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Reporting Dose-To-Water (Dw) Vs Dose-To-Medium (Dm) of the Acuros-XB Algorithm in the Treatment of Lung Stereotactic Body Radiosurgery/radiotherapy

D Pokhrel

D Pokhrel*, D Cheek , E Johnson , J Molloy , University of Kentucky, Lexington, KY


SU-I-GPD-T-598 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Historically, treatment machines/ion-chambers were calibrated in water phantom, conventional treatment planning system (TPS)/algorithms were commissioned in water-equivalent phantom, and hence, dosimetry protocols were based on Dw. However, clinical availability of an advanced Monte Carlo-based algorithms demands particles transport in patient-medium and hence, absorbed dose to be computed in the voxel-medium (Dm). Due to single/few-fractions dosing-schemata/smaller-target-size that are proximal to higher-density ribs for lung stereotactic body radiosurgery/radiotherapy(SBRS)fields, we sought to assess dosimetric-differences of AXB-Dm vs AXB-Dw and AAA plans in Eclipse-TPS.

Methods: A total of 15 stage-I NSCLC patients with peripherally located lung tumors, who underwent AXB-Dm based SBRS treatment were analyzed. Prescription dose was 30Gy×1=30Gy(n=5)and 18Gy×3=54G(n=10) delivered to the planning target volume(PTV=ITV plus 5-10mm non-uniform margin) with at least PTV(D95%)=100%. PTV ranged from 5.1-49.30cc(mean=21.6±16.8cc).Optimal clinical VMAT plans(2×2×2mm3)were generated using partial-non-coplanar-arcs for Truebeam using 6MV-FFF(1400MU/min) beam following RTOG-0915.Plans were re-computed with AXB-Dw/AAA utilizing identical geometry, MLC positions and total monitor units. PTV-coverage including target volume encompassed by prescribed percent dose (Vp) and organs-at-risk (OARs) doses were analyzed.

Results: CI/Vp were higher by 1%(max,2%) and 0.5%(max,1.7%) with AXB-Dw and by 3.4%(max,14.7%) and 0.7%(max,4.7%) with AAA, on average (p<0.03) compared to AXB-Dm, however, generally, GTV-coverage was statistically-insignificant. R50/D2cm were statistically-significant (p<0.01). Maximum dose/dose to 1cc of ribs were higher by 2.6%/3.7%(max,7.3%)with AXB-Dw and by 1.8%/2.7%(max,9.7%)with AAA,on average (p<0.001) compared to AXB-Dm. Maximum cord dose was higher by 1.3%(max,3.1%)with AXB-Dw and by 4.9%(max,8.3%)with AAA, on average (p<0.001) compared to AXB-Dm, although, AXB-Dm gave higher dose to skin.

Conclusion: AXB-Dw dose distributions lead to similar results of AXB-Dm for PTV-coverage,however, for smaller target-sizes, AAA significantly overestimated PTV-coverage-underdosing tumors.For OARs:Ribs/spinal cord (wrap-around with vertebral-body),AXB-Dw resulted up to 3.1-7.3% higher dose than AXB-Dm due to higher energy deposition ratio of water/bone-tissue. Ribs toxicity: chest-wall pain/ribs fracture due to AXB-Dw dose for lung SBRS could be modeled/investigated.

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