Answer: Malignant phyllodes tumor

Histology: At low magnification this lesion has a phylloidal appearance consisting of hypercellular stroma compressing glands into slit-like spaces. At higher magnification the glandular spaces are lined by benign appearing prostatic epithelium and focally by benign squamous epithelium. The surrounding stroma is moderately to markedly hypercellular with moderate nuclear atypia. Mitoses are infrequent (1-2/10 hpf). Ki-67 proliferation index is low to moderate (less than 5%).

Discussion: The majority of this tumor is classic for malignant phyllodes tumor of the prostate. It consists of an overall architectural pattern similar to phyllodes tumors of the breast. The epithelium is benign with the only unusual aspect of this tumor is that some of the benign prostatic glandular epithelial lining has been replaced by benign squamous metaplasia (image 3). The stroma is far too cellular for a benign phyllodes tumor of the prostate. Benign phyllodes tumors of the prostate have hypocellular fibrotic stroma. Sarcomatoid carcinomas (carcinosarcoma) would have malignant epithelium as well as malignant stroma.

Malignant phyllodes tumor is one of the patterns of stromal sarcomas of the prostate. Stromal sarcomas of the prostate may have other patterns such as storiform, epithelioid, pleomorphic giant cell, or high grade spindle features. In all these other patterns, it is necessary to rule out other types of sarcoma (ie. leiomyosarcoma, rhabdomyosarcoma, malignant SFT, etc) before diagnosing stromal sarcoma of the prostate, whereas the malignant phyllodes pattern is diagnostic of prostatic stromal sarcoma in and of itself. Prostatic stromal sarcomas, especially those that are higher grade, have the potential to metastasize and kill the patient.

Reference(s):
– Herawi M, Epstein, JI. Specialized Stromal Tumors of the Prostate: A Clinicopathologic Study of 50 Cases. Am J Surg Pathol. 2006 Jun;30(6):694-704.