in june 2014 , a 61-year - old female was admitted to keimyung university dongsan medical center because of a severe dry cough that had lasted for over 20 days . according to her medical history , she had undergone a hysterectomy in 2005 , and the pathologic diagnosis was leiomyoma of the uterus . in 2006 , chest computed tomography ( ct ) findings demonstrated multiple nodules in both lungs ( fig . microscopic examination revealed a proliferating spindle cell tumor showing relatively bland nuclei , no pleomorphism , and no mitoses . in addition , immunohistochemical staining analysis was positive for smooth muscle actin , desmin , vimentin , estrogen receptor ( er ) , and progesterone receptor ( pr ) , and negative for cd34 , pan - ck , and s100 proteins . the result of the pathologic study was benign leiomyoma , and the nodules were diagnosed as pulmonary benign metastasizing leiomyoma ( pbml ) . , the patient did not return to our outpatient clinic for follow - up . upon the patient s return , a posteroanterior chest x - ray showed a very large mass on the right cardiac border ( fig . chest ct demonstrated a 101-mm mass on the right lower lobe that compressed the posterobasal segmental bronchus ( fig . a ct - guided needle biopsy of the huge mass revealed it to be leiomyoma . after right thoracotomy , a very large mass , 100 mm in size , originating from the visceral pleura of the right lower lobe was observed ( fig . the mass was in contact with the parietal and diaphragmatic pleura , but no invasion was noted . near the mass , multiple fine , scattered , whitish nodules measuring less than 1 cm were found on the diaphragmatic , parietal , and visceral pleura . we performed an excision of the mass in the right lower lobe and excised fine nodules from the parietal and diaphragmatic pleura for biopsy . intrapulmonary nodules found in the middle lobe and upper lobe were not excised , because they had not increased in size . the necrotic lesion showed severe cellular atypia , frequent mitoses ( more than 15 per 10 high power fields [ hpfs ] ) , and continuity of the leiomyoma and leiomyosarcoma ( fig . 2c ) . immunohistochemical staining revealed that the tumor was positive for smooth muscle actin ( sma ) , er , and pr , but negative for cd34 , pan - ck , s100 , and hmb45 ( fig . the nodular lesions taken from the parietal and diaphragmatic pleura showed no malignant cells . the patient was discharged 8 days after the operation without complications . large heterogeneous hypermetabolic masses were found from the left para - aortic area to the left pelvic cavity ( fig . 3 ) , and we diagnosed them as metastatic leiomyosarcoma . additionally , increased fluorodeoxyglucose ( fdg ) uptake was observed in the peribronchial lymph nodes , the mediastinum , and the supraclavicular lymph nodes . two months later , en - bloc resection of the retroperitoneal masses with left nephrectomy was performed , and the pathologic diagnosis was metastatic leiomyosarcoma . we decided to continue observing fdg uptake in the mediastinal and peribronchial lymph nodes rather than prescribing chemotherapy , because metastasis of leiomyosarcoma to the lymph nodes is rare . a chest ct scan done 1 year after surgery demonstrated no changes in the pulmonary nodules and no recurrence of the mass . benign metastasizing leiomyoma ( bml ) results from metastasis of a uterine leiomyoma ; thus , bml patients have a history of uterine leiomyoma . the condition may occur at any age , but occurs most often in women from 30 to 74 years old . bml can occur in a number of different areas of the body , including but not limited to the lungs , para - aortic lymph nodes , abdominal lymph nodes , heart , breast , liver , and esophagus . the most common site of occurrence is in the lungs , in which case it is known as pbml . the pathogenesis of pbml is unknown , but the most common theory is hematogenous spreading of leiomyoma during uterine surgery . immunohistochemical staining of affected tissue is positive for desmin , sma , pr , and er . most cases of pbml do not require treatment , but pbml that metastasizes or increases in size does need to be treated , as it can cause respiratory failure , chest pain , or hemoptysis . surgical excision is the treatment of choice , but this is only possible in cases of resectable pbml . in some cases of pbml that a repositive for er or pr , reducing hormone release via hysterectomy , bilateral adnexectomy , and/or hormone therapy may decrease the tumor s size . the incidence of the sarcomatous transformation of benign uterine leiomyomas is only 0.1%0.8% , and is most often seen in women in their 50s . pbml can also undergo malignant transformation ; excluding our case , just 1 case of the malignant transformation of pbml has been reported . radiologically , it is difficult to differentiate between normal pbml and pbml that has become malignant . on fdg pet - ct , heterogeneous fdg uptake of the tumor may reflect necrosis within the leiomyosarcoma . in our case , pathologically , the current criteria used to differentiate a malignant smooth muscle tumor from a benign mass are the presence of necrosis , a high mitotic index ( > 5 mitoses seen in 50 hpfs ) , nuclear atypia , cellularity , and an ill - defined tumor border . however , in primary pulmonary leiomyosarcoma ( ppl ) , progress is slow , and metastasis of ppl occurs late in the disease process . if the condition is diagnosed early , wide excision with negative margins ( r0 ) is the standard treatment . the 5-year survival rate in ppl patients who undergo complete resection is over 50% , and recurrence is rare . lymph node resections are generally not necessary because ppl rarely shows lymph node involvement . for advanced lesions , depending on the size and grade of the tumor , radiation therapy , chemotherapy , or both may be indicated as an adjuvant therapy . however , the median survival of patients with an unresectable sarcoma or metastatic disease is about 12 months . overall , early diagnosis and complete mass resection are vital in cases of the malignant transformation of pbml . however , preventive resection or diagnostic excisional biopsy is not recommended for all pbml patients , since the occurrence of a malignant change is extremely rare if there is a very large , newly developed mass that is increasing in size rapidly , or if a malignant mass is found on pet - ct , diagnostic mass resection should be considered .
pulmonary benign metastasizing leiomyoma ( pbml ) is defined as metastasis of a leiomyoma to lung tissue . 
 it was first reported in 1937 . 
 p bml is known as a benign disease , but can undergo malignant transformation . only 1 case of the malignant transformation of pbml to leiomyosarcoma has been reported previously . in this report , we present a case of malignant transformation of pbml .