pit displays the characteristic histological features of a thymoma , with the additional characteristic of being surrounded by lung or visceral pleura without evidence of a thymic lesion in the anterosuperior mediastinum .
due to its rarity , variety , unclear natural course , and non - specific features on diagnostic imaging , a formal diagnosis can be made only based on a pathological examination .
therefore , surgical resection should be considered for an exact diagnosis , unlike is the case for mediastinal thymomas .
herein , we report the case of a patient who was initially suspected to have lung cancer on the basis of a bronchoscopic biopsy but was later diagnosed with pit after surgery followed by a pathological examination .
written informed consent was obtained from the patient for the publication of this case report and all accompanying images . a 59-year - old asymptomatic female with a history of partial thyroidectomy for papillary thyroid cancer visited for the evaluation of pulmonary nodules observed in a low - dose chest computed tomography ( ct ) scan .
there was a 1.1-cm nodule in the right upper lobe ( rul ) and a 5-mm small ground glass nodule in the left upper lobe .
the 1.1-cm rul nodule was suspected to be primary lung cancer , and a contrast ct scan was performed . in the contrast ct scan ,
the rul nodule was observed as a 1.5-cm finger - shaped lesion with a smooth margin in the subsegmental bronchus of the posterior segment .
in positron emission tomography ( pet ) with 18-f fluorodeoxyglucose ( fdg ) , the maximum standardized uptake value ( suvmax ) of the rul endobronchial nodule was 4.1 ( fig .
lung cancer was strongly suspected from the findings of the radiological studies , and fiber - optic bronchoscopy was performed to confirm the tissue diagnosis .
the bronchoscopy revealed a smooth surface mass obstructing a subsegmental bronchus of the posterior segment ( fig .
however , spindle cell proliferation with moderate cellularity , mild nuclear pleomorphism , and mitosis over 10 per high - power field were identified in the biopsy specimen .
the clinical diagnosis was ct1an0m0 primary lung cancer of the salivary gland type or a spindle cell tumor showing epithelial differentiation ; subsequently , thoracoscopic rul lobectomy and mediastinal lymph node dissection were performed . in the macroscopic examination , the white - to - gray tumor measuring 1.4 cm1.0 cm0.8 cm showed no evidence of hemorrhage or necrosis .
a microscopic evaluation showed a relatively fascicular spindle cell pattern with round - to - oval nuclei ( fig .
immunohistochemical studies revealed diffuse immunoreactivity for pan cytokeratin , cytokeratin 5/6 , p40 , and p63 ( fig .
2d ) , but negative for vimentin , s-100 protein , thyroid transcription factor-1 , napsin a , and smooth muscle actin . according to the world health organization ( who ) classification , these histopathological findings are consistent with type a thymoma .
pit is a rare disease , and thus far , only 37 cases of this condition have been reported . according to the epidemiological data reported by myers et al .   in 2007 ,
autoimmune paraneoplastic syndromes , such as myasthenia gravis or good syndrome , can be accompanied and are associated with a poor prognosis .
masses were located in the right lung in approximately 65% and in the left lung in approximately 35% of the patients .
the size of the masses ranged from 9 to 129 mm in diameter . on the basis of the who classification
, there were 7 cases of type a , 8 of type ab , 6 of type b1 , 5 of type b2 , and 7 of type b3 thymoma .
it has been suggested that pit involves thymomas with an ectopic location , tumors arising from an uncommitted germinative cell , and migration of a mediastinal thymoma into the lung . since the theory of ectopic location
is considered to have the most support , pit has been classified as a tumor of an ectopic origin in the recently revised who histological classification of lung tumors .
the radiological findings of pit are non - specific . in ct , most pits present as well - circumscribed hetero geneous masses that are not diagnostic . in the present case ,
the nodule was a cylindrical lesion with a smooth margin and showed a homogeneous enhancement pattern on the ct .
the homogeneous enhancement pattern in this case may have been related to the small size of the lesion .
a thymoma can show an increased fdg uptake in pet , as in the case of lung cancer , and the amount of the fdg uptake has been reported to have a correlation with the who subtype . however , pet is non - specific with respect to the differential diagnosis of pit because it is not possible to make a histological diagnosis by using suvmax .
the fdg uptake in the present case was not high , and the nodule was presumed to be a low - grade malignancy .
however , it was not possible to suspect pit until a pathological diagnosis was performed using a surgical specimen .
