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Does Intensity Modulation Increase Heterogeneity-Induced Dose Calculation Error Over Conformal Treatments for Lung SBRT?
D Zheng1*, (1) University of Nebraska Medical Center, Omaha, NE
Presentations
PO-BPC-Exhibit Hall-16 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: Heterogeneous tumor/lung interface in lung SBRT may cause large target dose calculation errors using conventional algorithms. Small field sizes have been reported to exacerbate the problem. With recent study results easing previous concerns about the motion interplay effect, intensity modulated radiotherapy (IMRT) is gaining popularity for lung SBRT due to superior normal tissue sparing. Therefore, our study aims to assess whether IMRT increases the conventional algorithm target dose calculation error over conformal treatments.
Methods: Ten lung SBRT patients were retrospectively parallel-planned with both IMRT and dynamic conformal arcs (DCA) using pencil beam (PB) and recalculated with Monte Carlo (MC). Taking MC calculations as the ground truth, percent PB target dose errors for PTV and GTV Dmin, Dmax, D50, D95 and D5 were recorded. The errors of IMRT vs. DCA were compared using Mann-Whitney U tests(P< 0.05). The levels of intensity modulation were evaluated using the ratios of monitor units used in IMRT plans to those in corresponding DCA plans.
Results: For the 10 studied patients, low intensity modulation ratios of 1.3±0.1 were needed for IMRT. As expected, large PB errors up to 35% for PTV Dmin and 20% for GTV Dmin were found. However, comparing IMRT and DCA plans, IMRT actually resulted in lower dose errors in target high-dose regions than DCA, with median(range) errors of 4%(1-8%) vs. 5%(3-10%) for PTV D5 (P=0.001), 3%(2-6%) vs. 4%(3-7%) for GTV D5 (P=0.002), 3%(1-7%) vs. 4%(2-8%) for PTV Dmax(P=0.02), and 3%(2-6%) vs. 4%(2-8%) for GTV Dmax(P=0.03), respectively. Differences were not significant for Dmin, D95, or D50.
Conclusion: Intensity modulation does not increase PB target dose calculation errors. On the contrary, it decreases dose errors in the target high-dose regions. This observation may be because the small beamlets in IMRT mostly expose the high-density target instead of low-density target edge or margin.
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