ESR Essentials: imaging of melanoma-practice recommendations by the European Society of Oncologic Imaging.

Staging and response assessment of melanoma is a challenging radiological task as the tumor entity is characterized by atypical metastatic phenotypes and response patterns. Radiologists should scrutinize diagnostic images for unusual metastatic sites, including, e.g., gastrointestinal, endobronchial, or splenic metastases. Routine imaging modalities must include body CT or PET/CT staging and brain MRI. Response assessment is complicated by pseudoprogression and dissociated response, which radiologists must identify and communicate with the treating physicians. Criteria-based reporting with immune-adapted response criteria, such as iRECIST, should be used. The abundant use of checkpoint inhibitor therapy should make radiologists aware of immune-related adverse events, which must be detected early. In addition, imaging professionals must carefully identify potential side effects that mimic progression, in particular sarcoid-like reactions with newly developed mediastinal and hilar lymphadenopathy, as this will ensure that progressive disease is not overcalled. Additionally, radiologists should be aware of the emerging field of intratumoral immunotherapy, which can result in local inflammatory changes at injection sites that may also mimic progression. For this, a dedicated adaptation of the response criteria may be applied with itRECIST. KEY POINTS: The staging of melanoma should factor in the possibility of atypical metastatic patterns. For response assessment, radiologists should be aware of the differential response patterns. Common side effects that may mimic progression should be carefully identified.