Systematic improvement of SBRT dose gradients.

Abstract

154 Background: Stereotactic body radiation therapy (SBRT) offers excellent local control for early stage lung cancer. SBRT utilizes high fractional doses of radiation in 5 or fewer treatments. Normal tissue risk is substantial necessitating steep dose gradients and conformal plans. The Radiation Therapy Oncology Group (RTOG) has established quality metrics for conformality and dose gradients for SBRT. The conformality ratio is the prescription isodose volume to the planning target volume (PTV). Dose falloff is represented by the ratio of the 50% prescription isodose volume to PTV volume (R50%). We evaluated our approach and identified a systematic method to improve our SBRT plans in a community cancer center setting. Methods: 26 lung nodules were treated with SBRT over 30 months at our institution. RTOG 0915 benchmarks were evaluated. Conformality ratios were excellent in all cases. Dose falloff was measured by the R50% and the 7 cases with the largest R50% were re-planned systematically. The technique included creation of a customized ring structure: PTV volume was multiplied by the maximum allowable ratio to establish an ideal 50% isodose volume and a 2 cm ring structure was created around this: the 50 IDV Ring. Intensity modulated radiation therapy (IMRT) optimization specified a max dose to the 50 IDV Ring of 50% of the prescription. Beam geometry and IMRT objectives were held constant from the initial plan. Results: Conformality ratios were maintained and were excellent in all cases. The 50 IDV Ring resulted in R50% improvements in 5 of 7 cases, 4 of which had at least a 15% improvement (15%, 20.6%, 19.2%, and 20.4%) and 1 of which had a 3.3% improvement. Two cases resulted in 1.8% and 0.5% increases in the ratio representing a small deterioration in dose falloff. Conclusions: A strategically designed ring structure can have an impact on dose falloff around a target volume resulting in improvements in dose gradients for SBRT plans. Whether this benchmark is clinically significant is uncertain but it does provide a useful metric for comparison and quality assurance of SBRT plans. A systematic approach to SBRT planning may help provide the highest possible therapeutic ratio in the setting of optimal beam geometry.