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Experimental Validation of Clinical as Well as Monte Carlo Dose Calculation for Proton Therapy of Lung Cancer Patients

C Grassberger

C Grassberger1 2*, J Daartz1, S Dowdell1, J Verburg1, G Sharp1, H Paganetti1, (1) Massachusetts General Hospital, Boston, MA, (2) Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland

MO-G-137-2 Monday 5:15PM - 6:00PM Room: 137

Purpose: Validate clinical pencil beam (PB) and Monte Carlo (MC) based dose calculations using a heterogeneous lung phantom and assess the clinical significance of these findings in lung patients.

Methods: A modified IMRT lung phantom and a matrix-array of ionization chambers was used to measure the distal fall-off of the proton beam. The treatment was planned according to clinical protocols. We evaluated absolute and relative dose differences and the range where dose decreases to 50% of prescribed dose (range50). 10 patients treated with 32 beams were subsequently simulated using MC.

Results: In the entrance region behind the ribs, the average deviation from the measured dose for MC and PB is 1.2% and 2.0%. Averaged over the 5 investigated depths in the distal fall-off, the average error is 7.1% and 12.5% of prescribed dose for MC and PB respectively. Relative to the experimental point dose, the deviations in the distal falloff are 25.6%(MC) and 71.9%(PB). Discrepancies in relative dose in the distal falloff appear large due to the low absolute dose, sharp dose gradient and range differences. The average absolute error in range50 is 3.9mm(MC) and 7mm(PB) in lung tissue, which equals 1.3/2.3mm in water. Apart from differences in multiple Coulomb scattering caused by lateral inhomogeneities, MC generally predicts a larger low-dose-penumbra and lower target dose, confirmed by the measured dose profiles. These features also arise in the patients: mean dose and D95 in the clinical target volume are on average 1.8% and 2.3% lower for MC than PB, while normal lung V5 and mean lung dose increase by 5.1% and 2% respectively.

Conclusion: For lung, MC increases dose calculation accuracy in the distal fall-off significantly and reproduces measurements within experimental uncertainty. Differences in patient cases are small in the target, though at times significant in organs distal to the beam.

Funding Support, Disclosures, and Conflict of Interest: This project was supported by the National Cancer Institute Grant No. R01 CA111590

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