I recently posted a note about what I thought was an obscure pathological process -- heterotopic ossification, also known as myositis ossificans (see: Heterotopic Ossification, a Common Pathophysiologic Process). The lesion turns out to be not so uncommon, at least for radiologists. A reader, who is also a radiologist, Dr. Brian Sabb, submitted the following comment:
I see [hetrotopic ossification; HO] (also known as myositis ossificans)... quite frequently. As you correctly noted, a very common cause of HO is orthopaedic surgery. After total hip arthroplasty (THA) every patient will get serial X-rays. While many of these patients do develop HO, in my experience they usually develop it to a mild degree (usually Brooker class 1 or 2). Additionally, it is usually not symptomatic.
I believe the main reason that you (and most pathologists) do not see many cases of HO is that it is infrequently biopsied. This is a good thing since the histology can be confused with a sarcoma....[Here is a link to a relevant article about this topic] : http://www.ncbi.nlm.nih.gov/pubmed/8147783.
[This report presents the] cautionary tale of [a pathologist] who misdiagnosed a pseudosarcoma [resulting in unnecessary surgery and the death of the patient]. Fortunately, this entity should be easily diagnosed with radiography (and, when needed, CT) by an experienced radiologist.
There are two important teaching points contained in Brian's short comment: (1) new bone formation can frequently look very wild under the microscope and be misinterpreted as a malignant lesion; (2) HO is relatively common after total hip arthroplasty but rarely biopsied because of the interposition of an additional diagnostician, the radiologist, between the surgeon/patient and the pathologist.
If you link to the original short article cited above, you will also learn the following: a pathology trainee initially and correctly diagnosed the biopsied lesion as a pseudosarcoma, was overruled by a more senior pathologist, and subsequently signed out the case as an osteosarcoma. The surgeon then performed radical cancer surgery on the patient who died of postoperative hemorrhage.
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