Author: Arjit K. Baghwala, Devin J. Olek, Parth K. Patel, Ramiro Pino, Adam Yanez 👨🔬
Affiliation: Houston Methodist Hospital, Kaiser Permanente 🌍
Purpose: The Halcyon beam model in Eclipse is immutable and only includes output factors (OF) for fields down to 1x1 cm2. The TPS must extrapolate OFs for smaller fields used in stereotactic (SRS/SBRT) and highly-modulated IMRT/VMAT plans, which can introduce errors. This work quantifies the error in those small fields.
Methods: Measurements were taken with the SNC EDGE and the PTW microDiamond detectors. The measured fields ranged from 1.4x1 to 0.2x1 cm2 and 3x0.5 to 0.2x0.5 cm2 and were measured in water at 95 cm SSD and 5 cm depth. Field output correction factors from IAEA TRS 483 were then applied. OFs were measured for two Halcyon linear accelerators and calculated in Eclipse v16.1.2 with AAA and AcurosXB algorithms.
Results: Comparison of average OFs measured by both detectors to reference TPS values for 1 cm field lengths showed relative differences ranging from 0.51%±0.28% for a 1.4-cm wide field using AcurosXB to -38.72%±0.46% for a 0.2-cm wide field using AAA. For field lengths of 0.5 cm, the relative differences range from -7.62%±0.67% for a 3-cm wide field using AcurosXB to -47.81%±0.08% for a 0.2-cm wide field using AAA. AcurosXB OFs were 1.72% to 10.82% less than AAA OFs and closer to measured values. SNC EDGE factors were 0.41%±0.18% less than microDiamond factors for 1 cm fields but were 0.78%±0.15% greater for 0.5 cm fields.
Conclusion: Measurements show a difference <2% between the TPS for the larger fields. As field size decreases, the differences increase for both AAA and AcurosXB with the latter having slightly better agreement. This result, backed by close agreement between both linacs and between both detectors, indicates that the discrepancy in OF values largely originates from inaccurate TPS extrapolation for fields <1x1 cm2.