PBML is a rare disease that occurs in reproductive-age women with a history of uterine leiomyoma. The etiology of PBML is controversial. Previous reports have suggested three possibilities: (1) hormone-sensitive in situ proliferation of smooth muscle bundles; (2) benign smooth muscle cells transported from a uterine leiomyoma and colonized in the lung or metastasis of a low-grade leiomyosarcoma; and (3) surgically induced mechanical displacement from a preexisting uterine tumor [1].

Although PBML generally presents as solid and well-circumscribed lung nodules of various diameters, some atypical cases such as cystic tumor or military pattern have also been reported [2]. To the best of our knowledge, there have been only four cases of fluid-containing PBML (summarized in Table 1) [3,4,5,6].

Table 1 Reported cases of pulmonary benign metastasizing leiomyoma with fluid-containing cystic change

Although malignant transformation of PBML has been reported in two cases previously, only one of them presented with fluid-containing tumor, which was concluded in the leiomyosarcoma (Table 1) [2, 6]. Moreover, the diameter of the fluid-containing tumors in the four cases was 101–190 mm, which tended to be larger than typical PBML. Fluid build-up in the tumor was thought to contribute to the increase of tumor size. In the previous four cases, not only was the mechanism ambiguous, the timing of the tumor size increase was also unclear, because these cases were already symptomatic at the time of detection. The present case differed from the previous cases in that we were able to observe the course of the tumor progression. Our experience with the present case suggests that fluid-containing PBML may have a latent risk of rapid tumor size increase and symptomatic lesions. In other words, we should carefully consider the surgical indications of fluid-containing PBML in light of the risk of malignant transformation and rapid enlargement. When operating on PBML, prioritizing the resection of fluid-containing lesions might be a valuable surgical consideration.

In the present case, hormonal therapy was administrated following pathological diagnosis (via left lung biopsy in consideration of multiple metastases) of PBML with positive expression of estrogen receptor and progesterone receptor. Hormone treatments were reported as an effective therapy for PBML with positive hormonal receptor [7, 8]. At the same time, with complete resection of all tumors being another possible treatment strategy, resection of the fluid-containing tumor in the right upper lobe arguably could have been performed instead of the left lung biopsy in the current case. As a result of our treatment strategy, the fluid-containing tumor in the right upper lobe showed rapid growth, while the residual non-fluid-containing tumors in the left lung remained stable for 4 years. These facts might add weight to the argument for surgical resection of fluid-containing PBML.

The rapid enlargement of uterine leiomyoma may result from infarction or cystic change in the leiomyoma. In addition, peri-nodular hydropic degeneration has occasionally been observed as a pathological feature [9]. However, only monolocular cystic change in the tumor was observed in the present case. We speculated two possible reasons for the rapid enlargement: (1) it was caused by fluid secretion into the tumor and (2) it resulted from intracystic hemorrhage. The fact that the fluid in the cyst was serous and contained a high level of CA125, which is generally related to uterine and ovarian disease, suggested the former to be more probable. Moreover, minute bleeding in the solid components was observed only in the RUL lesion, not in the right S9 lesion. Therefore, we speculated that this bleeding ensued secondarily to the rapid enlargement of cystic component of the tumor.

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