Hamartoma is a kind of pathological change between malformation and benign tumor. It is the abnormal differentiation of cells and produces a large number of scattered but mature tissues. Although the mature cells are similar to the surrounding organs and tissues, they are not normal tissues. Generally, intrapulmonary hamartomas are more common than retroperitoneal hamartomas. Only few retroperitoneal hamartomas have been reported in PubMed, Embase, and Scopus databases (Gupta et al. 1980; Hamilton and Mclnerney 1981; Javery and Lee 2010; Lange et al. 1988; Mallory and Spink 1968; Sigdel et al. 2012).
Retroperitoneal hamartoma usually does not cause any symptoms unless it grows to a certain extent and oppresses the surrounding tissue and organs. Hamartoma also contains blood vessels, smooth muscle, fat, and other mature tissue components which is similar to the histological components of AML, causing overlaps in imaging findings and identification difficulty. Although Wang et al. (2002) summarized imaging features of retroperitoneal angiomyolipoma such as linear vessels, aneurysmal dilatation, bridging vessel sign, hematoma, beak sign, and discrete intrarenal fatty tumors, these were not unique to AML. Moreover, in the earlier years, AML had been considered as hamartoma (MMKAB 1999) until perivascular epithelioid cells were found in it, and the differentiation characteristics of AML cells were shown by immunohistochemistry and electron microscopy. Molecular biology showed clonal proliferation, indicating that the lesion was a real tumor rather than a simple hamartoma (Takahashi et al. 2003). It was classified into the PEComa tumor family (Lienert and Nicol 2012).
When the retroperitoneal hamartoma containing abundant blood vessels is enlarged to a certain extent and the boundary with the surrounding tissue is unclear, the CT and MRI imagings just mimic retroperitoneal AML (Lemaitre et al. 1995; Safak et al. 2018) that will increase the difficulty of surgical resection for the high risk of excessive bleeding. As in this case, there is a large amount of blood exudation during the operation. Preoperative interventional embolization was a valuable choice (Javery and Lee 2010; Tseng et al. 2004) in reducing the size of the tumor and the risk of intraoperative bleeding. Only complete resection of the tumor is the radical treatment to avoid possible recurrence in the future.
Preoperative needle biopsy is helpful to judge the nature of retroperitoneal mass, but it should be cautious because there is a risk of bleeding. It is only suitable for low-risk patients, and the patient's physical condition should be comprehensively evaluated before operation. In our case, the blood supply in the mass is abundant, which is not suitable for needle biopsy.
Although the preliminary diagnosis of AML by rapid frozen section is different from that of hamartoma by paraffin section and immunohistochemistry, they are not contradictory. Because the most important difference between them is the finding of perivascular epithelioid cells, while the latter does not contain, the other tissue components could be similar and that is beyond the capability of pathological frozen section. Immunohistochemistry was also inconsistent with AML (Kodzo-Grey 2016). AML has unique immunohistochemistry, which can express human melanin-associated antigen (HMB-45) and smooth muscle actin (SMA) at the same time.
The latest gene detection found that hamartoma is associated with phosphatase and tensin homolog (PTEN) (Javery and Lee 2010), which is called the PTEN hamartoma tumor syndrome and has the potential to develop into various benign and malignant tumors (Tan et al. 2007). PTEN gene on chromosome 10q23.31 encodes a tumor suppressor protein. It regulates the phosphoinositide 3 kinase (PI3K) pathway, which is involved in cell cycle regulation, angiogenesis, cell growth, and proliferation. It was found and isolated in 1997 (Li and Hong 1997; Li et al. 1997). PTEN is now found to be associated with two common diseases: Bannayan–Riley–Ruvalcaba syndrome and Cowden syndrome.