tissue medium

“…Correctionbased algorithms are stated that are not suitable for stereotactic treatments with small treatment fields. In regions with tissue heterogeneity, they may cause inaccurate dose calculations in the target volume and surrounding healthy tissues [7]. In addition, the dose distributions calculated in the radiotherapy treatment plans are based on the correction of the measurements previously obtained in the water phantom.…”

Dosimetric comparison of treatment planning systems using collapsed cone convolution and pencil beam algorithms

“…Dose calculation algorithms used in treatment planning systems (TPS) directly affect dose delivery accuracy which will lead to better tumour control as well as better critical organ sparing. 1 To meet the International Commission on Radiation Units criteria, dose calculation accuracy must be within 2-3%. 2 Contemporary TPS have been introduced for achieving optimal treatment plans in the modern era of advanced technology.…”

“…To achieve this goal, medical linear accelerators with the capability of various photon energy options and multileaf collimators (MLC) are used in generation of optimal treatment plans. Dose calculation algorithms used in treatment planning systems (TPS) directly affect dose delivery accuracy which will lead to better tumour control as well as better critical organ sparing 1 . To meet the International Commission on Radiation Units criteria, dose calculation accuracy must be within 2–3% 2…”