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Image of the Week
Edited and moderated by Stacey Mills, MD, Pathology Network's Image of the Week Blog is a forum for the discussion of interesting and often diagnostically challenging pathology images.
Thursday, May 01, 2014
A 42-year-old Man with a Parotid Gland Mass
A 42-year-old man presented with a several year history of an enlarging mass at the angle of the left side of his jaw.  More recently, this had been associated with pain and facial nerve weakness.  Physical examination demonstrated a mass in the superfical left parotid gland with some adjacent clinically suspicious lymph nodes.  A superficial parotidectomy was performed and a partial lymph node dissection was done.
Part of the resected tumor looked like this:

A second component looked like this:

The two components were immediately juxtaposed but not intermingled.

The clinically suspicious lymph nodes contained metastastic tumor identical to that seen in the second image above. 

What is your diagnosis?

This is an example of an adenoid cystic carcinoma that has undergone "dedifferentiation" or perhaps more mechanistically correctly, "high-grade transformation."   Part of the tumor has the classic cribriform and tubular appearance of an adenoid cystic carcinoma as seen in the first and and also the third images.  The high-grade component consists of patternless sheets of pleomorphic basaloid cells with a high mitotic rate and frequent apoptotic bodies.  Areas of necrosis (not illustrated) were also present. 
Virtually all salivary adenocarcinomas, as well as some benign salivary gland tumors, have been documented to rarely undergo high-grade transformation.  The more commmon scenario is for this to happen metachronously.  The tumor recurs several times with a conventional appearance before the high-grade component develops.  The high-grade component in this case has a distinctly basaloid appearance but this component also may assume the appearance of a more conventional appearing high-grade adenocarcinoma without basaloid cells.
In the current case it is critical to not label the high-grade component as the solid pattern of conventional adenoid cystic carcinoma.  Although the solid pattern of adenoid cystic carcinoma may show slightly more pleomorphism and mitotic activity than the cribriform and tubular patterns, prominent pleomorphism, mitotic activity and necrosis/apoptosis should lead to the diagnosis of high-grade transformation.  Although conventional adenoid cystic carcinomas almost never embolically metastasize to regional lymph nodes, the high-grade transformation component is perfectly capable of doing so as it did in this case.  Several studies have shown that the solid pattern of adenoid cystic carcinoma is associated with a worse prognosis.  It is tempting to speculate that this might be due, at least in part, to lumping of some high-grade transformation cases into these series.
Great case. As an ophthalmic pathologist, we see ACC in the lacrimal glands not infrequently and I recently had a case of basaloid appearing tumour with extensive necrosis including comedo pattern arising in the gland with extensive invasion into the temporalis fossa and cranial cavity without perineural invasion or lymphatic metastases in a neck dissection. There was no conventional ACC in the exenteration specimen after exhaustive search. Myoepithelial markers were negative; LMW keratins and CD117 showed focal positivity and the Ki-67 was positive in over 80% of cells. My ENT colleagues queried NUT midline carcinoma (testing was negative), and after presenting it at a meeting, an expert suggest a basaloid variant (not sure if that equals solid). But do you think Ki-67 is a useful marker for the HGT type? [Dr. Mills' reply: Ki-67 will definitely be increased in the high-grade component. Usually the necrosis and increased pleomorphism will make the high-grade component obvious. Ki-67 has also been reported to be somewhat increased in the "solid variant" of adenoid cystic. I'm always cautious making that diagnosis. Some tumors labelled as such are more likely high-grade transformations.]
About the Author

Stacey E. Mills, MD
Stacey E. Mills, MD, a graduate of University of Virginia (UVA) and the UVA Medical Center, has authored nearly 230 articles, 20+ books, atlases and monographs—including the renowned Sternberg's Diagnostic Surgical Pathology. He has been a practicing Professor and Staff Pathologist at UVA for 30+ years and is Director of Surgical Pathology and Cytopathology. His clinical specialty is general surgical pathology with emphasis on neoplasms and neoplasm-like lesions. Dr. Mills is also Editor-in-Chief of The American Journal of Surgical Pathology.