Anatomic pathology selected abstracts

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Chan E, McKenney JK, Hawley S, et al. Analysis of separate training and validation radical prostatectomy cohorts identifies 0.25 mm diameter as an optimal definition for “large” cribriform prostatic adenocarcinoma. Mod Pathol. 2022;35(8):1092–1100.

Correspondence: Dr. E. Chan at emily.chan@ucsf.edu

Use of GRM1 IHC to distinguish chondromyxoid fibroma from histologic mimics

Chondromyxoid fibroma is a rare benign bone neoplasm that manifests histologically as a lobular proliferation of stellate to spindle-shaped cells in a myxoid background. It exhibits morphologic overlap with other cartilaginous and myxoid tumors of bone. Chondromyxoid fibroma (CMF) is characterized by recurrent genetic rearrangements that place the glutamate receptor gene GRM1 under the regulatory control of a constitutively active promoter, leading to increased gene expression. The authors conducted a study in which they explored the diagnostic utility of GRM1 IHC as a surrogate marker for GRM1 rearrangement using a commercially available monoclonal antibody. The study involved 230 tumors, including 30 CMF cases represented by 35 specimens. GRM1 was positive by IHC in 97 percent (34 of 35) of CMF specimens and exhibited moderate to strong staining in more than 50 percent of neoplastic cells. Staining was diffuse (more than 95 percent of cells) in 25 (71 percent) specimens. Among the nine CMF specimens that had documented exposure to acid decalcification, four (44 percent) exhibited diffuse immunoreactivity (more than 95 percent) for GRM1. All 15 CMF specimens that were not exposed to decalcification reagents were diffusely immunoreactive (P=.003). High GRM1 expression at the RNA level was previously observed by quantitative reverse transcription polymerase chain reaction in nine CMF cases that were also positive by IHC. Low GRM1 expression was observed by quantitative reverse transcription polymerase chain reaction in the single case of CMF that was negative by IHC. GRM1 IHC was negative (less than five percent) in histologic mimics of CMF, including conventional chondrosarcoma, enchondroma, chondroblastoma, clear cell chondrosarcoma, giant cell tumor of the bone, fibrous dysplasia, chondroblastic osteosarcoma, myoepithelial tumor, primary aneurysmal bone cyst, brown tumor, phosphaturic mesenchymal tumor, CMF-like osteosarcoma, and extraskeletal myxoid chondrosarcoma. These results indicate that GRM1 IHC may have utility in distinguishing CMF from its histologic mimics.

Toland AMS, Lam SW, Varma S, et al. GRM1 immunohistochemistry distinguishes chondromyxoid fibroma from its histologic mimics. Am J Surg Pathol. 2022;46(10):1407–1414.

Correspondence: Dr. Gregory W. Charville at gwc@stanford.edu

Impact of liver biopsy size on histopathologic evaluation of liver allograft rejection

Allograft liver biopsy is the gold standard for assessing transplant recipients for graft dysfunction. The impact of biopsy sample size on the diagnosis of acute cellular rejection (ACR) has not been studied, according to the authors, who assessed the relationship between biopsy sample length and diagnosis and determined optimal biopsy sample size in the transplant setting. They retrospectively reviewed core biopsies from 68 patients who had a history of liver transplant. Each biopsy sample was read on five occasions using differing lengths to assess for ACR per Banff criteria. Categorical agreement was calculated using rejection severity. The length of biopsy samples strongly correlated with the number of portal tracts. ACR rates increased from 73.5 to 79.4 percent with an increase in length from 1 cm to 2 cm, and moderate rejection increased from 27.9 to 33.82 percent. No cases of severe rejection were detected at 1 cm and 1.5 cm; one case was detected at 2 cm; and two cases were detected at 3 cm. The major error rate was reduced to less than 10 percent with a length of 2 cm, at which length the average number of complete and partial portal triads was 10 and 13, respectively. The likelihood of diagnosing ACR and rejection grade rose substantially with an increase in biopsy sample length. This study suggests that a minimum length of 2 cm and 10 complete portal triads or 13 partial/complete portal triads should be obtained to confidently exclude or grade ACR.

Agarwal AN, Nania J, Qiu L, et al. Impact of liver biopsy size on histopathologic evaluation of liver allograft rejection. Arch Pathol Lab Med. 2022;146(12):1530–1534.

Correspondence: Dr. Apeksha N. Agarwal at agarwala@uthscsa.edu