Very low dose radiation therapy for indolent lymphoma: comparing “big boom” (4gy × 1) versus “boom boom” (2gy × 2)

Purpose/Objectives: Indolent lymphomas are exquisitely sensitive to radiation therapy (RT). Programs of 2Gy × 2 were shown to be highly effective in controlling irradiated site(s). During the COVID-19 pandemic, the International Lymphoma Radiation Oncology Group (ILROG) proposed guidelines that offered substitution of the Boom Boom (2Gy × 2) regimen with Big Boom of 4Gy × 1. This report compares our center’s experience with both regimens. Materials/Methods: We included patients with indolent lymphomas in this retrospective single institution study. After April 2020, both options of very low dose and choice of a standard full dose of 24Gy were discussed with the patients. Patients were treated with a definitive or palliative intent depending on disease stage and prior therapy exposure. Patients treated with 24Gy are not included in this report. Overall response rate (ORR) was assessed with Lugano PET criteria at the initial post-RT imaging. Differences between the two groups were examined using the Fisher’s exact test and Mann-Whitney test. Results: We evaluated a total of 471 lesions in 386 patients, including 172 lesions (37%) treated with 4Gy × 1 and 299 lesions (63%) treated with 2Gy × 2. Table 1 summarizes the patient and treatment characteristics. Age at the time of RT and sex were not significantly different between the two groups. The 2Gy × 2 cohort was more likely to have follicular lymphomas (FL) (66% vs. 54%, p = 0.011), though the proportion of higher-grade FL was similar between cohorts. The ORR was similar (4Gy × 1 = 86%, 2Gy × 2 = 87%) at the first post-RT evaluation (median of 2 months from RT for both cohorts). There was no significant difference in the rate of complete response, partial response, stable disease, or progressive disease between the cohorts at initial post-RT imaging. For both regimens, no directly related short-term side effects were observed. Conclusions: Both the 4Gy × 1 and 2Gy × 2 regimens demonstrated excellent ORR at the initial post-RT imaging assessment among patients with indolent lymphomas. While longer term follow-up is required to confirm durability of these findings, our initial experience suggests that 4Gyx1 regimen recommended by ILROG during the pandemic is an effective treatment approach. FL Grade 3A FL Grade 3B FL Grade 1-2 7 (4.1%) 0 86 (50%) 7 (2.3%) 2 (0.7%) 188 (63%) >5 cm ≤5 cm Unknown 15 (8.7%) 96 (56%) 61 (35%) 40 (13%) 259 (87%) 0 Complete Response Partial Response Progressive Disease Stable Disease Not assessed yet 93 (62%) 35 (23%) 6 (4%) 15 (10%) 23 190 (64%) 68 (23%) 16 (5.4%) 24 (8.1%) 1 Keywords: indolent non-Hodgkin lymphoma, radiation therapy No conflicts of interests pertinent to the abstract.