Treatment of Non-Hodgkin Lymphoma Involving Head and Neck Sites with a 1.5 T MR-Linac: Preliminary Results from a Prospective Observational Study

<b>Purpose</b>: Lymphomas are generally radiosensitive; therefore, disease volume tends to shrink during radiotherapy courses. As MRI-linac provides excellent soft tissue definition and allows daily re-contouring of gross tumor volume and clinical target volume, its adoption could be beneficial for the treatment of lymphomas. Nonetheless, at this time there is a lack of literature regarding the use of MR-linac in this context. <b>Methods</b>: A prospective observational study was conducted on patients affected by non-Hodgkin lymphoma (NHL) involving head and neck (H&N) sites and treated with Elekta Unity^® MR-Linac. The clinical and dosimetric data of the first eight patients were collected and integrated with relevant data from medical records. <b>Results</b>: Seven patients had B-cell lymphoma (three DLBCL, two MALT, one follicular, and one mantle-cell) and one T-cell/NK lymphoma. The intent of RT was radical for four patients, salvage treatment for three, and CAR-T bridging for one. Two patients presented orbital localizations and six cervical lymphonodal sites. Median GTV was 5.74 cc, median CTV 127.01 cc, and median PTV 210.37 cc. The prescribed dose was 24-50 Gy in 2 Gy fractions for seven patients and 24 Gy in 3 Gy fractions for one patient. All the patients experienced acute toxicity, the maximum grade was G1 for five patients and G2 for three at the end of RT. One month after radiotherapy seven patients still experienced G1 toxicity, but no toxicity grade ≥ 2 was reported. First radiological assessment was performed for all the patients after a median of 101.5 days, reporting complete response in all the cases. After a median follow up of 330 days, no patient experienced local disease progression, while one patient developed distant progression. <b>Conclusions</b>: radiotherapy for NHL with H&N localization using a 1.5 T MR-linac was feasible, with no >G2 toxicity and optimal response rate and disease control.