(P 313) SPACIALLY-FRACTIONATED RADIOTHERAPY (LATTICE) TREATMENT IN INOPERABLE NON-EXTREMITY BULKY SOFT-TISSUE SARCOMAS

Objective: LATTICE is a technique of spatially-fractionated radiotherapy that allows to deliver a high dose of radiation (10-20Gy) to small volumes inside a tumor while simultaneously reducing the dose to normal tissue.

Our objective was to evaluate its feasibility, clinical outcome and toxicity in treatment of inoperable non-extremity soft-tissue sarcomas.

Methods: This retrospective observational study includes 9 histologically proven inoperable non-extremity sarcoma patients without option or no response to systemic treatment, treated between 2020-2022.

Six patients were treated with normofraccionated external beam radiotherapy (EBRT) 45-50 Gy (EQD2) and 3 patients with PS 2 were treated with short course treatment of 20-25Gy/5Gy per session. The VMAT technique was performed in all of them. A boost with Lattice technique of 20Gy single dose (8 patients) and 13.5Gy x 5 fractions (1 patient) in spheres, localized within the tumor was administered previously or after the end of standard EBRT.

Results: The mean follow-up since the beginning of the treatment was 10 months (4-23). 6 patients were male and 3 female. Mean age 55 years. Seven treated lesions were primary tumors, 2 metastatic lesions (1 pelvic and 1 ganglionar with multiple metastasis before the beginning of LATTICE treatment).

The tumor location was following: 1 mediastinum, 4 abdomen, 4 pelvis. The histology was: spindle cell sarcoma NOS (3), leiomyosarcoma (2), dedifferentiated liposarcoma(2), synovial sarcoma (1) and endometrial stromal sarcoma (1). The mean GTV volume was 1133 cm³ (142-3580 cm³). Mean number of spheres was 13 (4-30).

No acute and chronic G3-G5 toxicity was observed. Only 1 patient had G2 enteritis, due to 50 Gy + LATTICE 20 Gy. All 5 patients with symptoms before the treatment (dyspnea, pain, anemia, edema and constipation) presented symptom’s relief.

Based on RECIST1.1 criteria, 8 patients (89%) had stable local disease. Only 1 patient (11%) with metastatic disease before the LATTICE had multiple progression including the local one at 3 months follow-up.

The surgery resection was feasible after the RT treatment in 3 patients (2 pelvic lesions and 1 bulky retroperitoneal sarcoma). 1 of them is alive without disease and 2 are dead due to metastatic progression.

3 patients are alive with stable disease (at 9,10 and 23 months of follow-up),1 patient with performed surgery is alive without disease and 5 patients are dead due to metastatic progression.

Conclusion: The LATTICE is a feasible radiation technique in inoperable bulky soft-tissue sarcomas without increment of the acute toxicity. It can be considered as a treatment option in bulky sarcomas for symptomatic relief and in some cases in the preoperative scenario.