Author: Leslie Bell, Kai Ding, Reza Farjam, Russell K Hales, Sarah Han-Oh, Hamed Hooshangnejad, Jina Lee, K. Ranh Voong 👨🔬
Affiliation: Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Johns Hopkins University 🌍
Purpose: Optimal beam arrangement is crucial in automation and standardization of thorax radiotherapy where substantial tissue heterogeneity and several critical organs at risks (OARs) exist. Here, we present a robust quantitative metric for optimal beam selection for stereotactic body radiotherapy (SBRT) of the peripheral lung lesions in volumetric modulated arc therapy.
Methods: The new beam optimization approach was based on maximizing the therapeutic gain, defined as ratio of the target to OARs averaged tissue maximum ratio (TMR), of the beam-set (a set of half-arcs) estimated by minimizing the average physical depth of the lesion from the beam’s eye view (BEV). The new metric was used for beam selection and replanning of twenty-five SBRT lesions retrospectively to assess if a better plan is achievable in improving the target coverage, conformity indices and OAR sparing. Difference in isodose line volumes (IDLV25Gy), IDLV20Gy, IDLV15Gy, IDLV10Gy and IDLV5Gy between the two plan cohorts were also calculated and fitted in a linear regression model against the changes in the lesion depth with respect to BEV.
Results: Maintaining similar coverage, beam optimization decreased the treatment depth and improved OARs sparing and conformity indices in all cases. Regression analysis revealed that lower the treatment depth, the better the OAR sparing would be. In overall, we observed a reduction of 4.64% ± 4.55 (0.02%-21.58%, p < 3.8 x 10-5), 5.16% ± 5.54 (0.03%-24.68%, p < 0.005), 6.46% ± 6.95 (-1.35%-29.05%, p < 0.009), 12.83% ± 9.06 (0.89%-37.65%, p < 0.0001), and 14.01% ± 9.87 (1.43%-41.84%, p < 4.5 x 10-6) in IDLV25Gy, IDLV20Gy, IDLV15Gy, IDLV10Gy, and IDLV5Gy, respectively.
Conclusion: Physical depth of the lesion with respect to beam’s eye view is inversely proportional to the therapeutic gain of a beam-set and can be used as a robust quantitative metric for standardization of lung SBRT.