FINAL DIAGNOSIS AND DISCUSSION
Final diagnosis: Decidualized endometriosis
Endometriosis is reported to occur in 6–10% of the female population and is defined as endometrial-like glands and stroma outside of the uterus.1 Severe pelvic pain, dyspareunia, and infertility can develop. There appears to be a hereditary component with first-degree relatives at a higher risk, but the exact genetic mechanism has not been resolved.2 The natural history remains unclear. The most common anatomical locations for endometriosis are pelvic peritoneum and ovaries; however, numerous other sites have been encountered including lymph nodes.3,4 Cutaneous endometriosis is often secondary to iatrogenic seeding during prior caesarian section.4 Estrogen stimulates the growth of endometriosis while progesterone therapy and pregnancy inhibit the progression, which causes decidualization and can stop endometriosis-associated pain. Endometriosis can even occur in women without endometrium.5 Interestingly, growth of these lesions has been seen even in men and postmenopausal women who have no increased circulated estrogen.5
When it presents as a nodule, cutaneous endometriosis is usually firm and may appear tan, brown, erythematous, violaceous, or black. The nodule may vary in size throughout the menstrual cycle, and exhibit tenderness, or bleed with menses. Endometriosis under the influence of progesterone (pregnancy or exogenous) can become decidualized. Decidualization can cause a change in the size and appearance of endometriosis.3
The histologic diagnosis of endometriosis requires the presence of endometrial glands with a surrounding endometrial stroma. There are often erythrocytes extravasated into the stroma and hemosiderin-laden macrophages. The endometrial glands of endometriosis may display finding similar to uterine endometrium including: tubal, oxyphilic, hobnail, mucinous and papillary syncytial metaplasias.6 Under the effect of progesterone the endometrium becomes decidualized. The stromal cells convert epithelioid and the glands become atrophic and flattened. When the glands become inconspicuous and the stromal cells appear atypical, carcinoma or metastatic disease becomes the most important competing differential diagnosis, especially in women with known history of malignancy. Perineural invasion can be seen further obscuring the benign nature of the disease process.4 It is important that these features are not misinterpreted as malignancy, although it must be kept in mind that malignant transformation of endometriomas rarely occurs.7,8
Immunohistochemistry is a useful tool in diagnosing endometriosis. CD10 is a valuable marker for uterine stromal cells. In our case, CD10 could be conceivably useful, but in cases without decidualized stromal cells, it is difficult to interpret as the fibroblastic cells in the skin are positive that could hamper the interpretation of the cells of interest. The stromal cell is not reactive to cytokeratin markets, but the glandular component is. Pax-8 is positive in both components, but it is weaker or can be negative in the stromal cells. Both components are positive for estrogen and progesterone receptors.4
Endometriosis is a puzzling disease. Familiarity with the morphology is key for pathologists to avoid misinterpretation.
List of References
1. Giudice LC, Kao LC. Endometriosis. Lancet (London, England). 2004;364(9447):1789-1799.
2. Bulun SE. Endometriosis. The New England Journal of Medicine. 2009;360(3):268-279.
3. DeClerck BK, Post MD, Wisell JA. Cutaneous decidualized endometriosis in a nonpregnant female: a potential pseudomalignancy. The American Journal of Dermatopathology. 2012;34(5):541-543.
4. Kyamidis K, Lora V, Kanitakis J. Spontaneous cutaneous umbilical endometriosis: report of a new case with immunohistochemical study and literature review. Dermatology online journal. 2011;17(7):5.
5. Koninckx PR, Ussia A, Adamyan L, Wattiez A, Gomel V, Martin DC. Pathogenesis of endometriosis: the genetic/epigenetic theory. Fertility and Sterility. 2019;111(2):327-340.
6. Kazakov DV, Ondic O, Zamecnik M, et al. Morphological variations of scar-related and spontaneous endometriosis of the skin and superficial soft tissue: a study of 71 cases with emphasis on atypical features and types of mullerian differentiations. Journal of the American Academy of Dermatology. 2007;57(1):134-146.
7. Markopoulos C, Gogas H, Eleftheriou G, Floros D. Endometrioid carcinoma arising in a scar of caesarean section. Case report. European Journal of Gynaecological Oncology. 1996;17(6):520-521.
8. Miller DM, Schouls JJ, Ehlen TG. Clear cell carcinoma arising in extragonadal endometriosis in a caesarean section scar during pregnancy. Gynecologic Oncology. 1998;70(1):127-130.