Smooth Muscle Tumor of Uncertain Malignant Potential Arising from Renal Vein

Demographics and presentation

STUMPs are very rare tumors of smooth muscle origin, much rarer than leiomyomas and leiomyosarcomas.^[5] They almost always involve women with the mean age of 43 years^[5] and present as uterine tumors. Still, there have been cases reported in the retroperitoneum and other parts of the genitourinary tract such as the bladder.^[4,6] SMTs may be asymptomatic, and they can be discovered incidentally on imaging. The retroperitoneal SMTs may present with chronic or acute onset abdominal or flank pain, which can be relieved, aggravated, or not impacted by eating.^[7] Our patient had chronic pain that was not affected by meals. There have been multiple reported cases of leiomyomas and leiomyosarcomas arising from the renal vein,^[8-11] although, to the best of our knowledge, there are no reported cases of STUMP originating from the renal vein.

Imaging findings and differential diagnoses

CT and MRI are the primary imaging modalities for the diagnosis of SMTs. Hypo- to isointense signal relative to normal myometrium on T2-weighted images on MRI is an important clue to suggest SMTs. It may be difficult to differentiate uterine STUMP from leiomyoma and leiomyosarcoma.^[12] The STUMP may present as a large mass with heterogeneous signal on T1 and T2-weighted MRI images.^[13,14] Uterine leiomyosarcomas commonly manifest as large infiltrating myometrial mass with irregular and ill-defined margins, heterogeneous hypointensity on T1-weighted images, central hyperintensity on T2-weighted image indicative of extensive necrosis, and early heterogeneous enhancement on post-contrast images.^[13] They may have hemorrhage and foci of calcifications. Malignant lesions can present as hyperintense areas on diffusion-weighted imaging (DWI), and combined DWI and apparent diffusion coefficient values are useful to differentiate between uterine leiomyosarcoma, STUMP, and benign leiomyoma.^[14]

The same rule may apply to SMTs arising in the renal vein, although little is known about renal vein SMTs. MRI of the abdomen in our patient with renal vein STUMP showed a mass of isointense on T1- and T2-weighted images with low-level enhancement and heterogeneous signal on DWI without a definite restricted diffusion. A study analyzing five renal vein leiomyosarcomas cases showed that the tumor size ranged from 4 to 8 cm in the renal vein. Four of the five patients had masses on the left side, unlike our patient, who presented with a right-sided tumor. Contrast enhancement varied across the five subjects, with three subjects having homogenous low-level enhancement, one subject with streaky enhancement throughout the entire mass; and one subject with peripheral enhancement only. Three of the five patients had renal hilum involvement.^[10] The renal vein leiomyomas may present as a well-circumscribed non-enhancing to slightly enhancing lesion centered in the renal vein.^[11] They may rarely contain central cystic changes which may represent necrosis^[9] or degeneration similar to uterine degenerated leiomyomas. There are no clear data that how commonly the renal vein SMTs can be intraluminal and extraluminal. Other differential diagnoses for renal vein/hilar mass include RCC with renal vein invasion, renal lymphoma, adrenal carcinoma, and bland thrombosis. Table 1 summarizes the imaging findings of the differential diagnoses of lesions involving renal vein in general.^[15-19]

Pathologic findings

In the literature, there are no standard pathological characteristics of STUMPs, but they have been described in various ways. STUMPs are tumors that cannot be classified as either leiomyoma or leiomyosarcoma.^[5] Studies have shown that molecular and cytological markers have shown that Ki-67 and p53 can be useful, significantly when differentiating STUMPs from leiomyosarcomas.^[3,20] The positive p53 and Ki-67 markers tend to be linked to leiomyosarcoma.^[20] Uterine SMTs that are positive for estrogen and progesterone receptors may respond to hormone therapy.^[21] Our patient’s pathology result showed moderate atypia, <4 mitosis/10 HPF, and no coagulative necrosis, most compatible with a diagnosis of STUMP.

Treatment and outcome

Since STUMPs are sporadic tumors, there is not a set protocol on how to manage them.^[6] Resection is commonly practiced, especially if they are symptomatic. This is not followed by chemotherapy or radiotherapy. However, the recurrence risk for STUMPs is not low, as it ranges up to 36% and can present as another STUMP or even a leiomyosarcoma.^[5,22] It has been shown that STUMPs with a high level of coagulative necrosis are more likely to recur in STUMPs with >10 mitosis/10 HPF or atypia.^[23] The patients with STUMPs should be monitored with cross-sectional imaging every 6 months for at least 5 years for leiomyosarcoma risk.^[22]