350 Editorial Commentary Page 1 of 5 Classification of solid renal tumor with oncocytic/eosinophilic cytoplasm: is hybrid oncocytic/chromophobe renal tumor a subtype of oncocytoma, chromophobe renal cell carcinoma, or a distinct tumor entity? Shuji Mikami1, Naoto Kuroda2, Yoji Nagashima3, Chisato Ohe4, Hiroyuki Hayashi5, Ryuichi Mizuno6, Mototsugu Oya6, Kaori Kameyama1 1Department of Diagnostic Pathology, Keio University Hospital, Tokyo Japan; 2Department of Diagnostic Pathology, Kochi Red Cross Hospital, Kochi, Japan; 3Department of Surgical Pathology, Tokyo Women’s Medical University, Tokyo, Japan; 4Department of Pathology, Kansai Medical University Hospital, Osaka, Japan; 5Department of Pathology, Fukuoka University Hospital, Fukuoka, Japan; 6Department of Urology, Keio University Hospital, Tokyo, Japan Correspondence to: Shuji Mikami, MD, PhD. Department of Diagnostic Pathology, Keio University Hospital, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. Email: [email protected]. Provenance: This is an invited article commissioned by the Section Editor Dr. Xiao Li (Department of Urology, Jiangsu Cancer Hospital, Jiangsu Institute of Cancer Research, Nanjing Medical University Affiliated Cancer Hospital, Nanjing, Chnia). Comment on: Ruiz-Cordero R, Rao P, Li L, et al. Hybrid oncocytic/chromophobe renal tumors are molecularly distinct from oncocytoma and chromophobe renal cell carcinoma. Mod Pathol 2019. [Epub ahead of print]. Submitted Sep 01, 2019. Accepted for publication Sep 12, 2019. doi: 10.21037/atm.2019.09.77 View this article at: http://dx.doi.org/10.21037/atm.2019.09.77 Pathological diagnosis of oncocytic renal tumor is usually or a distinct renal tumor entity. Therefore, we comment on difficult task, and their differential diagnosis involves important studies of HOCT including recently published chromophobe renal cell carcinoma (ChRCC), renal paper by Ruiz-Cordero et al. (9). In addition, we briefly oncocytoma (RO), clear cell renal cell carcinoma (CCRCC), describe the differential diagnosis of solid renal tumors with epithelioid angiomyolipoma (EAML), etc. Hybrid oncocytic/eosinophilic cytoplasm from a pathological point oncocytic/chromophobe tumor (HOCT) of the kidney is of view, which includes oncocytic variant of ChRCC. an emerging entity, and there is no description on HOCT Clinically, the median age of the HOCT patients in the WHO classification of the urinary system, third at the diagnosis was seventh and eighth decades, and edition (1). HOCT was first used to describe tumors male patients tended to be slightly more common than showing pathological features of both RO and ChRCC. At female (7,9,10). Characteristic BHD clinicopathological the beginning, HOCT has been reported in patients with background is observed in HOCTs associated with BHD, Birt-Hogg-Dubé syndrome (BHD) (2,3), or those with renal but no characteristic symptoms have been reported in oncocytosis (4,5). Recently, HOCTs have been also reported HOCTs of non-BHD patients. Many patients with sporadic in patients without background diseases (6,7). Thus, HOCT HOCT had solitary masses, but multiple tumors including has been recognized as a renal tumor, but there is only brief bilateral ones occurred in some patients, particularly those description on HOCT in the histopathology section of with BHD. Majority of HOCT patients seemed to behave ChRCC in the 2016 WHO classification of renal tumors as indolently without evidence of malignant behavior, but below (8); “A small subset of tumors have overlapping histology distant metastasis and unclassified renal cell carcinoma between RO and ChRCC, and which are commonly seen in BHD (RCC), WHO/ISUP grade 4, arising within background of and renal oncocytosis”. It is currently a field of undergoing HOCT were recently reported (9). intense study whether HOCT is a subtype of RO, ChRCC Macroscopically, HOCT is well circumscribed and non- © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2019;7(Suppl 8):S350 | http://dx.doi.org/10.21037/atm.2019.09.77 Page 2 of 5 Mikami et al. Is HOCT a distinct tumor entity? HE CK7 HE Colloidal iron stain Figure 1 Histopathological features of HOCT. At low magnification, venous invasion was identified (arrows). Oncocytic tumor cells showed perinuclear clearing at high magnification, but no raisinoid nuclei were observed. Focal CK7-positive cells were observed, but most tumor cells were negative for CK7 staining. Therefore, this case was diagnosed as HOCT not ChRCC, eosinophilic variant. Hale’s colloidal iron stain was mainly observed in apical portion of the tumor tissue, and focal diffuse staining was also observed. Bars 100 μm. HOCT, hybrid oncocytic/chromophobe tumor; CK7, cytokeratin 7; ChRCC, chromophobe renal cell carcinoma; HE, haematoxylin