Dose calculations accuracy of TiGRT treatment planning system for small IMRT beamlets in heterogeneous lung phantom

Background: Accurate dose calculations in small beamlets and lung material have been a great challenge for most of treatment planning systems (TPS).  In the current study, the dose calculation accuracy of TiGRT TPS was evaluated for small beamlets in water and lung phantom by comparison to Monte Carlo (MC) calculations. Materials and Methods: The head of Siemens Oncor-impression linac was simulated for 6 and 18 MV photon beams using MCNPX MC Code. The model was validated using measured percentage depth dose and beam profiles. Then, the validated model used for dose calculations for small beamlets in water as well as lung phantoms. For treatment planning purposes, the lung phantom was scanned and imported into the TPS, and then the percentage depth dose values were obtained from plans for small fields of 1×1, 2×2, 3×3 and 4×4 cm2 in water and lung phantom. Results: For small fields in water phantom, there was a good agreement between TPS and MC for 2×2 to 4×4 cm2 field sizes. Nevertheless, the depth doses in lung phantom showed large discrepancies between TPS and MC calculations for points inside lung and lung-soft tissue interfaces. The TPS underestimated the lung dose up to 67% and 110% for 6 and 18 MV beams compared to MC results. Conclusion: Our findings revealed that the TiGRT TPS was not able to account for lung inhomogeities in small beamlets. Besides, the TPS calculated depth doses were not accurate enough to be used for small beamlets used in IMRT of lung region.