Leukemia/Lymphoma/Hematologic

SS 25 - Hematologic 2 - Translating Better Technology to Reduced Toxicity

175 - Comparative Photon and Proton Dosimetry for Patients with Mediastinal Lymphoma in the Era of Monte Carlo Treatment Planning and Variable Rbe

Tuesday, October 23
1:00 PM - 1:10 PM
Location: Room 004

Comparative Photon and Proton Dosimetry for Patients with Mediastinal Lymphoma in the Era of Monte Carlo Treatment Planning and Variable Rbe
Y. D. Tseng1,2, S. M. Maes1, G. Kicska3, E. Traneus4, T. P. Wong1, R. D. Stewart2, and J. Saini1; 1Seattle Cancer Care Alliance Proton Therapy Center, Seattle, WA, 2University of Washington, Department of Radiation Oncology, Seattle, WA, 3Department of Radiology, University of Washington, Seattle, WA, 4RaySearch Laboratories, Stockholm, Sweden

Purpose/Objective(s): Existing pencil beam algorithms (PBA) for proton therapy (PT) treatment planning are not ideal for sites with heterogeneous tissue density and do not account for the spatial variations in PT relative biological effectiveness (vRBE). This may lead to inaccurate estimates of treatment response in the target volume and organs at risk (OAR). Using a Monte Carlo (MC) treatment planning algorithm (TPA) that incorporates a published vRBE model, we compared the RBE-weighted dose (RWD) across three TPA: 1) PBA with constant RBE (cRBE=1.1), 2) MC with cRBE, and 3) MC with vRBE. We evaluated the effects of MC and vRBE TPA on target coverage and OAR dosimetry among patients with mediastinal lymphoma. We hypothesized that with MC and vRBE TPA, proton RWD to the heart, breast, and lung would be lower compared to photons.

Materials/Methods: Five mediastinal lymphoma patients with free breathing (FB) and deep inspiration breath hold (DIBH) CT simulation scans were analyzed. We retrospectively recalculated the original PBA-based PT plan with MC (MC FB recalculated). To ensure equivalent coverage of the CTV/ITV (V99=99-100%), PT plans were re-optimized using the MC with cRBE TPA for FB (MC FB optimized) and DIBH scans (MC DIBH optimized). These MC optimized plans were then recalculated applying the vRBE model. Comparative photon plans (1 3D conformal, 4 “butterfly” IMRT) were generated on the DIBH scan. IMRT was used for patients with mediastinal involvement that extended below the pulmonary artery.

Results: All 5 patients were female. Median RWD was 30 Gy (range, 20-30.6). The median CTV/ITV volume receiving 99% RWD or higher (V99) dropped from 99.5% to 48.6% when PBA plans were recalculated using MC. In contrast, the mean of mean RWD to the heart, breast, and lungs were similar across PBA, MC, and MC with vRBE TPA (Table). Mean of mean RWD to the lungs, heart, left main coronary, left circumflex, and left ventricle appeared lower with PT compared with photon DIBH. Mean of mean dose (Gy)
Orignal PBA FB MC FB: recalculated MC FB: optimized MC FB: optimized (vRBE) MC DIBH: optimized MC DIBH: optimized (vRBE) Photon DIBH
Lung 6.7 6.7 7.0 7.1 6.4 6.5 9.8
Breast L/R 3.8/1.5 3.8/1.6 4.0/1.7 3.8/1.6 3.5/2.0 3.3/1.8 4.0/3.0
Heart 10.9 10.7 10.9 10.5 10.4 10.2 14.5
R coronary 20.1 19.8 20.2 19.8 18.5 18.0 20.0
L main coronary 21.1 20.7 21.2 20.3 22.4 21.6 27.7
L anterior descending 15.1 14.9 15.3 14.7 16.9 16.2 17.7
L circumflex 12.7 12.4 12.5 12.2 10.2 10.0 17.1
R ventricle 10.5 10.3 10.4 10.1 9.7 9.7 12.9
L ventricle 6.6 6.5 6.6 6.5 6.7 6.5 11.2

Conclusion: MC should be used for PT treatment planning to ensure adequate coverage of mediastinal target volumes. Mean dose estimates to the heart, lungs, and breast were similar across PBA, MC, and MC with vRBE and were similar, if not lower, with PT compared with photons. Long term follow-up is needed to correlate with toxicity.

Author Disclosure: Y.D. Tseng: None. S.M. Maes: None. E. Traneus: Stock; RaySearch Laboratories, Elekta, ViewRay. R.D. Stewart: None. J. Saini: None.

Yolanda Tseng, MD

University of Washington

Disclosure:
Employment
University of Washington: Assistant professor: Employee

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