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MR-Based Contour Density Override for Proton Dose Calculation: A Dosimetric Evaluation On Head and Neck Cases

C Chang

C Chang*, O Cahlon, E Hug, D Mah, ProCure Proton Therapy Centers, Somerset, NJ

SU-E-T-727 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To examine the feasibility of using MR-based contour density override for proton radiotherapy dose calculation and to evaluate the sensitivity of MR-based dose calculation in current treatment planning algorithms.

Methods: Ten head and neck patients with both CT and MR images available were selected for this study. Target volumes, organs-at-risk and other tissues were manually contoured on MR, and patient outline from CT was used to minimize geometric distortion. Two separate bone contours, one according to MR and the other according to CT, were defined to examine the effect of heterogeneity. Treatment plans were generated on CT following standard clinical protocol. MR-based comparison plans were generated by overriding the proton relative stopping power (RSP) of individual contours. RSP values used for adipose tissue, air cavities, nerves, soft tissues and bones were 0.95, 0.04, 1.04, 1.0 and 1.5, respectively.

Results: Agreement between CT and MR-based proton dose calculations was observed when the bone contour is defined by HU>200. Difference in target coverage evaluated using CTV V99% had a median value of 0.6%, with a maximum deviation of 6.9%. Difference in maximum dose to critical structures such as optical nerves, chiasm, brainstem and cord had a median value of 0.9Gy(RBE), or 1.5% of Rx dose. However, the maximum dose difference in these critical organs can be up to 20% Rx dose in one of the ten cases. Difference between CT and MR-defined bone contours contributed to large discrepancies observed between CT and MR-based proton dose calculations in both target coverage and organ sparing.

Conclusion: Proton dose calculation is more sensitive to heterogeneity than using photons. Delineating a bony structure in MR that follows CT is critical in reaching agreement between CT and MR-based proton dose calculations for current pencil-beam algorithm.

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