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Per the NCCN guidelines version 4.2020 (which is unfortunately pay-walled), for invasive ductal carcinoma (IDC), the recommended margins are no tumor on ink, while isolated ductal carcinoma in situ (DCIS) margins are 2 mm. This alone never made sense to me, as I would imagine an already invasive cancer is more aggressive and likely to cause local recurrence if microscopic residual cancer was left behind. So if no tumor on ink was adequate enough for IDC, shouldn't it be adequate for DCIS?

Some authors (such as in this 2018 review article) suggest that there is a difference in the growth pattern of DCIS compared to IDC that may warrant the margin of 2 mm:

Faverly and colleagues examined the growth pattern of DCIS and found that while 90% of poorly differentiated lesions grew continuously, 70% of well-differentiated lesions had a multifocal, skip pattern, with 82% of skip lesions measuring between 0mm to 5mm, and only 8% having skip lesions >10mm. These studies suggest that that a small negative margin may lie within a skip lesion and may be associated with a substantial residual tumor burden.

Ok - I'm not a pathologist, oncologist, or surgical oncologist so I would of course defer to their expertise. At least this explains the apparent discrepancy I perceive in the recommended surgical margins.

But the guidelines further clarify that in circumstances where the specimen contains both IDC and DCIS, the negative margin definition remains no ink on tumor based on the invasive margin guideline regardless of the extent of DCIS.

I am not sure how to fit this into my mental model - if DCIS has fundamentally different growth patterns and underlying physiology such as to warrant wider margins, why does this change when co-present with IDC?

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