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Monte Carlo Evaluation of Tissue Inhomogeneity Corrections in the Treatment of Liver Cancer Patients Using Stereotactic Body Radiotherapy

D Pokhrel

D Pokhrel*, S Sood , R Badkul , C McClinton , H Jiang , H Saleh , C Lominska , University of Kansas Hospital, Kansas City, KS


SU-F-T-606 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To evaluate the dosimetric performance of X-ray Voxel Monte Carlo(XVMC) algorithm in effort to clinically validate Monte Carlo-approach in heterogeneous liver phantom and liver SBRT plans.

Methods:An anthropomorphic RPC liver phantom incorporating a liver structure with two cylindrical targets and organs-at-risk (OARs) was used in phantom validation. Subsequently, five patients with metastatic liver cancer were treated using heterogeneity-corrected pencil-beam(PB-hete)algorithm were analyzed following RTOG-1112 criteria.ITV was delineated on MinIP and OARs were contoured on MeanIP images of 4D-CT.PTV was generated from ITV with 5-10mm uniform margin.Mean PTV was 81.3±46.4cc. Prescription was 30-45Gy in 5 fractions,with at least PTV(D95%)=100%.Hybrid SBRT plans were generated with non-coplanar/3D-conformal arcs plus static-beams at Novalis-TX consisting of HD-MLC and 6MV-SRS.SBRT plans were re-computed using XVMC algorithm utilizing identical beam-geometry, MLC-positions, and monitor units and subsequently compared to clinical PB-hete plans.

Results:Our results using RPC liver motion phantom validation were all compliance with MD Anderson standards. However, compared to PB-hete, average target volume encompassed by the prescribed percent isodose(Vp)was 9.1% and 8.5% less for PTV1 and PTV2 with XVMC. For the clinical liver SBRT plans, PB-hete systematically overestimated PTV dose(D95, Dmean and D10)within ±2.0% (p<0.05) compared to XVMC. Mean value of Vp was about 3.8% less with XVMC compared to PB-hete (ranged 2.9-5.7%(p<0.003)).However,mean liver dose (MLD)was 3.2% higher(p<0.003),on average, with XVMC compared to clinical PB-hete (ranged -1.0to-3.9%). OARs doses were statistically insignificant.

Conclusion:Results from our XVMC dose calculations and validation study for liver SBRT indicate small-to-moderate under-dosing of the tumor volume when compared to PB-hete. Results were consistent with phantom validation and patients plans.However, Vp was less by up to 5.7% for some liver SBRT patients with XVMC–suggesting under-dosing of the target volume and overdosing of MLD by up to 3.9% occurred with PB-hete plan. These differences between PB-hete and XVMC dose calculations may be of clinical interest.

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