most pits are diagnosed postoperatively on the basis of the histopathological features of the surgically obtained specimens .
a cytologic diagnosis based on a biopsy with a small amount of the thymoma tissue is extremely challenging .
this is in part attributed to the fact that epithelial cells are difficult to recognize in lymphoid - rich aspirate smears and there is an inherent sampling error in a tumor that frequently displays heterogeneous histopathology .
therefore , most cases , including ours , are typically clinically diagnosed as primary lung cancer and staged accordingly before surgery . in the fourth edition of the who classification ,
the diagnostic criteria of thymomas have been revised to include histological features using hematoxylin and eosin - stained specimens , which are justified as obligatory criteria .
optional criteria and immunohistochemical features for the diagnosis are also introduced in order to reduce previous ambiguities , improve the unsatisfactory reproducibility of the who classification , and help to classify thymomas with ambiguous histology .
immunohistochemical markers such as cytokeratin 19/20 , p20 , p63 , p40 , and tdt are routinely used for this purpose . after a pathological examination of the surgical specimen
, we confirmed it to be type a thymoma because of its well - circumscribed and spindle - shaped epithelial cell - rich appearance with relatively low mitotic activity and minimal atypia in comparison with its histological findings .
further , in the immunohistochemical study of our case , the surgical specimen was positive for ae1/3 , indicating the epithelial keratin origin of the tumor .
we assumed it to be the epithelium of the salivary gland or a lung - origin malignancy because of the positive result of the cytokeratin ( ae1/3 ) staining of the biopsy specimen , which is also strongly associated with spindle - shaped epithelial - cell neoplasms , such as type a thymoma .
moreover , the positive result for p40 staining , which is a p53 marker that is positive in 97% of thymomas , supported this result .
additionally , p63 , another maker of the spindle - shaped epithelial cells of thymoma , was positive in our case .
the antigen cd20 reveals the existence of focal micronodular areas with a lymphoid stroma and is helpful in the diagnosis of type a thymoma .
however , 50% of type a thymomas show a negative result for cd20 , and our result was also negative .
the fourth edition of the who classification states that tumors should be diagnosed on the basis of hematoxylin and eosin staining ; thus , we concluded that our case was type a thymoma , although some immunohistochemical studies did not follow the usual pattern of type a thymoma .
pit should be staged according to the lung cancer staging system because pit is classified as a malignant tumor of the lung according to the who histological classification .
the masaoka staging system , which is based on the extent of invasion into mediastinal structures , is not appropriate for the staging of pit , because pit exists inside of a lung , and not in the mediastinum , and has an identical anatomical condition to that of lung cancer .
further , nodal involvement was described in several cases , which shared an anatomy similar to that of primary lung cancer .
surgical resection , lobectomy , and systematic lymph node dissection should be the treatment of choice in the case of pit .
a significantly better survival rate was reported in patients who underwent surgical treatment than in those who were conservatively treated in a systematic review by veynovich et al . .
they suggested that adjuvant radiation therapy be considered in cases of incomplete resection or expansion of the tumor tissue beyond a circumscribed capsule .
in particular , the need for lymph node dissection has not been adequately discussed because of the rarity of pit . as previously mentioned , terashima et al .  
reported a case of pit with lymphatic spread ; the sites of nodal metastases were similar to those of primary lung cancer .
they argued that systematic mediastinal lymph node dissection according to the lymph node map for primary lung cancer be considered for better staging and most likely , better prognosis . in our case
, video - assisted thoracic surgery lobectomy and mediastinal lymph node dissection were performed , and no adjuvant therapy was conducted subsequently , because the mass was confirmed to be type a thymoma without invasion . in conclusion , pit is a very rare lesion with uncertain pathogenesis .
pit should be included in the differential diagnosis when the pathology of a lung tumor shows uncommon features .
since the natural history of the disease has not yet been fully elucidated , regular long - term follow - ups should be performed in accordance with primary lung cancer .
 primary intrapulmonary thymoma ( pit ) is a very rare lesion of uncertain pathogenesis . 
 pit should be considered when the histopathological appearance of a lung tumor shows features that are uncommon but similar to those of a thymoma . in this case report 
 , we discuss the case of a 59-year - old female with a solitary pulmonary nodule that was confirmed to be pit on the basis of pathological tests . 
 treatment with complete resection showed good results . 
Case report
Discussion