Prospective Study of 4 Gy Radiotherapy for Orbital Mucosa-Associated Lymphoid Tissue Lymphoma (FORMAL)

SIMPLE SUMMARY: Marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALToma) is a slow-growing lymphoma with a good prognosis. This study was designed to evaluate the effectiveness of radiotherapy at a very low dose of 4 Gy (2 Gy × 2 fractions) in stage I orbital MALToma. Patients with complete remission after a very low dose of 4 Gy (2 Gy × 2 fractions) radiotherapy were closely monitored, and those who did not achieve remission received an additional 24 Gy radiotherapy. Using 4 Gy radiotherapy for orbital MALToma, 11 out of 17 lesions achieved complete remission. There was no transformation of diffuse large B-cell lymphoma, and there was only one local failure. Radiation therapy at a low dose of 4 Gy could be performed effectively and safely with a planned second-line treatment. ABSTRACT: External beam radiotherapy is effective for stage I orbital mucosa-associated lymphoid tissue lymphoma (MALToma). Hence, very-low-dose radiotherapy is increasingly being investigated. We conducted a single-center prospective phase II trial to evaluate the effectiveness of very-low-dose radiotherapy of 4 Gy (2 Gy × 2 fractions) in pathologically confirmed stage I orbital MALToma. In this first prospective trial, patients with complete response were observed after 3–6 months of follow-up. For patients without complete remission, a radiation dose of 24 Gy/12 fractions was additionally delivered. The primary endpoint was complete response rate; secondary endpoints were overall survival, local control, and progression-free survival. Seventeen patients were screened and three patients refused enrollment during October 2018–October 2021. Thus, 14 patients (17 eyes) were analyzed (median follow-up, 28.2 months). The overall response rate was 100% (complete remission: 11 lesions; partial remission: six lesions). In all lesions with residual disease, additional radiation therapy (dose: 24 Gy) was performed. One local failure was observed. Therefore, 4 Gy ultralow-dose radiation therapy for orbital MALToma was safely performed with a planned second-line treatment in patients without complete remission. This is the first prospective study to report the effectiveness of ultralow-dose radiotherapy of 4 Gy for stage I orbital MALToma treatment.