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HGAP is listed as a separate classification and is not a subtype of the diffuse gliomas. WHO Classification of Tumors of the Central Nervous System, 5th edition, has two categories dealing with non-pediatric astrocytic tumors:

Adult-type diffuse gliomas

Circumscribed astrocytic tumors

HGAP falls into the second category as a result of updates to the 4th edition WHO classification in 2016 with advances in the role of molecular diagnostics with the 5th edition. All astrocytic tumors were previously grouped together whereas not all diffuse gliomas (astrocytic or not) are grouped together on the basis of growth pattern and behaviors, and shared IDH1 and IDH2 genetic status. The new classification separates astrocytomas that have a more circumscribed growth pattern, lack IDH gene alterations, and sometimes have BRAF mutations (i.e., pilocytic astrocytoma). The impact of molecular advances has driven classification changes as described in the 5th edition.

Review of site/histologiy combinations for CNS neoplasms is currently being performed by Cancer PathCHART experts. It's possible they will recommend HGAP be moved to a subtype/variant of astrocytoma, NOS. 

Assign 8013/3 (combined large cell neuroendocrine carcinoma). There are two histologies present: large cell NEC and urothelial. Literature search found primary large cell NEC of the bladder is extremely rare with less than 20 reported cases. This case does not fall into the site-specific rules and given it's raity, a specific rule for this situation was not and will not be added to the Bladder rules. See #1, Example 2, in the general instructions for coding histology.

The correct term is diffuse leptomeningeal glioneuronal tumor listed as a synonym in Column 2. 

We will add the term diffuse in Note 2, Column 1 with the 2025 updates. In the meantime, you can add "diffuse" to your pdf version until the update is published.

Assign code 9431/1 for angiocentric glioma. This has been corrected in the 2025 update. Angiocentric glioma was removed from the 9421/1 row and remains in its own row.

Assign 9380/1 for low grade glioma diagnosed 1/1/2018 forward and for low grade glioma diagnosed prior to 1/1/2018 assign code 8000/1 on the advice of our expert neuropathologists. The site/type combination of C71 _ and 9380/1 will flag histology/site/behavior edits which should be overridden.

Low grade glioma is an umbrella term or non-specific diagnosis, primarily seen on radiologic reports such as CT scans and MRIs. Often, the patient is actively followed with scans and surgical intervention delayed or not recommended. WHO Classification of Central Nervous System Tumors, 5th edition, does not recognize this term and indicates that tissue diagnosis (including genetic testing) is needed to provide a specific diagnosis. Since biopsy of these “neoplasms” is not routinely done, a definitive diagnosis is not available. Literature searches yielded conflicting information with some stating low grade gliomas are malignant with an indolent clinical course while others felt they were benign. Until such time as WHO proposes a code for this neoplasm, our expert neuropathologists recommend coding glioma, NOS with borderline behavior 9380/1. 

Assign histology as 9505/3.  WHO Classification of Central Nervous System (CNS) Tumors describe ganglioglioma as a well-diffferentiated and slow-growing glioneuronal neoplasm.  While WHO does not recognize the histology/behavior combination 9505/3, the 2021 CNS Solid Tumor Rules identify non-malignant tumors that have the potential of transforming to a malignant tumor (new primary).  Ganglioglioma (9505/1) is listed with the transformed histology and instructs us to code as anaplastic ganglioglioma (9505/3).

Assign 8230/2 (ductal carcinoma in situ, solid type/intraductal carcinoma, solid type) using Breast Solid Tumor Rules Table 3 as instructed in Rule H2 for in situ tumors. The carcinoma, NST row lists this histology in the subtype/variant column 3.

Coding histology for in situ breast tumor differs from invasive. While the majority of in situ breast primaries will be coded to DCIS 8500/2, there are others that are listed in Table 3 that should be coded according to the specific histology. Some codes have the word subtype or type as part of their histologic term so these can be coded based on the histologic term as listed in the table. We suggest you routinely review the histology tables to see if a term is listed.

Lymphoepithelial carcinoma is a subtype of SCC usually seen in skin or H&N sites and often associated with EBV. CPC SME review determined 8082/3 invalid for breast but did not recommend a substitute code. There were only 45 cases coded 8082 2001 to 2019. For this case, it's possible the lesion originated in the breast skin and progressed to breast tissue. SCC is a subtype of metaplastic breast carcinoma so one could argue it code be coded either 8575 or 8070. For this case, we recommend assigning 8500/3. Use text fields to record the details.

Assign histology as adenocarcinoma, HPV-associated (8483/3) as designated in Table 17, Uterine Cervix Histologies, of the Other Sites Solid Tumor Rules. The WHO Classification of Female Genital Tumors, 5th edition, states that p16 immunohistochemistry is an effective (yet flawed) indirect test for HR-HPV infection, in line with the STRs that state p16 is a valid test to determine HPV status and can be used to code HPV-associated and HPV-independent histologies. In this scenario, "negative for high-risk human papilloma virus (HR-HPV) on Pap smear" would be cytology-based, and may have missed cytologically malignant cells. A subsequent, more definitive biopsy was performed and was found to be strongly positive for p16, therefore, the tumor should be coded as 8483/3.

Assign code 8742/2 (lentigo maligna) for “early lentiginous melanoma in situ.” ICD-O-3.2 lists the preferred term for 8742/2 as lentigo maligna (C44._).