Voxelized Second Cancer Risk Calculation - Comparison of Proton and Photon Radiotherapies for Hodgkin Lymphoma
K Homann1*, W Newhauser2, R Howell3, J Eley4, (1) UT MD Anderson Cancer Center, Houston, TX, (2) Louisiana State University, Baton Rouge, LA, (3) UT MD Anderson Cancer Center, Houston, TX, (4) UT MD Anderson Cancer Center, Houston, TXSU-E-T-275 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: The purpose of this study was to compare the risk of developing a second lung or breast cancer for a female Hodgkin Lymphoma patient treated with passively scattered proton therapy (PSPT) and photon intensity modulated radiotherapy (IMRT). Particular objectives of this work were to fully account for stray dose, perform voxel by voxel risk calculations, and to visually display risk in a manner analogous to dose visualization used in commercial treatment planning systems (CTPS).
Methods: Clinically approved PSPT and IMRT treatment plans were designed using a CTPS and prescribed 36 Gy to the target volume. Neutron and stray photon dose were calculated using an in house Monte Carlo code and a TLD measurement fitted analytical model for PSPT and IMRT, respectively. For lung and breast, therapeutic and stray doses were summed and input into an in-house program that calculates Excess Absolute Risk (EAR) per voxel in the patient based on the RadRAT risk model. The radiation weighting factor (Wr) varied for neutron doses (Wr =10 and 30) to account for biological uncertainties.
Results: Neutron equivalent doses added an additional 3% (Wr =10)/9% (Wr =30) of the prescribed dose to the CTV and near field organs. Scatter dose added an additional 5% (Wr =10 )/15% (Wr =30) of the prescribed dose to organ portions far out of field. PSPT registered a lower EAR compared to IMRT except for lung when Wr =30. Larger differences were seen in low dose regions of organs.
Conclusion: EAR was lower for PSPT versus IMRT unless neutron Wr values were very high, e.g.> ~20. More work incorporating uncertainties must be done before definitive answers are determined. In the future, similar types of risk visualization tools could be incorporated into treatment plan analysis to prospectively select plans with the lowest predicted risk of second cancers.
Funding Support, Disclosures, and Conflict of Interest: This research was supported in part, by the National Cancer Institute award 1R01CA131463-01A1 (W.D. Newhauser, P.I.) and a subcontract of that award (R.M. Howell, P.I). No conflicts of interest to disclose.