Stereotactic ablative body radiation therapy with dynamic conformal multiple arc therapy for liver tumors: Optimal isodose line fitting to the planning target volume

Purpose: To assess optimal relative prescribed dose values in stereotactic ablative body radiation therapy (SABR) using dynamic conformal multiple arc therapy (DCMAT) for liver tumors. Methods and Materials: We generated SABR plans for 8 typical liver tumors that received SABR with 50 Gy in 5 fractions. The prescribed dose had previously been defined as 80% of the maximal dose ("80% isodose plan"). Alternatively, 20%-90% isodose plans were created to compare dosimetric factors. Results: The mean liver volume values (%) that received >. 20 Gy (V20) and the mean liver dose were both the lowest with a 70% isodose plan and were the second lowest with a 60% isodose plan. The V20 dose was 5.19% lower (11.14%) with a 70% isodose plan and 4.51% lower (11.22%) with a 60% isodose plan compared with the value with an 80% isodose plan (11.75%). Mean planning target volume (PTV) dose increased as the % isodose decreased. The mean PTV dose was 10% higher (62.4 Gy) with a 70% isodose plan and 21% higher (68.9 Gy) with a 60% isodose plan compared with the value with an 80% isodose plan (56.8 Gy). Conclusions: During SABR treatment planning using DCMAT for liver tumors, target doses increased as the percentage isodose value decreased, which could result in better outcomes. In contrast, a 70% isodose plan had the lowest normal liver dose and a 60% isodose plan had the second lowest. An optimal percentage isodose level might be adjusted depending on tumor radiation sensitivity and liver function reserve. Further investigation is warranted to determine whether these dosimetric advantages result in improved outcomes.