I love questions from our readers. If one person has the question, undoubtedly many others do as well. Recently, someone asked the following question:
Q. Is there a fixed number of mitotic figures that implies a diagnosis of leiomyosarcoma instead of leiomyoma?
A. I’ll start with a little background on the two tumors and then get to the answer to the question.
Everyone who has been in a pathology gross room (where we cut specimens open) for more than 10 minutes has seen a leiomyoma or two. Leiomyomas are benign smooth-muscle tumors that occur in the uterine wall. They are very, very common, and are often found incidentally in a uterus removed for unrelated reasons.
It’s important, obviously, to be able to differentiate between leiomyoma and its malignant counterpart, leiomyosarcoma. Fortunately, leiomyosarcomas are not as common as leiomyomas. In fact, they’re pretty rare. Which is good, because they are fairly nasty malignancies, with a tendency to metastasize early (often to the lung) and recur after surgery. The overall 5 year survival rate is 40%; cases that are poorly differentiated have a dismal 5 year survival rate of 10-15%.
One thing to settle right away is the misguided idea that leiomyosarcomas arise from leiomyomas. They don’t! Or at least, they virtually never do. The incidence of malignant transformation of leiomyomas is vanishingly rare. Leiomyosarcomas arise de novo from myometrial cells or endometrial stromal precursor cells.
Gross appearance
Leiomyosarcomas are usually either bulky, invasive masses within the uterine wall, or polypoid masses projecting into the uterine lumen. Leiomyomas are also either within the uterine wall or bulging into the lumen – but they are not invasive (they look like well-circumscribed, rubbery masses).
Leiomyomas generally have a whorled, firm cut surface, and they are usually not necrotic or hemorrhagic (unless they are large, in which case they may have areas of reddish brown softening. Leiomyosarcomas, on the other hand, are often necrotic and hemorrhagic, and they don’t have a whorled, firm, creamy white cut surface like leiomyomas do.
Microscopic appearance
Of course, to really be sure whether you’re dealing with a leiomyoma or a leiomyosarcoma, you have to look under the microscope. Leiomyomas are composed of bundles of smooth muscle cells that look very much like those of the normal myometrium. The cells have an oval nucleus and are long and slender, and usually there is very little atypia (although there are some benign variants of leiomyoma that have atypia). Atypia or not, they have a very low mitotic index.
Leiomyosarcomas may be well-differentiated or poorly-differentiated. The well-differentiated ones look a lot like leiomyomas, and the poorly differentiated ones have a lot of atypia and pleomorphism. Either way, you should see an increased mitotic rate in order to call it leiomyosarcoma.
If the tumor is pretty well-differentiated, then you need to see 10 mitoses per 10 high power (400x) fields. If atypia is present, or if you see large epithelioid cells, then you only need 5 mitoses per 10 hpf. Necrosis, if present, pushes you towards leiomyosarcoma too.
Sometimes, you just can’t tell which it is: leiomyoma or leiomyosarcoma. In that case, you call it “smooth muscle tumor of uncertain malignant potential.”
Ow, it’s been a long time since my last comment here. Sorry! This post has made my day, THANK YOU!
“Tell me and I forget, teach me and I may remember, involve me and I learn.†Benjamin Franklin
So much to the point ..thanks .
Thanks. It’s very helpful.
This is what my results stated in the report. “Uterine fundal mass, probably a leiomyoma, 2.2 cm”, my concern is the “probably” part.
I wonder if this could be an imaging study report (ultrasound, for example). Imaging studies are great for getting a sense of what is going on – and sometimes the results are clear-cut enough that you don’t need a tissue sample for diagnosis. But in other cases, the best you can say on an imaging study is “most consistent with” – and in those cases, you’d need to get a tissue sample to make a diagnosis.
I hope that this is not a pathology report! Because if it is, it should not have “probably” in the bottom line.
There are specific things that you look for under the microscope to determine whether a mass is a leiomyoma or a leiomyosarcoma. So the bottom line of the pathologic report should say “leiomyoma” or “leiomyosarcoma.”
In rare cases, it can be difficult to tell the two apart microscopically. But in those cases, the bottom line would be more descriptive than just “probably” – and there would most likely be suggestions for how to proceed in order to make a more definitive diagnosis.
Back to your question. Assuming this is an imaging report, I’d discuss this comment with your doctor and see what he or she says. I wish I could give you more information than that, but I won’t pretend to know more than I do!
This is a clinical decision, and I hope that you have a physician that listens to you and that you trust. That is really important! If you don’t feel comfortable, I would get a second opinion. Don’t worry about what anyone thinks – it’s a common thing to do, and you need to feel comfortable that you understand the results of whatever studies are done, and that you understand and agree with the plan for moving forward.