Room: AAPM ePoster Library
Purpose: To clinically implement and comprehensively evaluate two independent methods for beam monitor calibration of scanning proton beam.
Methods: Single energy layer circular fields of diameter 15 cm with 2.5 mm spot spacing and 10 times of repainting (FS15cm) were designed for seven proton energies. The effective measurement points of Bragg peak chambers (BPC), advanced Markus chamber (AMC) and farmer chamber (FC) were all aligned to 2 cm depth in water using SSD setup. The uniformity of BPC’s lateral response was quantified by delivering a collimated narrow proton beam (5.8 mm diameter) to the active area and edge of the BPC. The dose area product (DAP, Gy-mm2/MU) was measured using two methods by two BPCs (BPC1 and BPC2), one AMC and one FC. For method 1, a single spot proton beam was delivered to the geometric center of the BPC. For method 2, the fields FS15cm were delivered to FC and AMC, respectively. The quantitative difference of DAPs between both methods was calculated.
Results: BPC1 has a uniform response from the center up to a radius of 3.5 cm. BPC2 has a uniform response only to 2 cm and the response dropped 1% to 2% at 3.5 cm from center. BPC2 has 2.2% over-response when it was used directly for DAP measurements. After BPC’s lateral response heterogeneity was taken into account, the DAPs measured by method 1 with two BPCs and by method 2 with FC and AMC were consistent to 0.5%.
Conclusion: The two independent methods for DAP were shown to give consistent results, given the sources of errors, in particular the BPC’s lateral response uniformity, were carefully minimized. Direct measurement of DAP with BPC is very efficient, but it may be subject to more than 2% systematic error if the BPC lateral response is not carefully evaluated.