3097 - Reporting Treatment Planning, Delivery and Dose Evaluation Guidelines Method for 3-Dimensional MLC-Based SFRT: A Single Institution Clinical Experience

Purpose/Objective(s): 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 guidelines for treatment planning using this SFRT method for unresectable bulky tumors.

Materials/Methods: A total of 137 large tumors treated across 131 patients with different histopathology were treated with 3D MLC-based SFRT method by MLC-fitting for 1 cm cylindrical dose and 2 cm center to center distance at isocenter. For SFRT target, physical dose and biological equivalent dose metrics and maximum EQD2 to nearby critical organs were reported. Clinical follow up data included tumor local control evaluated in post treatment CT scan, 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 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, 121.47-7879.87 cc). Patient cohort was predominantly male (61.8%), with most of treated sites in the abdomen pelvis areas (37.2%). Furthermore, the most common SFRT prescription utilized was 15 Gy in 1 fraction (85.4%) with most tumors receiving follow up treatment (126/137) for palliative courses (75/126). SFRT dose metric values for D5%, D10%, D50%, Dmean, D90%, D95%, and PVDR as 12.2, 11.8, 6.9, 7.3, 3.5, 2.4 Gy, and 3.4. For target BED metrics, BED5%, BEDmean, and BED95% as 71.5, 54.6, and 44.4 Gy. Tumor local control rate of 76.5% (75/98), and a pain relief rate of 72.0% (59/82) was found. Using multivariable logistic regression, these dose metrics were statistically significant with TCP or pain relief outcomes. Total, 95 SFRT patients were available for toxicity study, with 65 patients (68.4%) reporting no toxicities and 7 patients (7.4%) had grade 3+ events. Skin toxicity and xerostomia were statistically significant with maximum EQD2 of skin (p = 0.022) and parotid glands (p = 0.038). Cox hazard model showed that increases BED metrics were statistically significantly associated with longer OS; however, SFRT dose metrics were insignificant with longer OS. We reported OS rates of 51.8%, 36.3%, and 30.3% at 6 months, 12 months, and 18 months, respectively. Moreover, we found differences in OS between patients who received combination palliative dose of 30 Gy in 10 fractions and those patients who received curative dose of 60-70 Gy in 30-35 fractions (p = 0.002).

Conclusion: Our institutional outcome prediction of SFRT plans via same day 3D MLC-based crossfire method aligned well with our clinical observation for tumor local control, OS and toxicity data. Further refining of this predictive model for SFRT of large cohort of different histopathology is ongoing.