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Assessment of Monthly Linear Accelerator Beam Profile Constancy Based On AAPM TG-142 Recommendations

E Han

E Han*, S Gupta, X Zhang, M Hardee, V Ratanatharathorn, S Morrill, University of Arkansas Medical Science, Little Rock, AR

SU-E-T-154 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: AAPM Task Group 142 (TG-142) gives new Quality Assurance (QA) recommendations for clinical linear accelerators to insure beam profile constancy. The beam profile constancy can be calculated as an average value of chosen off axis points in the beam profile. Since this assessment tool was not commercially available, an in-house program was developed.

Methods:Five photon energies and five electron energies were included in this study. Our selected baseline measurement setup for photon beams used a 20x20 cm² field size, 100 cm SSD, and 10 cm measurement depth in water. Sun Nuclear IC Profiler and solid water slabs (9.1 cm solid water and 0.9 cm inherent buildup) were used for monthly measurement. Six points at different off axis positions were selected within the central 80 % of the profile. The selected baseline electron beam measurement setup used a 25x 25cm² cone, 100 cm SSD, and no additional buildup.

Results:The selected off axis points were 3, 6, and 8 cm from the central axis(CAX) for photon beam and 3, 6, and 9 cm from CAX for electron beam. For 6 MV photon beam, average constancy was 0.57 % (± 0.14 %) and average flatness was 2.53 % (± 0.1 %) for four months of period. For 6 MeV electron beam, average constancy was 1.26 % (± 0.22 %) and average flatness was 2.63 % (± 0.11 %) for three months of period. However beam constancy was not increased as a photon beam profile was intentionally changed by increasing the beam flatness from 2.4 % to 3.5 %.

Conclusion:The in-house program works well and overall beam constancy were less than 1 % over four months of period. However, there is a great need to further investigate for sensitivity study on whether the constancy can detect significant beam profile changes from the machine baseline.

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