Monte Carlo dose calculation of segmental IMRT delivery to a moving phantom using dynamic MLC and gating log files

Abstract

Respiratory gating is emerging as a tool to limit the effect of motion for liver and lung tumors. In order to study the impact of target motion and gated intensity modulated radiation therapy (IMRT) delivery, a computer program was developed to simulate segmental IMRT delivery to a moving phantom. Two distinct plans were delivered to a rigid-motion phantom with a film insert in place under four conditions: static, sinusoidal motion, gated sinusoidal motion with a duty cycle of 25% and gated sinusoidal motion with duty cycle of 50% under motion conditions of a typical patient (A = 1 cm, T = 4 s). The MLC controller log files and gating log files were retained to perform a retrospective Monte Carlo dose calculation of the plans. Comparison of the 2D planar dose distributions between simulation and measurement demonstrated that our technique had at least 94% of the points passing gamma criteria of 3% for dose difference and 3 mm as the distance to agreement. This note demonstrates that the use of dynamic multi-leaf collimator and respiratory monitoring system log files together with a fast Monte Carlo dose calculation algorithm is an accurate and efficient way to study the dosimetric effect of motion for gated or non-gated IMRT delivery on a rigidly-moving body.

Amplitude gating for a coached breathing approach in respiratory gated 10 MV flattening filter‐free VMAT delivery

“…Qian et al (7) observed gamma passing rates $>95\%$ using only sinusoidal breathing traces for VMAT plans delivered on a TrueBeam linac. Similar dosimetric results were reported by Oliver et al (11) for IMRT plans delivered on a Varian 2100 C/D. One observation of note by Riley et al (13) was that irregular breathing could be responsible for poor dosimetric results seen in his phase gated study.…”

“…No trend is seen with PTV volume, for volumes ranging from 60 cc to 2388 cc. Our study was based on actual patient breathing traces, while most previous studies 7 , 11 , 13 used a sinusoidal breathing pattern, which may mask problems associated with realistic breathing patterns. These previous studies also used a phase gating approach.…”

“…Gated IMRT has been verified with film measurements on dynamic phantoms undergoing sinusoidal motion 11 , 12 . Oliver et al (11) developed a Monte Carlo program to simulate segmental IMRT delivery to a moving phantom, while Lee et al (12) tested gated IMRT with different gate window widths using film measurements on a phantom undergoing sinusoidal motion.…”

“…Oliver et al (11) developed a Monte Carlo program to simulate segmental IMRT delivery to a moving phantom, while Lee et al (12) tested gated IMRT with different gate window widths using film measurements on a phantom undergoing sinusoidal motion. The dosimetric impact due to the interplay effect (defined as the simultaneous motion of the target and the dynamic multileaf collimator) has been evaluated to be of lesser significance than the variability in the respiratory motion during gated VMAT (13) .…”

Dynamic gating window for compensation of baseline shift in respiratory‐gated radiation therapy

“…In fixed-duty cycle experiments, the efficacy of duty cycles ranging from 20% to 50% have been studied. 1,9,11,15,21,22,24,[26][27][28] In experiments with fixed gating windows, duty cycles ranging from 12.2% to 69% have been seen. 4,7,8,17,20 Various studies have sought to improve the performance of respiratory gating by provid-ing audio or visual coaching to patients so as to increase the reproducibility and stability of their breathing patterns, especially while the tumor is in the gating window.…”