Semester of Graduation

Summer 2020

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: This study assessed the benefits to the heart and lung of using scanned electron beams and continuous energy spacing (ΔR90=0.1 cm) for left-side post-mastectomy radiotherapy (PMRT) patients previously treated with volumetric modulated arc therapy (VMAT). Such beams offer a sharper distal falloff (R90-10) than do currently available scattered, discrete energy beams, which increases sparing of healthy organs distal to the target.

Methods: Seven left-side PMRT patients previously treated with VMAT at the Mary Bird

Perkins Cancer Center were planned in this study. The patients were divided into two sets; Patient Sets 1 (three patients) and 2 (four patients) used one and two fields, respectively, to plan chest wall irradiation. Four and five intensity modulated bolus electron conformal therapy (IM-BECT) plans, respectively, were created per patient using combinations of scattered/scanned and discrete/continuous energy beam data. For Patient Set 2 the inferior edge of the upper field was feathered to match the penumbra of the superior edge of the lower field. Dose distributions and dose volume histograms (DVHs) were used to evaluate plan quality, to calculate physical dose metrics for the target, heart, and lung, and to calculate the biological metrics, normal tissue complication probability (NTCP), and secondary cancer complication probability (SCCP) for the heart and lung.

Results: Scanned and/or continuous energy electron beams showed patient-dependent, sometimes significant reductions in both physical and biological dose metrics for the heart and lung. For patient CW1, as compared to scattered, discrete energy beams, scanned, continuous energy beams reduced V22.5Gy from 10.4% to 2.3%, V30Gy from 3.1% to 0.1%, Dmean from 8.3 Gy to 4.3 Gy, and NTCP from 0.4% to 0.1% for the heart. The comparison reduced V20Gy from 17.2% to 10.7% and SCCPlin from 14.4% to 8.7% for the lungs. Such reductions tended to increase as R90 increased. Compared to VMAT, IM-BECT plans with scanned beams on average reduced V22.5Gy from 9.4% to 3.4%, Dmean from 9.3 Gy to 4.4 Gy, and NTCP from 1.0% to 0.2% for heart and Dmean from 8.8 Gy to 7.1 Gy, NTCP from 2.8% to 0.2%, and SCCPlin from 15.1% to 10.2% for lung.

Conclusion: Scanned and/or continuous energy electron beam treatment plans showed reduced physical and biological dose metrics for heart and lung compared to scattered, discrete energy beams. These improvements were patient dependent, although patients requiring higher energy beam(s) tended to show the greatest benefits.

Committee Chair

Carver, Robert

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