ESTRO 2021 Abstract Book

S277

ESTRO 2021

Conclusion We have identified DTRT gantry-table-collimator path selection strategies enabling substantial OAR sparing compared to state-of-the-art VMAT for a number of common HN cases. This work was supported by Varian Medical System. PH-0378 How to achieve the sharpest dose fall-off for hypo-fractionated radiosurgery of large brain lesions? T. Nano 1 , O. Morin 1 , B. Ziemer 1 , D. Raleigh 1 , L. Boreta 1 , J. Nakamura 1 , S. Fogh 1 , P. Sneed 1 , S. Harvey-Jumper 2 , P. Theodosopoulos 2 , S. Braunstein 1 , L. Ma 1 1 University of California, San Francisco (UCSF), Radiation Oncology, San Francisco, USA; 2 University of California, San Francisco (UCSF), Neurological Surgery, San Francisco, USA Purpose or Objective Previous studies have characterized dose gradient patterns for various hypo-fractionated brain radiosurgery (hSRS) treatment platforms. The Gamma Knife Icon (GKI) has consistently exhibited either superior or non- inferior peripheral dose fall-off and normal brain sparing characteristics when compared to other Linac-based hSRS treatment platforms. Given the prevalence of Linac-based treatments, identifying planning techniques to optimize treatment parameters is desirable. In this study, we investigated a novel Linac-based treatment approach that aimed to create the sharpest dose fall-off for hSRS of large brain lesions. Materials and Methods A cohort of patient cases (n=10) with single brain lesions (volume 27.6±8.1 mL, range 20.-42.1 mL) treated with GKI at our institution were selected as sample test cases for our study. A non-coplanar unconstrained VMAT (NCU-VMAT) treatment planning approach was developed, and its script was implemented on a commercial treatment planning system (RayStation, RaySearch Americas, CA, USA) for clinical Linac treatment (Varian TrueBeam STx) equipped with the latest high-definition multi-leaf collimators (MLCs). Treatment plans produced via the NCU-VMAT approach were then compared against further optimized treatment plans from GKI, as well as conventional coplanar and non-coplanar VMAT treatment planning approaches. The comparison was carried out using DVH-derived parameters including target volume coverage, target dose conformity, modified dose fall-off index defined as the volume of 50% prescribed target dose divided by the target volume (PIV 50 ). Results For each case studied, NCU-VMAT achieved practically identical target coverage (0.98±0.01) and Paddick dose conformity index (0.87±0.02) compared to optimized GKI treatments. The mean PIV 50 values were 2.99±0.14, 3.77±0.22, and 3.36±0.19 for optimized GKI, conventional coplanar, and non-coplanar VMAT treatment plans, respectively. These results were in excellent agreement with previously published studies. However, the NCU- VMAT technique yielded a mean PIV 50 of 2.41±0.07. This represents an improvement of approximately 40% (p<0.001, paired two-tailed Student t-test) over all existing approaches including GKI. Further analyses revealed that PIV 50 of NCU-VMAT approached the theoretical minimum for all the studied cases.