Published online Jun 6, 2022. doi: 10.12998/wjcc.v10.i16.5343
Peer-review started: July 18, 2021
First decision: October 16, 2021
Revised: November 4, 2021
Accepted: April 26, 2022
Article in press: April 26, 2022
Published online: June 6, 2022
Processing time: 318 Days and 19.6 Hours
Mammary-type myofibroblastoma (MTMF) is a rare benign extramammary soft tissue tumor with myofibroblastic differentiation. Although 160 cases of MTMF have been reported in the literature since 2001, no cases of infarction or atypical mitosis have been reported so far. Herein, we report an unusual case of MTMF in the pelvic cavity, which mimicked some malignant features, including infarction, atypical mitosis, infiltrative growth, and prominent cytologic atypia, making it difficult to ascertain whether the tumor was benign.
A 49-year-old man complained of pain and discomfort in the right buttock for more than 4 mo and did not receive any treatment. Nuclear magnetic resonance imaging (MRI) showed a 13-cm-sized mass in his right pelvic cavity. Histologically significant differences were atypical mitosis figures and multiple necrotic foci in the tumor. In addition, smooth muscle and skeletal muscle were invaded within and at the edge of the tumor. These morphologic features are often reminiscent of malignant tumors and therefore pose a diagnostic challenge to pathologists. The tumor cells were strongly positive for both cluster of differentiation 34 and desmin, and the loss of retinoblastoma 1 shown by immunohistochemical and fluorescence in situ hybridization results confirmed the pathological diagnosis of MTMF. Currently, the patient is alive and in good condition without tumor recurrence or metastasis after 2.5 years of follow-up by telephone and MRI.
The two pseudo-malignant characteristics of infarction and atypical mitosis broaden the morphological lineage of MTMF, a rare mesenchymal tumor.
Core Tip: Mammary-type myofibroblastoma (MTMF) is a rare extra-mammary soft tissue tumor with myofibroblast differentiation. Since 2001, more than 160 cases have been reported in the literature. Almost all reports have identified MTMF as a benign tumor except for three cases of recurrence. Here, we report a case of MTMF, which mimicked some malignant features, such as infarction, atypical mitosis, infiltrative growth, and cytologic atypia. Although infiltrative growth and cytological atypia have been reported in previous cases, there are no reports of infarction and atypical mitosis in MTMF, and this is the first report that these pseudo-malignant features appear simultaneously in the same case. Taken together, these characteristics pose a diagnostic pitfall that must be avoided. The purpose of this report is to alert pathologists if they encounter a tumor with these pseudo-malignant features and myofibroblastic differentiation, they must not forget that they need to rule out the diagnosis of MTMF first.