Relapse localization in Danish pediatric patients with Hodgkin lymphoma.

Background: Pediatric Hodgkin lymphoma (pHL) is highly curable. However, a minority experience relapse and are subjected to toxic salvage regimens. Investigating the patterns of relapse could help to select the patients and/or the involved sites that would benefit from consolidating radiotherapy.

Material And Methods: The Danish Childhood Cancer Registry was used to identify children <18 years with relapsed pHL from 1990-2018. The lymphoma volumes involved at diagnosis and at relapse were contoured on the patients' original scans. Rigid image co-registration was used to merge the scans enabling a visual assessment of the anatomical relapse localization relative to the initially involved lymph nodes, and if irradiated, to the radiotherapy field.

Results: From 185 patients with pHL, 24 patients with relapse were available for analysis. All patients received combination chemotherapy and seven had consolidating radiotherapy. Relapses exclusively in initially involved sites occurred in 14 patients. Relapses exclusively in new sites were rare and only observed in three irradiated patients. Seven patients relapsed in both initially involved and new sites. The median time to relapse was 6 months (range 2-59 months), however, in-field relapses in irradiated patients occurred later (54 months, range 10-59 months). Neither risk group, initial bulky disease, early response, or metabolic activity seemed to be associated with the site of a later relapse.

Conclusion: The number of relapses were small, and conclusions regarding the selection of patients for radiotherapy could not be drawn. Relapse exclusively in initially involved sites were the most common, most often in the exact same initially involved lymph nodes. Hence, modern involved site radiotherapy, focusing on the initially involved lymphoma volume and minimizing the radiation doses to normal tissues, should be applied when consolidating radiotherapy is used in patients with pHL.