Hidradenoma; ultrasonographic findings of a rare skin tumour

Clinical History

The patient presented to the outpatient department with a solitary, asymptomatic 2 × 2.5 cm nodule on the medial humeral region. The lesion had appeared some years ago but gradually increased in size during the last months, causing a palpable skin lump with red coloration.

Imaging Findings

Ultrasound examination of the lesion was performed with a linear high frequency (13MHz) transducer and using enough quantity of ultrasonic gel in order to avoid contact of the transducer with the skin. As a result, the skin layers were adequately imaged. The palpable lesion was located in the dermis and extending to the subcutaneous tissue, appearing on greyscale technique as a mass with mixed echogenicity including both solid and cystic parts. The mass measured 2.2x1.5 cm and had no microcalcifications. (Fig. 1) On colour, power Doppler and eFlow technique, the mass showed marked vascularity in both the periphery of the mass and within its solid parts. (Fig. 2) The lesion was surgically removed and histology established the diagnosis of a hidradenoma. (Fig. 3)

Discussion

Clear cell or nodular hidradenomas represent rare benign tumours of the skin originating from the eccrine sweat glands. They can be found in any age but show a slight predilection for the fourth decade and female patients. Hidradenomas are usually located in the head, face, extremities and trunk. [1, 2, 3] There is only a limited number of published cases located in the breast. This type of tumour is typically located in the skin layer of dermis or the subcutaneous adipose tissue and is covered by a normal epidermis. Skin symptoms like thickening, ulceration or serous discharge may occur. [1]
Due to its overall rarity and the fact that most lesions are surgically removed and diagnosed based on histology, imaging findings are rarely reported in the literature. [3] Ultrasound is the primary imaging modality for evaluation of any palpable soft tissue lump, as it is the case in hidradenomas. Mammography has been used to evaluate breast lesions. Hidradenomas appear on ultrasound as single, well-demarcated, rounded nodules measuring 0.5 cm to 3 cm. In terms of echogenicity, hidradenomas usually present as cystic lesions with mural solid nodules or alternatively as solid tumours with areas of hypoechogenicity. Colour Doppler technique is useful and demonstrates hypervascularity within solid lesions or the solid mural nodules of a complex mass. Hidradenomas echogenicity may be further complicated by calcifications or hyperechoic areas representing haemorrhagic changes. [1, 2, 4] All these ultrasonographic findings have been found to correlate with histological findings. [3]
On MRI, hidradenomas are identified as subcutaneous complex cystic and solid masses showing low to intermediate signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images. As it happens with ultrasound, haemorrhagic changes of the cystic part of the lesions may complicate their appearance by altering signal intensity. Following intravenous administration of contrast material, the solid parts of the tumour show enhancement. [1, 2] Cystic parts of hidradenomas may sometimes demonstrate fluid levels. [2]
Hidradenomas only rarely show malignant transformation. [1, 5] The differential diagnosis between hidradenomas and malignant hidradenocarcinoma relies on the aggressive local invasion and the distant metastases of the latter. Treatment of hidradenomas include complete surgical excision with clear margins to prevent recurrence due to tumour remnants by an inadequate excision. [1]
Although rarely investigated with imaging, hidradenomas should be included in the differential diagnosis of a subcutaneous complex and hypervascular lesion with solid and cystic parts.

Differential Diagnosis List

Final Diagnosis

Hidradenoma

URL:	https://www.eurorad.org/case/13370
DOI:	10.1594/EURORAD/CASE.13370
ISSN:	1563-4086