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Purpose: This study aimed (1) to demonstrate the ability of mixed beam (MB) therapy, intensity modulated x-ray therapy (IMXT) optimized on top of a bolus electron conformal therapy (ECT) plan, to deliver a uniform dose to the planning target volume (PTV) in a homogenous phantom and (2) to evaluate the accuracy of the pencil beam algorithm (PBA) and pencil beam redefinition algorithm (PBRA) in calculating the dose distributions for the bolus ECT and MB plans. Methods: PTVs and critical structures from parotid and post-mastectomy chest wall patients were modeled in a 27cm ö polystyrene phantom. Bolus ECT treatment plans using 20MeV and 16MeV beams, respectively, used .decimal p.d software to conform the 90% isodose surface to the distal PTV surface. IMXT was optimized over the bolus ECT dose distributions to homogenize dose to the PTV (ECT:IMXT weightings of 2.08:1 and 1.73:1 for the parotid and chest wall, respectively). Multiple in-phantom radiographic film measurements of the dose delivery were acquired for each of the five transverse and one sagittal planes for the bolus ECT, IMXT, and MB plans. The bolus ECT component of the MB dose distribution was computed using the Pinnacle PBA and an in-house PBRA. Calculated and measured dose distributions were compared. Acceptability criteria for dose points in the bolus ECT, IMXT, and MB dose distributions was set at either ±4% dose difference or ±2mm distance to agreement. Results: Measured dose distributions (36 planes) had an average precision of less than 1% or 1mm. Results for the parotid ECT, parotid MB, chest wall ECT, and chest wall MB showed pass rates (either criteria) for the PBA of 98.9%, 97.5%, 97.3%, and 95.7%, respectively. Pass rates for the PBRA were 98.9%, 97.2%, 98.7%, and 98.0%, respectively. The IMXT dose component of the MB plans showed pass rates of 93.7% and 95.8%, respectively. Conclusions: This study provided confidence that MB therapy planned using the Pinnacle system can be delivered accurately for a homogenous phantom. Also, the PBA and PBRA calculated dose distributions have similar, clinically acceptable accuracy for the bolus ECT and MB plans in homogenous material. |
