Abstract PO5-18-05: Assessment of the Potential of Photon mini-GRID Therapy for Pre-operative Partial Breast Cancer Treatment

Abstract Purpose We propose a new spatially fractionated radiation therapy (SFRT) technique, called mini-GRID therapy, to reduce normal tissue toxicities in pre-operative partial breast cancer radiotherapy. Mini-GRID therapy, an optimized implementation of GRID therapy, utilizes very narrow beams (widths ~ 1-2 mm) which significantly increase normal tissue tolerances. The concept, proposed by our team in 2017 [1], has been successfully implemented at Hospital de Santiago de Compostela (Spain) showing excellent results in terms of brain toxicity reduction [2]. Profiting from this remarkable increase in tolerance, this study aims to evaluate the feasibility and efficacy of photon mini-GRID therapy for pre-operative partial breast cancer treatment. Methods Ten unbiased clinical cases were selected for the study, comparing photon mini-GRID therapy with conventional broad beam radiotherapy. Monte Carlo simulations (TOPAS v.3.6) were performed based on the mini-GRID therapy implementation (megavoltage X-rays, Varian flattening-filter-free LINAC) realized at the University Hospital in Santiago de Compostela. A mean dose of 24 Gy was delivered in the PTV using three photon mini-GRID arrays with mini beams of approximately 1.8x1.8 mm² at the isocenter. The study aimed to achieve better tumor coverage than conventional pre-operative radiotherapy while ensuring comparable skin toxicity. Valley doses, determining tissue-sparing effects, were compared against organs-at-risk (OARs) tolerance limits. Results The mini-GRID therapy treatments ensured partial tumor coverage and met the dose tolerance limits for all the considered OARs, the most challenging of all being the skin [3]. Peak-to-valley dose ratio (PVDR) in the skin was higher than 3.5 for all the considered patients. Compared to standard radiotherapy (SRT) plans, mini-GRID therapy significantly reduced mean doses to the lungs, heart, and chest wall in the studied cases. Conclusion In this proof-of-concept study, we evaluated photon mini-GRID therapy with megavoltage X-rays as an alternative treatment approach for pre-operative partial breast cancer irradiation, demonstrating its feasibility and straightforward implementation. Because of its high skin and normal tissue-sparing capacities, photon mini-GRID therapy has great potential to improve pre-operative treatment outcomes. Complementary studies and clinical validation are needed to fully establish the optimal protocols and assess long-term effects, but the results encourage the integration of photon mini-GRID therapy as a promising modality for breast cancer therapy. References [1] Martínez-Rovira I et al., Dose evaluation of Grid Therapy using a 6 MV flattening filter-free (FFF) photon beam: A Monte Carlo study. Med Phys, 2017. [2] Sanchez-Garcia M et al., Implementation and dosimetry of an optimized spatially fractionated radiation therapy technique at a clinical medical accelerator. Physica Medica: European Journal of Medical Physics, 2022. [3] QUANTEC 2010, Quantitative analyses of normal tissue effects in the clinic Int. J. Radiat. Oncol. Biol. Phys. 76 S1–160. Citation Format: Angela Corvino, Tim Schneider, Jeremi Vu-Bezin, Youlia Kirova, Yolanda Prezado. Assessment of the Potential of Photon mini-GRID Therapy for Pre-operative Partial Breast Cancer Treatment [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-18-05.