Diagnostic imaging in Merkel cell carcinoma: Lessons to learn from 16 cases with correlation of sonography, CT, MRI and PET

Abstract

Objective

The authors report imaging findings in a series of 16 patients with MCC, a rare tumour which is often managed primarily by a dermatologist. To our knowledge, no equivalent series of MCC has been described in the nuclear medicine literature.

Material and Methods

In this IRB-approved retrospective noncomparative case series 16 patients with biopsy-proven Merkel cell carcinoma were included between January 1999 and October 2007. Twenty-nine whole body PET scans (18F-FDG n = 24, 18F-FDOPA n = 5) in 16 patients were retrospectively reviewed with regard to tracer uptake in six anatomical sites per patient. For 127/144 of FDG-PET evaluated regions and 68/144 of regions depicted by conventional imaging methods, a valid standard of reference could be obtained. A combined standard of reference was applied, which consisted of histopathology (lymphadenectomy or biopsy) or clinical or radiological follow-up for at least 12 months. Results: the mean FDG uptake over the clinicopatholigical verified FDG avid areas was 4.7 SUV (1.5–9.9 SUV). The region based assessment of diagnostic value, in consideration of the standard of reference, resulted in a sensitivity of 85.7% and a specificity of 96.2% of FDG-PET (n = 127) and in a combined sensitivity of 95.5% and a specificity of 89.1% for morphological imaging methods (n = 68). Differences between methods did not reach statistical significance (p = 1.00, p = 0.18).

Conclusions

FDG-PET is a highly useful whole body staging method of comparable value compared to conventional imaging methods with restricted field of view. The lessons learned from case series are discussed.

Introduction

Merkel cell carcinoma (MCC), or neuroendocrine carcinoma of the skin, is a rare aggressive malignancy with a mortality rate of approximately 25%. Incidence rates are estimated to be 0,1–0,3/100.000/year (melanoma ∼15/100.000/year) [1]. MCC occurs most frequently in the head and neck region of the elderly or immunocompromised patient [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].

MCC usually appears as a fast-growing, painless, dome-shaped lump that is red or violet in colour and is often resected primarily by a dermatologist [10], [11]. At initial presentation MCC involves regional lymph nodes in 10–45%, and during the course of the disease 50–75% of patients will develop regional lymph node metastases [2], [3], [4], [6], [12], [13], [14].

In approximately 50% of patients with regional lymph node metastases distant involvement will be found, predominantly in the liver, bone, brain, lung, and skin. A commonly used staging system defines three stages [2]: Stage I describes localised disease without spread to lymph nodes or distant sites. In stage II, the MCC has spread to regional lymph nodes, but there is no evidence of distant metastases. Stage III is defined by the presence of systemic metastases. Treatment options depend on stage and location and include radiation therapy after wide local excision for stage I, additional adjuvant chemotherapy for stage II, or mere chemotherapy for patients with stage III MCC [15]. Hence, staging is crucial in therapy planning and to avoid surgery in incurable patients.

Due to the rarity of MCC, imaging findings have been reported only in case reports and small series and no universally accepted imaging algorithm for MCC exists [12], [16]. Morphological standard imaging methods, i.e. ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) often well visualize the primary MCC and its metastases, but are confined to their limited field of view and of limited accuracy because tumour involvement is not necessarily reflected by morphological changes. Literature about FDG-PET in patients with MCC is limited to 14 published cases [17], [18], [19], [20], [21], [22].

We hypothesised that PET imaging is useful in the staging of patients with MCC. Thus, the purpose of our study was to review the morphological and functional imaging findings and to compare their impact on clinical management in 16 patients with biopsy-proved MCC. The implications of these cases with regard to chose the best imaging modality for patients presenting with MMC are discussed in this article and a brief review of the relevant imaging literature on this topic is also presented.