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DCIS: The Measure of the Margin

What’s the best way to treat ductal carcinoma in situ (DCIS)? We’ve asked and tried to answer that question time and time again. On the surgical front, consensus guidelines published this month in three medical journals – Annals of Surgical Oncology, Journal of Clinical Oncology, and Practical Radiation Oncology – addressed the question of how large the margin of tissue surrounding the tumor should be for DCIS.

As I see it, we keep returning to the question of how to treat DCIS because we are applying standards that work for invasive breast cancer, which has the potential to be deadly, to DCIS, which does not. Invasive cancers generally present as a lump in a defined area of breast tissue. The lump occurs because the cancer has pushed its way out of the breast duct and invaded the breast tissue and the fat that surround the duct.  These lumps are easy to feel, to see on mammogram, and to remove. DCIS is a lesion that starts in the lining of the duct. It spreads inside the duct, not into the breast tissue. A duct is like the branches of a tree, and the DCIS can move through the branches as they divide. As it moves, it may settle in some areas but skip others.

Measuring Margins

The way we measure surgical margins – how close the disease comes to the edge of the tissue that has been removed – is pretty crude and was developed for the assessment of lumps. Basically, a piece of tissue is removed that contains the lesion seen on your mammogram or felt, and that piece of tissue is then painted with colored ink. Once the ink has dried, sections from each edge (top, bottom, superficial, and deep) are examined for tumor cells to see if there is a margin of normal tissue around the lesion. To really examine every edge of the tissue would require over 4,000 slides! But we do six to ten. This approach works for lumps. Yet we use the same approach for DCIS, which is biologically different. If there is a large area where DCIS crosses the margin, we will probably catch it. But if it’s just a small area, there is a large chance we will miss it. This means that if the margins are positive, they are really positive, whereas if they are negative, we can’t really be sure. 

Guidelines

Given how hard it is to determine margins and the extent of disease, it is really remarkable that three organizations came together to develop a recommendation that the pathologically measured margins around DCIS need to be 2 mm or more to reduce the risk of the DCIS coming back in the same breast as DCIS or invasive breast cancer. They didn’t decide on 2 mm arbitrarily. They looked at clinical studies of DCIS that followed up on women who had had surgery and radiation to see if they could determine a cutoff for the optimal margin that had the least risk of recurrence and left as much of the breast as possible. It is a statistical game, and in the end, 2 mm appeared to work as the minimum margin to guide decisions about further treatment.

The consensus statement does indeed give surgeons and radiation therapists a much-needed guideline. But it also points out the need for more research into how DCIS spreads along the ductal systems and the actual pattern of the ductal systems in women. If we had this knowledge in hand, we would be able to make surgical decisions about margins based on the biology of DCIS itself.