Purpose: To assess the most suitable value of a relative prescribed dose in clinical treatment plans of stereotactic body radiotherapy (SBRT) using dynamic conformal multiple arc therapy to treat lung tumors. Methods and Materials: We retrospectively generated alternative SBRT plans for typical examples of 8 patients who had been treated with SBRT for a lung tumor with a prescribed dose of 50 Gy in 5 fractions. The prescribed dose had been defined as 80% of the maximal dose in the planning target volume (PTV) ("the 80% isodose plan"). Alternative 20%-90% isodose plans at 10% intervals were generated (64 plans; 8 plans for each of the 8 patients), and factors related to leaf margins, target volume, normal lung volume, and monitor units were compared using dose-volume histogram analysis. Results: We could generate all the 64 plans. Compared with the 80% isodose plan, the V20 and mean lung dose (MLD) were both lower in the 60% plan; the V20 was approximately 19% lower (4.72% vs 3.84%) and the MLD was 13% lower (4.0 Gy vs 3.5 Gy). Mean PTV and ITV doses were higher in the lower percentage isodose plans. Compared with the 80% isodose plan, in the 60% isodose plan the mean PTV was 19% higher (56.1 Gy vs 66.8 Gy) and the mean ITV was 30% higher (59.6 Gy vs 77.4 Gy). The mean total monitor units increased more steeply than did the mean homogeneity index. The mean conformity index values in the 60% and 70% isodose plans were less than 1.15. Conclusions: The 60% isodose plan was considered the best plan in this analysis because of the lower comparative dosimetric factors in normal lung tissue (including V20 and MLD) and the higher comparative mean PTV and internal target volume doses achieved, along with good conformity index values. In clinical use, accurate estimation and commissioning should be performed for the dose distribution prior to selecting a plan. Further investigation is warranted to determine whether the calculated dosimetric advantages result in improved outcomes.
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