Reporting treatment planning, delivery, and dose evaluation for 3‐dimensional MLC‐based SFRT: A single institutional clinical experience

PURPOSE/OBJECTIVE: We report our institutional clinical experience of treatment planning, dose evaluation, and delivery via the same-day 3D MLC-based crossfire method for spatially fractionated radiotherapy (SFRT) in conjunction with combination therapy. Additionally, we aimed to model our clinical outcome data to provide clinical insights into treatment planning using this SFRT method for unresectable bulky tumors. MATERIALS/METHODS: A total of 137 large tumors in 131 patients with different histopathologies were treated with the 3D MLC-based SFRT method using MLC-fitting for a 1 cm cylindrical high-dose and a 2 cm center-to-center distance at the isocenter. Physical and biological effective dose metrics for the SFRT target were reported, as were the maximum EQD2 to nearby critical organs. Clinical follow-up data included tumor local control evaluated on post-treatment CT scans, pain relief, toxicity profiles, and overall survival (OS). Tumor control probability (TCP) and pain relief probability were modeled using multivariable logistic regression, while normal tissue complication probability (NTCP) was modeled using univariable logistic regression. Lastly, OS was modeled with a univariable Cox hazard model. RESULTS: This cohort had a median tumor volume of 477.55 cc (range, 21.47-7879.87 cc). The patient cohort was predominantly male (61.8%), with most treated sites in the abdominal-pelvic area (37.2%). Furthermore, the most common SFRT prescription was 15 Gy in 1 fraction (85.4%), and most tumors received follow-up treatment (126/137) with palliative intent (75/126). The median SFRT dose metric values for D5%, D10%, D50%, Dmean, D90%, D95%, EUD, and PVDR are 13.2 Gy, 11.8 Gy, 6.9 Gy, 7.3 Gy, 3.5 Gy, 2.4 Gy, 5.3 Gy, and 3.4. The resulting median target BED metrics, BED5%, BEDmean, and BED95% was 71.5, 54.6, and 44.4 Gy. A TCP rate of 76.5% (75/98), and a pain relief rate of 72.0% (59/82) was found. Using multivariable logistic regression, none of these dose metrics were statistically significant with TCP or pain relief outcomes. In total, 95 SFRT patients were available for toxicity assessment, with 65 (68.4%) reporting no toxicities and 7 (7.4%) reporting grade 3+ events. Skin toxicity and xerostomia were statistically significantly correlated with maximum EQD2 values to the skin (p = 0.022) and parotid glands (p = 0.038). The Cox hazard model showed that increases in BED metrics were statistically significantly associated with longer OS; however, increases in SFRT dose metrics were not. We reported OS rates of 51.8%, 36.3%, and 30.3% at 6, 12, and 18 months, respectively. Moreover, we found differences in OS between patients who received a combination palliative treatment to those who received a curative treatment (p = 0.002). CONCLUSION: Here, we report our single-institution results using the 3D-MLC-based crossfire technique for SFRT treatment and the resulting outcome models. Further refinement of this predictive model for SFRT in a larger, more diverse cohort is ongoing to validate our findings.