Assessment of dose perturbations for metal stent in photon and proton radiotherapy plans for hepatocellular carcinoma

Abstract

Background The present study aimed to investigate the dosimetric impact of metal stent for photon and proton treatment plans in hepatocellular carcinoma. Methods With computed tomography data of a water-equivalent solid phantom, dose perturbation caused by a metal stent included in the photon and proton treatment of hepatocellular carcinoma was evaluated by comparing Eclipse and RayStation treatment planning system (TPS) to a Monte Carlo (MC) based dose calculator. Photon and proton plans were created with anterior–posterior/posterior-anterior (AP/PA) fields using a 6 MV beam and AP/PA fields of a wobbling beam using 150 MeV and a 10 cm ridge filter. The difference in dose distributions and dosimetric parameters were compared depending on the stent's positions (the bile duct (GB) and intestinal tract (GI)) and angles (0°, 45°, and 90°). Additionally, the dose variation in the target volume including the stent was comparatively evaluated through dose volume histogram (DVH) analysis. And the comparison of clinical cases was carried out in the same way. Results Percentage differences in the dosimetric parameters calculated by MC ranged from − 7.0 to 3.9% for the photon plan and − 33.7 to 4.3% for the proton plan, depending on the angle at which the GB and GI stents were placed, compared to those without the stent. The maximum difference was observed at the minimum dose (Dmin), which was observed in both photon and proton plans in the GB and GI stents deployed at a 90° incidence angle. The parameter differences were greater in the proton plan than in photon plan. The target volume showed various dose variations depending on positions and angles of stent for both beams. Compared with no-stent, the doses within the target volume containing the GI and GB stents for the photon beam were overestimated in the high-dose area at 0°, nearly equal within 1% at 45°, and underestimated at 90°. These doses to the proton beam were underestimated at all angles, and the amount of underdose to the target volume increased with an increase in the stent angle. However, the difference was significantly greater with the proton plan than the photon plan. Conclusions Dose perturbations within the target volume due to the presence of the metal stent were not observed in the TPS calculations for photon and proton beams, but MC was used to confirm that there are dose variations within the target volume. The MC results found that delivery of the treatment beam avoiding the stent was the best method to prevent target volume underdose.

Monte Carlo Modelling for Photon and Proton Therapy in Heterogenous Tissue and Prosthesis Material

“…Many previous studies agreed that in areas with low heterogeneities, the difference between dose to medium and dose to water is less than 2%, which is not clinically significant (212,215,216). However, in areas of different densities, especially in the presence of bone material and air, the difference can reach 10% (212,215,216). To simplify the effect of the heterogeneous area in both calculation algorithms, the variation in density does not change the dose because the dose is a measure of absorbed energy per unit mass.…”

“…However, the conversion between the two modes introduced a systematic error that reached 8% in hard bone areas (221). Many previous studies agreed that in areas with low heterogeneities, the difference between dose to medium and dose to water is less than 2%, which is not clinically significant (212,215,216). However, in areas of different densities, especially in the presence of bone material and air, the difference can reach 10% (212,215,216).…”

“…However, protons interact differently to photons. Very few articles discuss the effect of dose distribution in the area surrounded the stent during proton therapy (214,215). The majority of studies tested the treatment effects on the stent for oesophageal cancer, and few studies tested the effect on pancreatic stents.…”