Chinook's Histology Report

I received Chinook's histology report today.  Histology is the microscopic anatomy of cells.  Here is what the veterinary pathologist discovered, and its implications.

This soft tissue mass is a spindle cell predominant soft tissue
sarcoma. It is demonstrating hemangiopericytoma pattern, and has
overall low grade histologic features, at least based on the examined
sections. The soft tissue sarcoma (STS) group includes tumor such as
malignant nerve sheath tumor, hemangiopericytoma, leiomyosarcoma, and
fibrosarcoma. These neoplasms are difficult to distinguish with light
microscopy alone, though the clinical behavior is similar; they are
locally invasive, frequently recur, but seldom metastasize. The
overall metastatic rate is reportedly 20%, but in a recent study, only
high grade tumors demonstrated metastases. For this reason, metastatic
disease is not anticipated for this particular sarcoma.

Though these neoplasms will grossly appear to be well demarcated at
the time of surgical excision, nearly all will
 demonstrate small
fascicles of neoplastic cells that invade into the surrounding
tissues, following the path of least resistance, along fascial planes.
This is the reason complete surgical excision is so difficult, and
eventual recurrence is so common. It has been estimated that up to 30%
of these sarcomas will recur within the first year, and up to 60% will
demonstrate eventual recurrence if followed long term.

Wide surgical excision is important to provide long term local
control, and margins measuring several centimeters in all directions,
including deep fascial plane are currently recommended, if feasible.
But, it is also been suggested that narrow margins (several
millimeters) may be adequate for some low grade soft tissue sarcomas,
combined with careful monitoring. Postoperative irradiation for
incompletely resected or recurrent tumors at sites prohibiting
additional surgery can also be
 helpful.

For soft tissue sarcomas, the following factors usually indicate
favorable long term outcome: small size (<5cm) limited to the
subcutis, location on extremity, low to intermediate grades, and clean
surgical margins. See Small Animal Clinical Oncology, third ed, pages
288-94.

Our vet informed me that the tumor will almost certainly grow back in the same location, though it isn't likely to spread.  When it recurs it will probably be more difficult to excise than this time.  These things aren't susceptible to chemotherapy at all, and though radiation treatment my delay the recurrence it is unlikely to "cure" the issue.

So, the good news is that it doesn't represent a death sentence, but the bad news is we can expect it to return bigger and badder sometime in the next couple of years.  It won't do any good to fret or worry over it.  For now I'm just going to enjoy my big buddy from day to day, and we'll deal with the future at the appropriate time.