220 - Direct-to-Unit Stereotactic Body Radiation Therapy Utilizing Adaptive Planning

Purpose/Objective(s): Adaptive radiation therapy (ART) is often used in the non-stereotactic body radiation therapy (SBRT) setting to omit computed tomography (CT) simulation prior to treatment. We proposed utilizing diagnostic positron emission tomography (PET)-CT scans to deliver SBRT through a direct-to-unit (DtU) ring-gantry linear accelerated-based adaptive-driven workflow. We hypothesized this workflow allows for efficient treatment (tx) with local control on par with established standards while eliminating the patient and clinical burden of CT simulation.

Materials/Methods: Patients traveling from afar presenting with non-vertebral bone or lymph node metastases without proximity to critical organs at risk (OARs) amenable to SBRT were assessed for DtU ART. Using a ring-gantry ART system, the DtU workflow utilized the patient's PET-CT images for target and OAR delineation to create a reference tx plan. At the adaptive tx, images from a cone beam CT (CBCT) were used to contour OARs and targets per daily variations. These contours were used to calculate a scheduled plan (new contours + reference plan) and an adapted plan (new contours + newly optimized plan). The adapted plan was selected if coverage and/or OAR sparing were improved. All demographic and outcome variables were extracted from the electronic health record.

Results: Thirty-four patients (97% male, median age 71 years (IQR 68—77)) with 41 sites were treated with DtU ART. Most (91%) presented on video or phone visits and lived a mean distance of 372 miles from clinic. Diagnoses included prostate (91%), melanoma (6%), and breast (3%) cancers with bone (93%) and soft tissue (7%) metastases. Single-fraction DtU ART was used for 71% of sites. Of the 41 sites, 68% had high and low dose target volumes with a median dose of 2400 cGy and 1600 cGy, respectively. In every case, ART plans were superior to scheduled plans due to better target coverage (58%), improved coverage and OAR sparing (27%), or a combination of other factors (15%). The mean time of DtU ART from the first CBCT to end of tx was 45 minutes (SD 10). Contouring and planning took an average of 12 (SD 6) and 5 (SD 2) minutes, respectively. The first fraction adaptive plan resulted in a high-dose volume D95 that was on average 4.8% higher than the prescribed dose; the scheduled plan D95 dose was on average 5.8% lower. Median follow-up was 8.9 months; median time to progression was 8.2 months. Of the 39 sites with available follow-up data, 25 (64%) had a complete metabolic response at a median of 6 months after tx. Progression-free survival for the 31 prostate cancer patients was 40% at one year. Overall survival was 97% at 12 months post-tx.

Conclusion: CBCT-based direct-to-unit adaptive radiation treatment utilizing diagnostic CT or PET-CT can be implemented to deliver SBRT in a safe, clinically efficient, and effective manner, especially for patients residing far from a radiation oncology center. DtU SBRT avoids conventional simulation session which can improve patient QOL, time off work, and travel costs.