A 77-year-old woman presented with a subcutaneous abdominal mass. She had a history of a resected parotid gland tumor an unknown number of years earlier. The abdominal mass was excised and histologic sections from it are shown below.
Microscopically, the tumor has a remarkably heterogeneous appearance and consists of nests, cords, sheets, and tubules of epithelial cells embedded in a stroma that varies from fibrous to myxo-chondroid. Well-formed hyaline cartilage is not seen, although several areas approach this appearance. The lesion is at least partially surrounded by a fibrotic capsule. The epithelial cells lack any significant nuclear pleomorphism and mitotic figures are extremely rare. The neoplastic cells are positive for smooth muscle actin and S100 protein (not shown).
What is your diagnosis?
This lesion has all of the microscopic features of a salivary gland-type mixed tumor. Histologically identical tumors may occur in the skin where they have been termed "chondroid syringomas" as well as "cutaneous mixed tumors." The appearance of this case and the history of a prior parotid gland tumor was intriguing. The parotid tumor was ultimately obtained and proved to be a histologically identical mixed tumor. This leads to the obvious question of whether the subcutaneous tumor is an independent cutaneous mixed tumor or whether the lesion represents a so-called benign metastasizing mixed tumor. The latter is an extremely rare by well-recognized phenomenon. We have seen only a small handful of such cases over the last three decades and often the secondary lesion involves the skin, raising the issue of whether these are indeed metastases or independent tumors.
Several studies have addressed the issue of benign metastasizing mixed tumor and have invariably concluded that there are no clinical or histologic features that allow one to predict which of these tumors might rarely pursue this clinical course. As the name implies, the presence of isolated metastases in this setting does not predict a malignant clinical course and the patients do extremely well. A number of other benign tumors have been noted to be associated with "bengn metastases" including, for example, uterine leiomyomas and giant cell tumors of bone.
Molecular techniques could potentially be brought to bear to compare these two lesions and possibly answer this question. My own prejudice in this instance is to fall back on "Occam's razor" and assume that the simplest answer, ie. one tumor with a "benign" metastasis is the correct one, but I have no proof as yet to support that interpretation. Regardless, this patient should have an excellent prognosis.