although the pathologic diagnosis of lymphoma traditionally depend on the major criteria of cellular cytological atypia and monomorphism , marginal zone b - cell lymphomas ( mzbl ) of mucosa associated lymphocytic tissue ( malt ) type occasionally lack significant cytological atypia . moreover , polymorphous lymphoid infiltrate of plasma cells , macrophages and centrocyte like - cells ( ccl - cells ) is frequently found in malt type lymphomas . in common with other sites where malt type lymphomas arise , the thyroid gland is devoid of native lymphoid tissue . lymphoid tissue acquired in the course of hashimoto s thyroiditis ( ht ) bears a close resemblance to malt . histologically , ht is characterized by the presence of b - cell follicles , infiltration of thyroid epithelium by b - cells , and plasma cell differentiation . moreover , in comparison with other malt type lymphomas , plasma cell differentiation is more prominent in thyroid malt type lymphoma . as a result we present here two cases of incidental malt type lymphoma showing prominent plasma cell differentiation associated with ht . an 82-year - old japanese male was admitted to our hospital with a six - month history of painless left neck mass . abnormal laboratory test results included an elevated thyroid - stimulating hormone ( tsh ) level of 73 iu / ml ( normal < 0.4 ) and a free thyroxine ( t4 ) concentration of 0.3  ng / dl ( normal 0.71.5 ) . a white well demarcated nodule measuring 1  cm was present in the lower pole of the left lobe ( fig . a white well demarcated nodule measuring 1  cm was present in the lower left lobe ( arrow ) case 1 . ( b ) on low - power field , the dominant nodule was well - circumscribed but unencapsulated ( arrow ) against background of sclerosis . ( c ) on high - power field , the nodule demonstrated numerous mature plasma cells , a few ccl - cells ( arrow ) and large transformed lymphocytes . ( d ) numerous mature plasma cells and a few ccl - cells ( arrow ) infiltrated the thyroid gland acini forming lymphoepithelial lesions . immunohistochemical study demonstrated that numerous neoplastic plasma cells and a portion of ccl - cells ( arrow ) were kappa light chain positive ( e ) but lambda light chain negative . note a few polyclonal plasma cells in the back ground ( f ) 40 case 2 ( a ) cut surface of the resected specimen . a white well demarcated nodule measuring 1  cm was present in the lower left lobe ( arrow ) case 1 . ( b ) on low - power field , the dominant nodule was well - circumscribed but unencapsulated ( arrow ) against background of sclerosis . ( c ) on high - power field , the nodule demonstrated numerous mature plasma cells , a few ccl - cells ( arrow ) and large transformed lymphocytes . ( d ) numerous mature plasma cells and a few ccl - cells ( arrow ) infiltrated the thyroid gland acini forming lymphoepithelial lesions . immunohistochemical study demonstrated that numerous neoplastic plasma cells and a portion of ccl - cells ( arrow ) were kappa light chain positive ( e ) but lambda light chain negative . note a few polyclonal plasma cells in the back ground ( f ) 40 case 2 a 78-year - old japanese female was admitted to our hospital with a painless right neck mass . a 1.5  cm white well demarcated nodule was present in the lower pole of the lobe an 82-year - old japanese male was admitted to our hospital with a six - month history of painless left neck mass . abnormal laboratory test results included an elevated thyroid - stimulating hormone ( tsh ) level of 73 iu / ml ( normal < 0.4 ) and a free thyroxine ( t4 ) concentration of 0.3  ng / dl ( normal 0.71.5 ) . a white well demarcated nodule measuring 1  cm was present in the lower pole of the left lobe ( fig . a white well demarcated nodule measuring 1  cm was present in the lower left lobe ( arrow ) case 1 . ( b ) on low - power field , the dominant nodule was well - circumscribed but unencapsulated ( arrow ) against background of sclerosis . ( c ) on high - power field , the nodule demonstrated numerous mature plasma cells , a few ccl - cells ( arrow ) and large transformed lymphocytes . ( d ) numerous mature plasma cells and a few ccl - cells ( arrow ) infiltrated the thyroid gland acini forming lymphoepithelial lesions . immunohistochemical study demonstrated that numerous neoplastic plasma cells and a portion of ccl - cells ( arrow ) were kappa light chain positive ( e ) but lambda light chain negative . note a few polyclonal plasma cells in the back ground ( f ) 40 case 2 ( a ) cut surface of the resected specimen . a white well demarcated nodule measuring 1  cm was present in the lower left lobe ( arrow ) case 1 . ( b ) on low - power field , the dominant nodule was well - circumscribed but unencapsulated ( arrow ) against background of sclerosis . ( c ) on high - power field , the nodule demonstrated numerous mature plasma cells , a few ccl - cells ( arrow ) and large transformed lymphocytes . ( d ) numerous mature plasma cells and a few ccl - cells ( arrow ) infiltrated the thyroid gland acini forming lymphoepithelial lesions . immunohistochemical study demonstrated that numerous neoplastic plasma cells and a portion of ccl - cells ( arrow ) were kappa light chain positive ( e ) but lambda light chain negative . note a few polyclonal plasma cells in the back ground ( f ) 40 case 2 a 78-year - old japanese female was admitted to our hospital with a painless right neck mass . physical examination demonstrated a nodule on the right side of the thyroid gland . a 1.5  cm white well demarcated nodule was present in the lower pole of the lobe . the tissue specimens were fixed in formalin solution , routinely processed and embedded in paraffin . for light microscopic examination , immunohistochemical studies were performed using the ventana automated ( benchmarktm ) stainer according to the manufacturer s instructions . the panel of antibodies included human immunoglobulin light chains ( kappa and lambda ) ( novocastra , newcastle , uk ) , ps-1 ( cd3 ; immunotech , marseille , france ) , 4c7 ( cd5 ; novocastra ) , 56c6 ( cd10 ; novocastra ) , l26 ( cd20 ; dako a / s , glostrup , denmark ) , 1b12 ( cd 23 ; novocastra ) , dft-1 ( cd43 ; dako ) , 1b16 ( cd56 ; novocastra ) , 5a4 ( p80 ; novocastra ) , sp4 ( cyclin d1 ; nichirei co. , tokyo , japan ) , 124 ( bcl-2 ; dako ) , ae1/3 ( cytokeratin ; dako ) and 137b1 ( human herpes virus type-8 [ hhv-8 ] , novocastra ) . sections with known reactivity for the antibodies assayed served as positive controls and the sections treated with normal rabbit- and mouse serum served as negative controls . in  situ hybridization ( ish ) with epstein barr virus ( ebv)-encoded small rna ( eber ) oligonucleotides was performed to test for the presence of ebv small rna in formalin - fixed paraffin - embedded sections using a ventana automated ( benchmarktm ) stainer . histological examination demonstrated that both lesions exhibited ht including lymphoplasmacytic infiltration with the formation of germinal centers , destruction of the normal thyroid follicular architecture , hrthle cell changes , and squamous metaplasia . the dominant nodule in both cases contained large , well - circumscribed but unencapsulated aggregation of mature plasma cells against a background of sclerosis ( fig . 1b ) . in case  1 , a portion of the mature plasma cells demonstrated intranuclear inclusions ( dutcher body ) ( fig . scattered small - to - medium lymphocytes and large lymphoid cells were also intermingled with plasma cells . in both lesions , medium - sized lymphocytes had round or slightly indented nuclei with small conspicuous nucleoli and a moderate amount of clear cytoplasm ( centrocyte - like cells ; ccl - cells ) ( fig . numerous mature plasma cells and a few ccl - cells infiltrated the thyroid gland acini forming lymphoepithelial lesions ( lels ) ( fig . the majority of mature plasma cells and a portion of ccl - cells in the main nodules showed intracytoplasmic kappa light chain ( fig . however , there were no cd5 , cd10 , cd23 , cd43 , cyclin d1 , hhv-8 positive b - cells in either lesion . other small b - cell lymphomas , plasmacytoma and plasmablastic lymphoma were excluded using stains for cd5 , cd10 , cd23 , cd43 , cd56 . cd23 immunostain demonstrated a few residual follicular dendritic cell network colonized by the neoplastic plasma cells and ccl - cells . barr virus ( ebv)-encoded small rna ( eber ) positive cells in any of the lesions on in situ hybridization using a ventana automated ( benchmarktm ) stainer . the plasma cells outside the main nodules showed polytypic nature . .   