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The Solid Tumor Rules instruct us not to use differentiation for coding histology unless it is specifically listed in the table. The terminology non-small cell carcinoma with neuroendocrine differentiation is not in lung histology Table 2.

However, SINQ 20150033, prior to Solid Tumor rules, indicates this diagnosis should be coded to 8574 (adenocarcinoma/carcinoma with neuroendocrine differentiation).

This presentation appears to represent distinctly different histologies. However, because the 2018 histology diagnosis is not in the table and the prior SINQ appears to disagree with current instruction, it is not clear how to apply the M rules to this case. The outcome of the histology coding will affect the number of primaries reported in this case.

ICD-O-3 Rule H states to use the topography code provided when a topographic site is not stated in the diagnosis. This topography code should be ignored if the tumor arose in another site. For the following brain and central nervous system (CNS) examples, should the suggested sub-site codes be assigned based on the histology, or should the primary sites be coded as C719 (posterior fossa or suprasellar brain) since the only information available was a tumor in these non-specific sites?

Example 1: Resection of a posterior fossa tumor proved medulloblastoma, WNT-activated. Although medulloblastoma has a site-associated code in the ICD-O-3 (C716, cerebellum), the only information available is that this was a posterior fossa tumor (C719).

Example 2: Resection of a suprasellar brain tumor proved pineoblastoma. The pathologist labeled this as a brain tumor, suprasellar. Although pineoblastoma has a site-associated code in the ICD-O-3 (C753, pineal gland), the only information available is that this was a suprasellar brain tumor (C719).

Example: Patient has a secretory carcinoma of minor salivary gland tissue (mammary analogue secretory carcinoma [MASC]) of the mucosal lower lip; it is unclear which table to use and how to arrive at the correct histology using the H Rules.

Rule H1 (code the histology when only one histology is present) states, Note 1: Use Tables 1-9 to code histology. There is no table that includes the lip. The correct histology should be 8502 which is listed in Table 6 (Tumors of Salivary Glands) however this does not correspond to minor salivary glands of the mucosal lip (site C003 per ICD-O-3 coding instruction).

The 2018 ICD-O-3 Update table does not include this histology, however Table 6 indicates code 8502 (secretory carcinoma) is a new code that was approved by IARC/WHO.

The ICD-O-3 only includes this histology as secretory carcinoma of breast. Therefore, in order to arrive at the correct histology, one must be aware of previous SINQ entries 20160036 and 20130003 that indicate secretory carcinoma (or MASC) is histology 8502. However, these are related to MP/H Rules, so registrars may be hesitant to apply this guideline to cases coded using Solid Tumor Rules.

The Extent of Disease (EOD) manual states that "Confined to thymus WITH mediastinal or pleural involvement" should be coded as regional by direct extension. I have EOD primary tumor coded as 200 and based on SEER*RSA, this is localized.

Core biopsy of left breast at 2:00: Invasive ductal carcinoma, Nottingham score 6/9.

Core biopsy of left breast at 4:00: Invasive mucinous carcinoma (variant of ductal carcinoma), Nottingham score 5/9.

Post neo-adjuvant mastectomy: Main (largest tumor): Invasive ductal carcinoma, upper outer quadrant grade 2. Secondary tumor: mucinous carcinoma, grade 1 at 4:00.

Patient had a thymectomy including pericardial fat for a mediastinal mass found incidentally during lung screening. Final diagnosis is WHO B3 thymoma. Staging Summary lists transcapsular invasion: "Present, as separate tumor nodules within peri-thymic adipose tissue." Tumor extension is stated to be "Confined to thymus, including peri-thymic adipose tissue." The pathologist staged this resection as pT1a pNX with no mention of mets. Clinically, there are no noted metastatic sites and no further treatment is planned.

The autopsy report indicated a diagnosis of: Skin: Hyperkeratosis and pseudoepitheliomatous hyperplasia as well as reactive angioendotheliomatosis indicating Elephantiasis Nostras Verrucosa.

There are two histologies provided in the final diagnosis, malignant spindle cell neoplasm (8004/3) and atypical fibroxanthoma (8830/3). There is a definitive diagnosis of the non-specific histology, but the more specific histology is only described using ambiguous terminology.

The external ear (C442) is included in the Head and Neck schema for diagnosis year 2018 and later. The Head and Neck Histology Rules indicate ambiguous terminology cannot be used to code a more specific histology. So ignoring the atypical fibroxanthoma, because it is modified by ambiguous terminology, we are left with a non-reportable site and histology combination (C442, 8004/3).

Diagnoses of malignant atypical fibroxanthomas are regularly diagnosed using the syntax above in our area. Follow-up with the physician or pathologist is generally not possible as these cases are received from dermatopathology clinics only. The pathology report is the only information that will be received. If the reportable diagnosis of malignant atypical fibroxanthoma is ignored per the current Solid Tumor Rules, incidence cases will be lost.

For example, pathologic diagnosis is non-small cell carcinoma with squamous features. The medical oncologist describes this as squamous cell carcinoma and begins treatment regimen. As I interpret the rules, we would use code 8046, non-small cell carcinoma, because of instruction 2 and the fact that features is not included in the list of ambiguous terminology.