Magnetic Resonance Imaging findings in Primary Extracutaneous Melanoma from a Tertiary Cancer Care Centre: A Retrospective Cohort Study

Abstract

Introduction: Extracutaneous melanomas are rare, aggressive type of tumour, clinically and biologically distinct from their cutaneous counterpart. The two large broad categories of the extracutaneous melanomas are ocular and mucosal subtypes. Melanomas are classically hyperintense on T1 weighted images and hypointense on T2 weighted images due to the paramagnetic effects of melanin and presence of paramagnetic elements. Aim: To describe the Magnetic Resonance Imaging (MRI) findings in primary extracutaneous melanomas at various anatomic sites. Materials and Methods: This was a retrospective study in which 13 cases of primary extracutaneous melanomas were identified from Picture Archiving and Communication System (PACS) archive over a period of eight years (January 2013 - December 2020). Location and morphology of the tumour, signal intensity characteristics in T1 weighted, T2 weighted (hyperintense/ isointense/hypointense to adjacent muscle) and Diffusion Weighted Imaging (DWI) (presence or absence of diffusion restriction) were analysed. Results: The ocular melanomas (n=2) were seen as well-defined small intraocular mass attached to the choroid. The mucosal melanomas of the nasal cavity (n=2), rectum (n=4), vagina (n=3) and cervix (n=2) presented as large intraluminal polypoidal masses. Three categories of MRI appearances emerged in this study. Majority of the cases (n=8) showed hyperintense signals in T1- weighted images and hypointense signals in T2 weighted images (category 1). Diffusion restriction was seen in all cases (n=13) with low Apparent Diffusion Coefficient (ADC) values which ranged from 439-966 mm/sec with an average value of 755 mm/ sec. Conclusion: T1 and T2 shortening are typical of melanoma, the absence of these does not exclude the diagnosis. Although diffusion restriction and low ADC values help in the diagnosis of these tumours, they do not tend to play a specific role in the diagnosis.