Sci-Fri PM: Delivery - 03: Application of Advanced Dosimetry Techniques to Nonstandard Field Deliveries

Abstract

Purpose: To apply established reference dosimetry techniques for verifying nonstandard field deliveries. Method and Materials: A cylindrical PMMA phantom filled with water was constructed at the center of which absorbed dose to water was measured. Two candidate plan‐class specific reference (pcsr) fields were created for linac‐based and TomoTherapy®‐based dynamic head and neck IMRT deliveries. Three reference detectors (Gafchromic EBT films, a diamond detector, and a guarded liquid‐filled ionization chamber) and a PTW microLion chamber were used to measure the absorbed dose to water in the pcsr field normalized to that in a 10×10 cm2 field for the linac and TomoTherapy®‐based deliveries, respectively. Based on the new dosimetry formalism, pcsr correction factors k Qpcsr,Q fpcsr,fref were measured for five air‐filled ionization chambers, Exradin A12, NE2571, Exradin A1SL, Exradin A14 and PinPoint® 31006, in fully‐rotated and collapsed deliveries for the linac‐based IMRT delivery and only in a fully‐rotated delivery for the TomoTherapy®‐based IMRT delivery. Results: For the linac‐based IMRT delivery, k Qpcsr,Q fpcsr,fref was the same (0.9955–0.9986) within uncertainty (0.3 %) and systematically smaller than unity in the fully‐rotated delivery. k Qpcsr,Q fpcsr,fref was more chamber‐type dependent (0.9922–1.0048) in the collapsed delivery. For the TomoTherapy®‐based IMRT delivery, while it is above unity (1.0037–1.0076) with a 0.3 % uncertainty, k Qpcsr,Q fpcsr,fref differed by 0.33 % between the Farmer‐type and smaller ionization chambers.Conclusions: Four independent dosimetry techniques carried out the relative dose measurements in the pcsr fields to within 0.3 % 1σ uncertainty. These dosimetry techniques will be used to help develop a reference dosimetry protocol for nonstandard beams.