and eosin. encapsulated, and their cut surfaces are homogenous tan CoA racemase (AMACR), CK20, CD10, carbonic anhydrase to brown without necrosis. Generally, they are localized 9 (CA9) staining tended to be negative except for focal tumors without invasion to the adipose tissue nor vascular staining in some tumors. Based on morphological and invasion (7). Histologically, HOCTs are consisted of slightly immunohistochemical characteristics, HOCT is considered atypical nuclei and rich eosinophilic cytoplasm mainly to harbor a mixture of the immunohistochemical features that showing a solid alveolar pattern (Figure 1). Perinuclear halo overlap with those of RO and ChRCC. and binucleated cells are frequently observed (Figure 1), To elucidate the molecular characteristics of HOCT, but there are no raisinoid nuclei, characteristic to ChRCC. analyses were performed by 2 groups, but conflicting Perinuclear cytoplasmic clearing is observed in HOCT, results were obtained. Analysis of 14 sporadic HOCTs by resembling ChRCC closely. Tumor cells are similar to fluorescence in situ hybridization and Sanger sequencing those of RO. Some HOCTs might have morphological revealed that all HOCTs showed some chromosomal features both of RO and ChRCC. There might not be a imbalances, and the most common numerical aberration clear boundary between RO-like regions and ChRCC-like was found in chromosome 20 (7). This result is distinct regions. from those of both RO and ChRCC. Another analysis of Hale’s colloidal iron stain is only focally positive in the apical 10 sporadic HOCTs and 2 HOCTs associated with BHD portion of tumor cells of HOCTs, by contrast to ChRCC using array-comparative genomic hybridization showed with diffuse positive staining. This staining pattern is rather no chromosome imbalances in about half cases (58%) (10). similar to that in RO. Immunohistochemically, all HOCTs These findings and similar Hale’s colloidal iron staining are positive for pan-cytokeratin and mitochondrial antigen. pattern suggested HOCTs might be a variant of RO. Cytokeratin 7 (CK7), E-cadherin, and parvalbumin expression In recent study by Ruiz-Cordero et al., the mutational are also observed in most cases, but the proportion of positive and transcriptomic analysis of HOCTs was performed cells varies among cases. On the contrary, α-methylacyl- in addition to the DNA copy number analysis (9). This © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2019;7(Suppl 8):S350 | http://dx.doi.org/10.21037/atm.2019.09.77 Annals of Translational Medicine, Vol 7, Suppl 8 December 2019 Page 3 of 5 HE CK7 CD117 Colloidal iron stain Figure 2 Histopathological features of ChRCC, oncocytic variant. Tumor cells proliferated in predominant tubular pattern with focal solid arrangement. CK7 staining was diffusely positive, but no apparent CD117 staining was observed. Hale’s colloidal iron staining was observed in apical portion of tumor cells. Bars 100 μm. ChRCC, chromophobe renal cell carcinoma; CK7, cytokeratin 7; HE, haematoxylin and eosin. study revealed that up to 40% of HOCTs did not show iron stain, CD117, CK7 are observed in ChRCC. Although chromosomal arm-spanning imbalances, and the results perinuclear halo is common in HOCTs, no raisinoid nuclei is consistent with the previous study (10). Losses at is observed in HOCTs. Typical RO differs is distinct from chromosome 1 or X/Y were predominant copy number HOCT in that small tumor cells proliferate in solid alveolar aberrations and this is similar to these reported in RO and pattern. RO also shows cytoplasmic CD117 staining, but eosinophilic variant of ChRCC (7,10). Furthermore, even negative or only focally positive for CK7. In addition, Hale’s deep-targeted next-generation sequencing of HOCTs failed colloidal iron staining was limited to the apical portion of to find genetic abnormally in genes reported to be mutated RO tissue. EAML is sometimes misdiagnosed as high-grade in RO (ERCC2) or ChRCC (TP53 and PTEN). Finally, RCC or other tumors. Most importantly, EAML should be an analysis of differently expressed genes between RO suspected in cases where the morphological characteristics and ChRCC revealed that HOCTs show an intermediate of RCC and RO are poor. Immunohistochemically, EAML features between RO and ChRCC, and this results are is negative for CK and EMA, and positive for melanocytic compatible with the pervious study of three HOCTs differentiation markers, such as Melan A. associated with BHD (11). These data, altogether, indicates As for differential diagnosis of oncocytic/eosinophilic that HOCTs are clinicopathologically and molecular renal tumor, such as RO, ChRCC and HOCT, a new biologically distinct from RO and ChRCC. Therefore, variant of ChRCC, namely an oncocytic