Purpose: Lung SBRT is used in clinics routinely to treat early stage and oligometastatic tumors. This work investigates how treatment iso-center (Tx-iso) location with respect to PTV centroid, beam type (6MV vs 6MV FFF) and lesion size effect plan specific QA. This work aims to provide a framework to dosimetry team when planning for these cases in order to achieve a clinically deliverable plan.
Methods: An anthropomorphic phantom was scanned along the thorax region. An ITV (Internal Treatment Volume) was defined in posterior right lung having diameter of 0.5 cm and density of 1 g/cm3. PTV (Planning Target Volume ) was constructed by expanding the ITV by 0.5 cm. Four scenarios were explored with regards to iso-center location and beam type as below: 1) Tx-iso at PTV centroid using 6 MV FFF beam; 2) Tx-iso at PTV centroid using 6 MV flat beam; 3) Tx-iso away from PTV centroid anteriorly by 10 cm using 6 MV FFF beam; and 4) Tx-iso away from PTV centroid anteriorly by 10 cm using 6 MV flat beam. In all cases VMAT plans were developed using Pinnacle Treatment Planning System to deliver 48 Gy in 4 fraction to at least 95% of the PTV while meeting OARs constraints following RTOG 0915 protocol. The plans were then transferred to SNC ArcCHECK phantom for plan specific QA and delivered using a TruBeam ver. 2.7 machine.
Results: The gamma analysis pass rate for 3% dose, 3mm distance criteria and 10% threshold were obtained as 100%, 100%, 88.0% and 95.4% for scenarios 1 to 4 above respectively.
Conclusion: Our results indicate the best QA results are obtained when the tx-iso is positioned at the PTV centroid. Having the Tx-iso moved away from the PTV center results in a drop of pass rate specially for 6 MV FFF beam.