3695 - Spatially Fractionated Radiation Therapy for Bulky Head and Neck Malignancies: A Single Institution Experience

R. J. Megahed¹, S. Gholami¹, C. L. Chien¹, S. Samanta¹, M. Patel¹, A. Z. Kesaria¹, and G. D. Lewis²; ¹Department of Radiation Oncology, UAMS Winthrop P. Rockefeller Cancer Institute, University of Arkansas for Medical Sciences, Little Rock, AR, ²Department of Radiation Oncology, Mays Cancer Center, UT Health San Antonio, San Antonio, TX

Purpose/Objective(s): Spatially Fractionated Radiation Therapy (SFRT) is a safe and effective tool to debulk large head and neck tumors and improve local control. This technique offers ablative level doses to an intra-tumoral volume while sparing surrounding sensitive organs at risk. Different methods have been developed and proven to deliver this type of advanced radiation treatment including block based, VMAT-based, three dimensional MLC-based, and 3-D CRT and IMRT system-based. There appears to be no consensus as to what size volume would benefit from SFRT in this area. In this study we aimed to assess whether volume or treatment modality influenced plan quality. Materials/

Methods: Forty-eight consecutive patients treated with SFRT from a single institution to bulky head and neck tumors were evaluated and included in our analysis. Dose heterogeneity was measured as the peak-to-valley-dose ratio (PVDR). A one-way ANOVA was used to analyze if the PVDR is statistically significant (p<0.05) with different treatment methods and to check if the tumor size is statistically significant (p<0.05) with different treatment methods. A post-hoc Tukey’s test was done when the ANOVA result is significant.
Results: A total of 48 consecutive plans were evaluated with an average size of 256.8 cc (range of 39.66 to 861.5) GTV. All patients underwent a prescribed dose of 15 to 20 Gy in a single fraction. 43 of 48 patients (89.5%) underwent a prescription dose of 20 Gy, 2 (4.1%) underwent 18 Gy, and 6.3% underwent 15 Gy. The ANOVA result showed that the PVDR quality is correlated with different treatment methods while there is no significant differences in the usage of treatment methods for various size of tumors. The post-hock Tukey test showed that the difference of PVDR between block-based and 3-D CRT and IMRT system (p=1) is insignificant while 3D MLC-based (p<0.0061) and VMAT-based (p<0.001) have different PVDR than block-based and 3-D CRT and IMRT system-based. Furthermore, the PVDR of 3D MLC-based and VMAT-based are significantly different.
Conclusion: The PVDR was not sacrificed for lower volume tumors. With this result, SFRT appears feasible and be an effective tool for low volume tumors. While VMAT based therapy appears to have a higher PVDR compared to other modalities, it still results in clinically adequate dosimetry.