reported five cases of extramedullary plasmacytoma that showed characteristic histological findings of mzbl , including the presence of a marginal zone distribution pattern , ccl - cells , reactive lymphoid follicles with or without follicular colonization and lel at sites where epithelium was present . based on these observations , they suggested that a portion of extramedullary plasmacytoma can be regarded as an extreme plasmacytic differentiation of mzbcl . in both lesions reported here , numerous mature plasma cells and a few ccl - cells contained monotypic intracytoplasmic kappa light chain . moreover , immunostain highlighted a few lels and remnants of follicular dendritic cell network colonized by the neoplastic plasma cells and ccl - cells . .   suggested , both of our cases would best be classified as mzbcl of malt type showing prominent plasma cell differentiation . the present two cases should be differentiated from various benign and plasma cell lesions that affect thyroid gland . among these , inflammatory pseudotumor of plasma cell variant ( plasma cell granuloma ; pcg ) appears to be the most important lesion in the differential diagnosis . pcg is a non - neoplastic proliferation of plasma cells within a fibrous stroma with unknown etiology . however , pcg of the thyroid gland is occasionally associated with ht [ 79 ] . histologically , there is a diffuse and focally nodular infiltration of numerous mature plasma cells intermixed with lymphocytes and surrounded by fibrous tissue [ 79 ] . moreover , squamous metaplasia with lymphoplasmacytic infiltration somewhat resembling lel in malt type lymphoma was noted . overall histomorphological findings of the present two cases were similar to those of pcg associated with ht . however , immunohistochemical study demonstrated the polytypic nature of plasma cells in pcg , whereas plasma cells of both lesions reported here , demonstrated monotypic intracytoplasmic kappa light chain [ 79 ] . however , there were no cd20 positive tumor cells in the extramedullary plasmacytoma , whereas both lesions in this series contained a few cd20 positive ccl - cells . previously , hyjek and isaacson   reported that analysis of thyroid glands from 236 cases of surgically treated ht demonstrated five cases ( 2.1% ) of incidental malt type lymphoma . we also found two cases ( 3.8% ) of incidental malt type lymphoma showing prominent plasma cell differentiation among 53 cases of surgically treated ht treated by one of the authors ( m. k ) between 1982 and 2007 . as hyjek and isaacson   indicated , given the indolent nature of thyroid malt type lymphoma , as defined by light chain restriction of the b - cells and plasmacytic infiltrate arising in ht , it is conceivable that the true incidence of thyroid malt type lymphoma may be much higher than recognized .
we present here two cases of incidental extranodal marginal zone b - cell lymphoma of mucosa - associated lymphoid tissue ( malt lymphoma ) showing prominent plasma cell differentiation associated with hashimoto s thyroiditis ( ht ) . 
 histological examination demonstrated that both lesions exhibited ht including lymphoplasmacytic infiltration with the formation of germinal centers , destruction of the normal thyroid follicular architecture , hrthle cell changes , and squamous metaplasia . 
 the dominant tumor nodules of both cases contained large , well - circumscribed but unencapsulated aggregation of mature plasma cells and scattered centrocyte - like cells ( ccl - cells ) . 
 both lesions contained a few lymphoepithelial lesions . 
 moreover , immunohistochemical study demonstrated that plasma cells and ccl - cells of these two lesions contained monotypic intracytoplasmic kappa light chain . 
 other small b - cell lymphomas , plasmacytoma and plasmablastic lymphoma were excluded using stains for cd5 , cd10 , cd23 , cd43 , cd56 . 
 cyclin d1 , human herpes virus type-8 .