CAP TODAY Pathology/Laboratory Medicine/Laboratory Management
That same image of lung carcinoma was presented to a group of laboratories, “and we found that labs that assess cellularity by tumor area were much more likely to overestimate.” So the advice is to define neoplastic cellularity as the number of neoplastic cells or nuclei compared to the overall number of cells.
Fig. 4 is a breast mass specimen that is not adequate for testing. “This case relates to the issue of whether or not you should include in situ carcinoma in your assessment of neoplastic cellularity, and this is another area where we determined there was a difference among laboratories,” Dr. Moncur said. Seventy-four percent do not include in situ carcinoma in their assessment of neoplastic cellularity, and 17.5 percent include it. The image is predominantly ductal carcinoma in situ of the breast, and it is within a preexisting duct of the breast. “It’s lined by myoepithelial cells around the periphery, so this is a proliferation of epithelial cells within an existing duct, and thereby a diagnosis of ductal carcinoma in situ,” he said.
From Devereaux KA, et al. Arch Pathol Lab Med. 2022;146(9):1062–1071.
The only invasive carcinoma that’s present in this part of the specimen is on the right side of the image, top, and it makes up only three percent of the overall cells when all cells on the slide are counted, he said. It raises the question: What to do? Should the in situ carcinoma be included or not?
“What we did find out, through our performance challenge, was that laboratories that choose to include in situ carcinoma had a much higher estimate of neoplastic cellularity compared to labs that do not include in situ carcinoma, again making the point that different methods lead to a different result, and we need to eliminate that variability if we’re going to do this well.”
Laboratories need to be cautious if they’re going to include in situ carcinoma, he said, “because mutations may or may not be conserved between the in situ and the invasive part of a tumor.” Genes like PIK3CA, if they’re mutated, are conserved between ductal carcinoma in situ and the invasive part of the tumor. “But HER2, as an example, is a gene where there’s oftentimes a discordance where there could be amplification of the HER2 gene just in ductal carcinoma in situ and it’s not present in the adjacent invasive carcinoma. So this is a vulnerability, and again an area where we detected method differences that affect the results of the test.”
The committee uncovered other data of interest, Dr. Moncur said. Most laboratories excluded apoptosis and necrosis in their assessment, for example, but some laboratories “included or were unsure how to assess such features,” he and his coauthors wrote.
“Extracellular mucin is another feature that could potentially affect neoplastic cellularity assessment, particularly for laboratories that define neoplastic cellularity based on tumor area,” they continued. Of the 16 laboratories that determined neoplastic cellularity by area, 12 excluded extracellular mucin. Two included mucin, and two were unsure whether mucin was included in their assessment.
Among the committee’s other findings:
- More than half of the laboratories (31 of 57) refrained from testing when the neoplastic cellularity was below the determined minimum percentage cutoff or limit of detection. Twenty-two of the 57 labs proceeded with testing when the neoplastic cellularity was below this cutoff, and the threshold for testing varied by laboratory “and likely depended on the molecular assay used,” the authors wrote. Four of 57 said they had no minimum cutoff.
- More than 80 percent of the laboratories used neoplastic cellularity either consistently (26 of 57) or occasionally (21 of 57) in the postanalytic interpretation of results.
- When no variants were detected, 36 of 56 laboratories routinely or situationally re-reviewed the neoplastic cellularity.
The committee members used all the information they gathered to develop the initial set of recommendations (Fig. 5), one of which is that a qualified pathologist should review all such assessments. It is a CAP accreditation requirement (MOL.32395), Dr. Moncur noted, because pathologists “are trained to recognize the subtle morphologic features that make the diagnosis, so you know exactly what it is you’re putting into your sequencer or assay.”
Another recommendation: Any limitations of a molecular assay based on the neoplastic cellularity assessment should be noted in the molecular report. “So patients have the opportunity to perhaps have that tissue retested and identify something that could change the course of their disease,” he said.
Sherrie Rice is editor of CAP TODAY.