Room: AAPM ePoster Library
Purpose: Previously, at our institution SBRT lung patients were being treated using modulated conformal arc beams calculated in AAA. When Acuros XB (AXB) was adopted, the more accurate modeling of radiation transport of Acuros made it more challenging to meet protocol guidelines, especially in heterogeneous regions. To combat this issue we increased the MLC block margins and needed to add static oblique beams to the conformal arcs to help achieve better results. The purpose of this study is to quantify the resulting improvement in dosimetric plan quality.
Methods: A cohort of 25 patients treated with the new beam arrangement was retrospectively analyzed. These patients were planned initially using only modulated conformal arcs and up to 3 additional static beams were added to improve the plan. The plans without the added beams were compared to those with the added static beams. A subgroup of 12 patients with island-type tumors were analyzed separately. The plans were evaluated using the RTOG 0813 guidelines.
Results: On average 1.5 static beams were added per plan. The addition of the static beams lead to a small increase in maximum dose and V105 spillage, while staying within protocol guidelines. There was a significant decrease in deviations related to the conformity index (V100) and D2cm. V50 also improved in minor and major deviations. The total MU from the different approaches increased by less than 1%. In general, the island type PTVs were more challenging to make meet planning constraints, but saw similar improvements when static beams were added.
Conclusion: The addition of static oblique beams helps improve planning for SBRT lung lesions with the biggest benefit seen in the conformality index and D2cm. While total MU remained relatively unchanged, the added beams will increase total treatment time and therefore should be used only as needed.