The dilemma of diagnostic drift

5 minute read

One dermatopathologist’s moderately dysplastic naevus is another’s melanoma in situ. And that’s not necessarily a bad thing.

In 1997 the Red River in North Dakota flooded, threatening the city of Grand Rapids.

Dykes were ready to withstand a peak as high as 52 feet (16m). The National Weather Service predicted that the flood would crest at 49ft.

So when the flood peaked at 54ft, causing the evacuation of more than 50,000 people and billions of dollars in property losses, questions were asked.

Why was the prediction so far off? It wasn’t, said the NWS – they had a margin of error in their prediction of plus or minus 9ft! They just hadn’t told the public that, fearing a loss of confidence.

The old saying “better safe than sorry” was clearly a lesson the National Weather Service hadn’t absorbed in 1997.

Dr Blake O’Brien, a dermatopathologist with Sullivan and Nicolaides, told the Red River tale during his presentation at the recent Australian Melanoma Conference in Brisbane, to illustrate how uncertainty can lead to diagnostic drift.

“If you think that uncertainty might take you into a place where real harm might occur, if you feel that our report [to clinicians] needs to reflect that, then that encourages drift towards [diagnosing] malignancy,” Dr O’Brien said.

“If I’m putting in my report ‘I think this is a dysplastic naevus, however there’s some uncertainty and I can’t exclude, or am concerned about, the possibility of an early melanoma in situ’ – that will be treated [by the clinician] as early melanoma in situ. That might represent a diagnostic drift [towards malignancy] over time.”

Dr O’Brien works in a world where his ability to accurately diagnose a sample depends on the skin cancer GP or dermatologist’s ability to accurately excise and provide a good history. It’s not always ideal.

“What makes it easier for us is if the lesion is completely removed, with a rim of normal skin – the national guidelines suggest removing suspicious pigmented lesions with a 2mm margin,” Dr O’Brien told TMR.

“It’s very helpful to compare that background skin to your lesion to get a better sense of what the lesion looks like.”

In terms of depth, ideally, the dermatopathologist wants to see the entire lesion.

“If you partly take out a lesion, then the body starts responding to that attack of the surgery and inflammation, healing – it distorts whatever’s left,” said Dr O’Brien.

Is diagnostic drift towards malignancy a bad thing?

“We’re understanding more about melanoma as time goes on,” he says. “We’re understanding that sometimes really thin lesions can behave aggressively.

“What we’re trying to do as a profession is to get these lesions early and I think that’s a good thing. It’s hard to argue against that. Picking melanoma early and getting rid of those clearly seems to be a worthy goal.

“Where it might have a downside is overdiagnosis – at a population level [diagnostic drift] is potentially negative because a lot of people are being diagnosed with malignancy that wouldn’t have gone on to harm them.

“But until we can look at an individual lesion and better define its kinetics and how it would behave, I don’t think we have much of a choice but to continue trying to diagnose in the way that we feel is appropriate for that individual.”

He said thinking about possible overdiagnosis when looking at a possible melanoma in situ was “not fair” on the patient.

“The answers will come from better biologic markers. When we get to that point, then we can better say maybe this lesion has some atypical findings but the genetics of it suggests that it’s going to behave in a more indolent or benign fashion.

“Then we can better pick out those ones that might behave worse. So we can say, this is yes, an early lesion, but one that might behave more aggressively and warrants more aggressive treatment.”

Is it important to rein in diagnostic drift?

“It’s important for us as a profession not to drift [too far towards] calling everything malignant, because then it loses its meaning to the patient biologically,” said Dr O’Brien.

“What could really help to rein in the drift – excising lesions completely with a rim of normal skin, not excising them in a way where their removal is ambiguous, because it makes it so much harder to be to provide a confident, benign diagnosis.

“It’s important to move away from the dichotomy of benign or malignant.

“Forcing the dichotomy we’ll end up with more lesions being put into that malignant box. If you only have two boxes, benign or malignant, and you have to choose when there a grey zone and it’s a difficult case, then it’s really hard not to err on the side of caution.

“You’re not doing the patient any favours by pushing it into a benign box and taking a bit of a risk that way because the potential for missing the melanoma might be so great.”

But telling a patient they have a malignancy can do harm as well.

“I know there’s potential psychological damage and excessive surgery from an overcall. I do understand that and I’m cognisant of that, but I still feel that the negative outcome is heavily weighted towards a much worse outcome if you miss a melanoma and it goes on to really hurt the patient.”

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