Adaptive Radiation Therapy in the 0-7-21 Regimen: A Retrospective Study of Tumor Volumetric Response.

Purpose: A hypofractionated palliative radiation regimen referred to as "0-7-21," which involves 1500 to 2400 cGy prescribed in 3 fractions delivered over 3 weeks (ie, days 0, 7, and 21), has been well documented in literature as an effective way to alleviate symptoms in solid malignancies with minimal toxicity and logistics. Given the time between fractions, this study aimed to identify predictive factors of on-treatment tumor response in patients treated with the 0-7-21 regimen to help select palliative patients who may benefit from adaptive therapy. Methods and Materials: This single-institution retrospective review of patients treated with 0-7-21 from February 2017 to June 2022. Gross tumor volumes from cone-beam computed tomography at days 0 and 21 were collected. The calculated percent change in lesion volume was grouped into quartiles. Groups showing the greatest reduction (25th percentile) versus the greatest growth or least reduction (75th percentile) were analyzed. Univariate and multivariable linear regression models identified factors associated with the percent change in tumor volume. Results: A total of 105 patients with 193 lesions were included. The mean age was 77 years, the majority were men (n = 57, 54.3%), and several were nonsmokers (n = 42, 40%). Head and neck primaries (n = 125, 64.8%), head and neck anatomic sites (n = 160, 82.9%), and squamous cell carcinoma (n = 148, 76.7%) were the most common. Nine lesions (4.7%) were treated during concurrent systemic therapy. From days 0 to 21, the mean percent volume change was -9.9% (SD, 19.94). On multivariable analysis, Merkel cell histology (-24.24% [95% CI, -45.4 to -3.06]; Conclusions: Online adaptive radiation therapy may benefit selected 0-7-21 palliative regimen patients with Merkel cell histology and a smoking history, warranting consideration in future studies and resource allocation.