Film‐based dose validation of Monte Carlo algorithm for Cyberknife system with a CIRS thorax phantom

Abstract

Monte Carlo (MC) simulation, as the most accurate dose calculation algorithm, is available in the MultiPlan treatment planning system for Cyberknife. The main purpose of this work was to perform experiments to thoroughly investigate the accuracy of the MC dose calculation algorithm. Besides the basic MC beam commissioning, two test scenarios were designed. First, single beam tests were performed with a solid water phantom to verify the MC source model in simple geometry. Then, a lung treatment plan on a CIRS thorax phantom was created to mimic the clinical patient treatment. The plan was optimized and calculated using ray tracing (RT) algorithm and then recalculated using MC algorithm. Measurements were performed in both a homogeneous phantom and a heterogeneous phantom (CIRS). Ion‐chamber and radiochromic film were used to obtain absolute point dose and dose distributions. Ion‐chamber results showed that the differences between measured and MC calculated dose were within 3% for all tests. On the film measurements, MC calculation results showed good agreements with the measured dose for all single beam tests. As for the lung case, the gamma passing rate between measured and MC calculated dose was 98.31% and 97.28% for homogeneous and heterogeneous situation, respectively, using 3%/2 mm criteria. However, RT algorithm failed with the passing rate of 79.25% (3%/2 mm) for heterogeneous situation. These results demonstrated that MC dose calculation algorithm in the Multiplan system is accurate enough for patient dose calculation. It is strongly recommended to use MC algorithm in heterogeneous media.

Evaluation of newly implemented dose calculation algorithms for multileaf collimator‐based CyberKnife tumor‐tracking radiotherapy

“…The Ray‐tracing algorithm calculates doses using only density correction of the primary photon components obtained from measured data such as OCR, TPR, and OPF. This calculation method does not consider the contribution of the scattered radiation component along the off‐axis direction . In contrast, the FSPB algorithm considers this in the calculation.…”

“…In a similar study, Pan et al reported that the dose errors at the center of the tumor in the lung-equivalent tissue were 0.68% and 13.37% for the MC and Ray-tracing algorithms in MultiPlan TPS with fixed collimators. 30 The Ray-tracing algorithm calculates doses using only density correction of the primary photon components obtained from measured data such as OCR, TPR, and OPF. This calculation method does not consider the contribution of the scattered radiation component along the off-axis direction.…”

“…This calculation method does not consider the contribution of the scattered radiation component along the off-axis direction. [30][31][32][33] In contrast, the FSPB algorithm considers this in the calculation. Therefore, compared with the Ray-tracing algorithm, the dose error of the FSPB algorithm was considered to be smaller, and the results are confirmed in this study.…”

“…1 were irradiated with the MLC-defined field sizes of 30.8 9 30.8 mm 2 and 60.0 9 61.6 mm 2 and 250 MU. The PDDs were measured using the Semiflex 3D ionization chamber type 31021 with a sensitive volume of 0.07 cm 3 (PTW, Freiburg, Germany) at measured depths of 10,15,20,30,40,50,60,80,100,120,140,150,160,180,200, and 220 mm in the center of the slab phantoms. For the OCR evaluations, Types A and C in Fig.…”

Commissioning of two different algorithms for stereotactic Radiosurgery M6 FI+ CyberKnife system

“…Motion tracking mode includes the Xsight lung tracking system and Synchrony Respiratory Tracking System. CIRS-Xsight lung tracking phantom kit is used in motion tracking system [9]. There are two orthogonal Radiochromic films loaded in phantoms.…”