Epithelial malignancies frequently present with significant subcutaneous extent while manifesting the usual surface changes such as induration, crusting, or ulceration. Occasionally, they present with no epidermal changes, solely developing as subcutaneous tumors. These can be carcinomas arising in deep adnexal structures of the skin or within subcutaneous cysts. Spread may be entirely subcutaneous evading detection for some time.
It is critical to differentiate such tumors from either local or distant metastases. Local metastases without epidermal changes are commonly known as in-transit metastases and spread along the dermal lymphatics or perineural lymphatics. The photos below show an example of such a metastasis of a squamous cell carcinoma in an immunosuppressed patient. This cancer recurred after Mohs excision likely due to its non-contiguous spread along local or perineural lymphatics. Radiation therapy together with surgery is indicated for treatment.
Distant metastases to subcutaneous locations are usually associated with multiple other metastases such as to other subcutaneous locations, liver, lung, brain, or bones. Isolated subcutaneous metastatic tumors, although possible, are very rare and would pose a significant diagnostic challenge.
We presented a different case this month at out Skin Cancer Conference that poses the question of whether the subcutaneous tumor is the primary squamous cell carcinoma or a distant metastasis. The nature of subcutaneous primary (non-metastatic) carcinoma itself poses additional risks of regional lymphatic spread. This deserves further investigation of downstream lymphatic chains with CT or MR imaging and/or sentinel lymph node biopsy as was performed in this month’s case. However, with lack of obvious surface spread of the cancer, evaluation for a possible alternative primary carcinoma is required. CT/PET scan imaging is the choice modality for that purpose. The only caveat is that PET imaging will detect the hypermetabolic healing of the surgical site for up to 3 months after surgery making the findings surrounding the surgical bed suspect during that period. Therefore, we perform the CT/PET scan 3 months after surgery.
The primary tumors that would present entirely subcutaneously are carcinomas arising in benign adnexal tumors, such as cyst, or in deep adnexal structures of the scalp, such as pilosebaceous units. Although, ulceration inevitably follows, it is prior to that development that these cancers pose diagnostic questions.