Table of Contents
Definition / general | Bell criteria for problematic smooth muscle uterine tumors | Clinical features | TreatmentCite this page: Pernick N. Smooth muscle tumors of uncertain malignant potential. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/uterusSTUMP.html. Accessed January 19th, 2021.
Definition / general
- WHO: uterine smooth muscle tumor that cannot be histologically diagnosed as unequivocally benign or malignant
- Also known as STUMP
Bell criteria for problematic smooth muscle uterine tumors
- Note: criteria do not apply to extrauterine tumors
- Note: must rigidly apply following criteria for atypia, mitotic figures and coagulative tumor cell necrosis (Mod Pathol 2000;13:328)
Atypia:
- Classify as none / mild or moderate / severe, based on nuclear pleomorphism, nuclear size, nuclear membrane irregularities, chromatin density and nucleoli size/prominence
- No / mild atypia: uniform nuclei that may be enlarged, but with smooth nuclear contours, evenly distributed chromatin; minimal variation in nuclear size and shape, small nucleoli
- Moderate / severe should be detectable at low power
- Moderate atypia: large, plump and irregular nuclei with coarse chromatin; if 1 - 2 enlarged abnormal mitotic figures, call moderate atypia
- Severe atypia: obvious pleomorphism, numerous cells with enlarged bizarre nuclei with dense chromatin; frequent giant cells, often multinucleated, enlarged and sometimes atypical nucleoli
Mitotic figures criteria:
- Hairy extensions of chromatin must be present, extending from a central clot-like dense mass of chromosomes; hairy extensions from an empty center favor a non mitosis
- Count 4 sets of 10 fields in area of highest mitotic activity, and use the highest count
- No nuclear membrane
- Must rule out lymphocytes, mast cells, stripped nuclei, degenerated cells and precipitated hematoxylin
- Count only definite mitotic figures
Necrosis:
- Presence or absence is powerful predictor of outcome for patients with uterine smooth muscle tumors
- Must distinguish coagulative tumor cell necrosis and hyalinizing necrosis
- Coagulative tumor cell necrosis: abrupt transition between necrotic cells and preserved cells; ghost outlines of nuclei of necrotic cells are often seen in necrotic area, but inflammatory cells are uncommon; common in clinically malignant smooth muscle tumors - DON'T IGNORE
- Hyalinizing necrosis: zone of hyalinized collagen between dead cells and preserved cells, reminiscent of infarcted region organized by granulation tissue; eosinophilic collagen matrix common; if dead nuclei present, nuclei are uniform and chromatin is often faint, compared to nuclear hyperchromasia and pleomorphism in tumor cell necrosis; common in leiomyomas
- Necrosis secondary to ulceration in submucous leiomyomas features acute inflammatory cells and a peripheral reparative process, whereas ghost outlines of nuclei are usually inconspicuous or absent
Leiomyomas: no coagulative tumor cell necrosis, no significant atypia, but any degree of mitotic activity; can call "with significant mitotic activity" if 5+ mitotic figures/10 HPF, but have benign behavior
Atypical leiomyoma: moderate / severe atypia, < 10 mitotic figures/10 HPF, no coagulative tumor cell necrosis
Leiomyosarcoma: usually hemorrhagic and soft, marked pleomorphism, 15 - 30 mitotic figures/10 HPF with abundant abnormal mitotic figures; coagulative tumor cell necrosis
STUMP: minimally atypical smooth muscle neoplasms with a low mitotic index but with uncertainty about the histologic type (standard vs. myxoid or standard vs. epithelioid); combination of standard smooth muscle differentiation, marked diffuse severe atypia, low mitotic index and uncertainty about whether coagulative tumor cell necrosis is present; moderate to severe atypia plus uncertain mitotic index because possible mitotic figures may be degenerating nuclei mimicking mitotic figures
Algorithm:
- No / mild atypia, no tumor cell necrosis → leiomyoma
- Moderate / severe atypia, no tumor cell necrosis → atypical leiomyoma if < 10 mitotic figures/HPF or leiomyosarcoma if 10+ MF/10 HPF
- Moderate / severe atypia and tumor cell necrosis → leiomyosarcoma (mitotic figures don’t matter)
Clinical features
- Suggested to consider as tumor of low malignant potential because may recur (Am J Surg Pathol 2009;33:992), rarely as leiomyosarcoma (Gynecol Oncol 2009;113:324)
Treatment
- Conservative because low likelihood of leiomyosarcomatous transformation (Ann Acad Med Singapore 2010;39:625)