Author: Reza Farjam, Russell K Hales, Todd R. McNutt, Mohammad Rezaee, Ehsan Tajikmansoury, K. Ranh Voong 👨🔬
Affiliation: Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Johns Hopkins University 🌍
Purpose: Optimal beam angles and off-axis isocenter selection are important to achieve high quality plan and collision-free delivery in stereotactic body radiotherapy (SBRT) of the posteriorly located lung lesions. This study evaluates the safety of off-axis treatment in selection of optimal beam angles in SBRT of the peripheral lung lesions.
Methods: A novel beam optimization approach was implemented that maximizes the therapeutic gain of the beam-set by minimizing the average physical depth of the lesion with respect to the beam’s eye view (BEV). In an IRB-approved retrospective study, the quality of off-axis plans, essential to avoid collision, with optimal beam selection were evaluated and compared with that of on-axis plans and original ones without the optimal beam selection for 20 cases using RayStation treatment planning system. A custom design phantom is also under production to perform end-to-end test to assess the accuracy of off-axis treatments.
Results: Off-axis planning improved the organs at risk dose in all plans by reducing the treatment depth with an average of 27% compared to initial plans. We observed a significant reduction of 3.58% ± 5.02% (0.02-21.58, p < 2.3 × 10⁻4), 4.15% ± 6.27% (0.03-24.68, p < 0.03), 5.1% ± 7.42% (-0.65-29.05, p < 0.02), 11.87% ± 8.36% (0.89-37.65, p < 0.002), and 17.78% ± 10.92% (4.27-41.84, p < 1.9 × 10⁻5), in 25Gy isodose line volume IDLV25Gy, IDLV20Gy, IDLV15Gy, IDLV10Gy, and IDLV5Gy between the initial and off-axis plans, respectively. We observed no significant differences between the on-axis and off-axis plans in our study.
Conclusion: Off-axis treatment planning offers a potential solution to avoid collision for optimal beam selection in SBRT of the posteriorly located lung lesions. Using larger dataset and end-to-end phantom study warrants the safety of the off-axis treatment for SBRT of the thoracic lesions.