Metastatic tumors to the nail unit: subungual metastases.

BACKGROUND: Cutaneous metastases are variable in location and morphology. Metastatic tumor can present as a subungual lesion in either an oncology patient or a previously cancer-free individual. However, the diagnosis of a subungual metastasis is often not initially considered since the symptoms and appearance of the subungual tumor frequently mimic those of other conditions.
OBJECTIVE: To describe the clinical characteristics, radiographic changes, and pathologic findings of the subungual metastases in two women with metastatic carcinoma and to discuss the features of metastatic tumor lesions to the subungual area and distal digits previously reported in oncology patients.
METHODS: The clinical presentation, radiologic studies, and pathologic examination of metastatic subungual tumor lesions were described in two oncology patients: a woman with breast cancer and a woman with renal cell carcinoma. The published reports of cancer patients with subungual metastases were reviewed and the following variables were evaluated: the primary origin of the cancer, the histology of the primary tumor, the temporal relationship between the onset of symptoms or the appearance of subungual metastasis and the diagnosis of the visceral malignancy, the symptoms and the morphology of the subungual metastases, the clinical differential diagnosis of subungual metastases, the relationship between the site of origin of the primary tumor and the incidence of metastases either to the fingers and the thumbs or to the toes, the distribution of subungual metastases, the incidence of radiologically confirmable bony involvement of the distal phalanx by metastatic tumor in the digit containing the subungual metastasis, and the prognosis of patients in whom the diagnosis of a subungual metastasis has been confirmed.
RESULTS: Subungual metastases most frequently occur in patients with primary tumors of the lung (41%), genitourinary tract organs (17%, of which the kidney represents 11%), and breast (9%). The histology of the primary tumors that was most common included renal cell carcinoma and squamous cell carcinoma. The appearance of the subungual tumor was the first sign of a previously unsuspected primary malignancy in 44% of the patients with subungual metastases. Subungual metastases were frequently painful and most often presented as either an erythematous enlargement or swelling of the distal digit or a red to violacious nodule that distorted either the nail plate or the soft tissue of the distal digit, or both. The lesion was often initially mistaken as an acute infection. The lesion involved one or more digits of the hands in 92% of patients with subungual metastases; symmetrical subungual metastases and metastatic tumor restricted only to the great toes were less commonly observed. In patients with subungual metastases that involved the digits of their hands, the most frequent sites of primary tumor origin were the lung (35%) and the genitourinary tract organs (25%). Radiologic evidence of bony involvement of the respective distal digit was either initially present or subsequently developed in 92% of patients with subungual metastases. Patients with subungual metastases have a poor prognosis; their survival following the diagnosis of the subungual tumor is usually only a few months.
CONCLUSION: The clinical differential diagnosis of a new periungual or subungual lesion (with or without an associated nail plate dystrophy) should include tumor metastasis to the nail unit not only in oncology patients, but also in previously cancer-free individuals.