Diagnosis-Related Outcome Following PALLIATIVE Spatially Fractionated Radiation Therapy (Lattice) of Large Tumors

Background: Lattice Radiotherapy (LRT), a spatially fractionated stereotactic radiotherapy (SBRT) technique, has shown promising results in the palliative treatment of large tumors. The focus of our first analysis of 56 lesions >/=7cm was on the extent of shrinkage following palliative LRT (: mean 50%), and assessment of its effect duration (: mean >6 months). Herewith we present an updated analysis of our single center LRT cohort, with focus on LRT outcome across diagnoses and applied LRT regimens. Methods: We assessed the clinical outcome following LRT in 66 patients treated for 81 lesions between 01.2022-05.2025. LRT protocols included simultaneous integrated boost (sib-) LRT in 49 lesions (5x 4-5 Gy to the entire mass with sib of 9-13 Gy to lattice vertices). Alternatively -mainly in pre-irradiated and/or very large lesions- a single-fraction stereotactic LRT (SBRT-LRT) of 1x 20 Gy to vertices only was delivered to 26 lesions. In 6 cases with modest response to single fraction SBRT-LRT, the sib-LRT schedule was added 4-8 weeks later. Results: The median age was 68 years (18-93). Main tumor locations were abdomino-pelvic (n=34) and thoracic (n=17). Histopathological diagnosis included carcinoma (n=34), sarcoma (n=31), and melanoma (n=16). 31% of all lesions have been previously irradiated. 73% of cases underwent concurrent or peri-LRT systemic therapy. The mean/median overall survival (OS) time of the cohort was 7.6/4.6 months (0.4-40.2), 11.9/5.8 months in 16/66 alive, and 6.4/4.3 months in died patients, respectively. 82% of symptomatic patients reported immediate subjective improvement (PROM), with a life-long response duration in most cases. Progressive disease (PD: >10% increase of initial volume) was found in 9%, stable disease (SD +/-10% of initial volume) in 19% of scanned lesions, shrinkage (>10% reduction of initial volume) in 75%, with a mean/median tumor volume reduction of 51/60%. The extent of shrinkage was found 11-30% / 31-60% / 61-100% in 38/24/38% of lesions. Response rates (PD, SD, Shrinkage) following the two applied LRT regimens as well as related to sarcoma and carcinoma diagnosis were found comparable. Treatment tolerance was excellent (G0-1). Conclusions: Palliative LRT provides rapid subjective relief in ~80% of symptomatic patients. Radiologic shrinkage was stated in 75% of FU scanned lesions, with a life-long effect duration in most patients. LRT was found effective across histologies, with a similar extent of shrinkage in carcinoma and sarcoma and following 1F SBRT- and 5F sib-LRT regimens, respectively.