(P 285) MULTICENTER STUDY OF SPATIALLY FRACTIONATED RADIOTHERAPY (LATTICE) FOR LARGE-VOLUME RADIORESISTANT SARCOMAS

Objective: The management of bulky unresectable sarcomas with limited therapeutic options is challenging. Radiotherapy in this setting also has limitations. Most sarcomas are radioresistant, and treatment toxicities are often prohibitive. Spatially fractionated radiotherapy, also known as GRID or lattice, may be a viable alternative. This technique involves the intentional delivery of a heterogeneous dose distribution, alternating high and low doses across the tumor. Literature data on lattice therapy for sarcomas are limited. This study aims to evaluate the outcomes of this technique in large, radioresistant sarcomas.

Methods: A retrospective, international, multicenter cohort study was conducted. Eligible participants had a histopathological diagnosis of radioresistant sarcoma, lesions larger than 4.5 cm, were refractory to systemic therapy, or were ineligible for surgery. Two lattice protocols were used. A single-fraction (peaks) dose of 15–22 Gy, followed by 3–16 fractions of 2.66–8 Gy (24–42.56 Gy in the valleys). The other treatment used five fractions on alternate day of 4–5 Gy (20–25 Gy in the valleys), with an integrated boost of 12–13.25 Gy (60–65.25 Gy at the peaks).

Results: The study included 30 patients with 42 lesions treated. At first follow-up none of the lesions evaluated showed progression, with 43% remaining stable and 45% exhibiting reduction. The median survival was 10.0 months, with 33% of patients still alive at the time of analysis. Most patients reported significant pain relief, with limited and manageable side effects.

Conclusion: Lattice therapy demonstrated a good response rate and tolerability. These findings are valuable and hypothesis-generating, justifying further research to compare this strategy with current radiotherapy protocols for sarcomas in the palliative setting.