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Dose Uniformity of Arc-Based TBI Investigated Using a Commercial Treatment Planning System

S Peca1*, K Alexander1,2 , A Kerr1,2 , (1) Medical Physics Department, Cancer Centre of Southeastern Ontario, Kingston, ON; (2) Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, ON.


(Monday, 7/30/2018) 1:15 PM - 1:45 PM

Room: Exhibit Hall | Forum 3

Purpose: The goal of total body irradiation (TBI) is to deliver uniform dose to the entire body as part of a conditioning regimen preceding transplants for hematological malignancies. Recently, multiple centers have implemented linac-based TBI with non-patient-specific arc delivery, but there is great heterogeneity among practice. In addition, dose calculations are usually manual and measurements limited to few points, as commercial treatment planning systems (TPS) are not optimized for the extended distances required. Here we use a commercial TPS to investigate the dosimetry of extended-distance arc-based TBI, varying field aperture, energy, and arc segment weighting.

Methods: Ten TBI plans were created to encompass a virtual patient/phantom (16×56×200cm³). Each plan consisted of four 140° arcs, two (clockwise, counter-clockwise) with patient supine and two prone. The SSD at gantry 0° was 190cm. Energies 6MV and 15MV were considered; field apertures varied from 10×40 to 40×40cm². Each arc was modelled as 14 segments of 10°; the weight of each segment was calculated analytically based on inverse-square law (ISQL-weighted) and then modified using the TPS to produce a more uniform dose (TPS-modified). The plans were delivered to a plastic water phantom and TPS dose accuracy was assessed by ion chamber and film measurements.

Results: ISQL-weighted plans produced insufficient dose uniformity, while TPS-modified plans were greatly improved. The greatest uniformity was achieved with the 20×40cm² 15MV TPS-modified plan, delivered in 25 minutes, in which the virtual patient received 95%-105% of the prescribed dose.

Conclusion: In arc-based TBI, the TPS is a valid tool to quantify and improve dose uniformity, and to inform the choice of energy and field size. Measurements are also warranted to characterize the TPS accuracy. For centers transitioning to arc-based non-patient-specific TBI, we suggest to: use the TPS to determine optimal segment weighting, use a medium aperture, and consider using >6MV energies.


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