Introduction

The central goal of all skin cancer surgery is a complete tumor excision with the least possible morbidity. This seems like a simple goal, but the way margins are discussed can be confusing. Take for example Merkel cell carcinoma, for which a 1- to 3-cm excision margin is recommended. Does this mean 1 cm or 3 cm? Somewhere in the middle? Does it mean the preoperative clinical margin? Or is it the intraoperative margin? Histopathological margin? How are these margins different and what do they imply about the certainty that no tumor has been left behind?

Before tackling these questions, let us first consider the relatively basic tools most surgeons use to guide their skin cancer surgery.

Assessment of clinical margins is often the first step in any skin cancer excision. This entails a sort of mystical guesswork as to what lies beneath the surface based on prior experience, usually accompanied by a bit of a gut feeling combined with actual feeling or palpation of the lesion to be excised. The surgical marker is then put to work and the surgeon starts to draw. Most surgeons would struggle to remember if anyone taught them how to draw margins or if it was learned by osmosis, just watching it done by others. Like the famous joke of a man stopping a stranger in the street and asking, “How would you get to the pub from here?,” the answer to which is, “if I was going to the pub I wouldn’t start from here,” where we start to draw the margin is not often discussed and is poorly described in the literature. It naturally depends on what sort of tumor we are dealing with, and why a preoperative biopsy is so important. Some tumors are easy to spot and grow in a regular fashion with a nice cohesive advancing tumor front. They do not often exhibit perineural invasion or lymphovascular invasion and hence often what you see is what you get. But other tumors are highly unpredictable and the invasive fronts extend in different directions in a seemingly chaotic fashion. For these tumors, the surgeon is basically guessing when they decide on the shape and size of the excision.

As soon as you cut out the tumor, that clinical margin becomes a surgical or excision margin and there is an expected rim of “excess” tissue that is removed as a buffer zone around the tumor. Confusingly, there are a variety of guidelines defining how wide the clinical buffer zone needs to be. Let us take high-risk basal cell carcinoma (BCC) as an example. Any literature search will come up with 3 to 4 mm as an adequate excision margin for a low-risk lesion, but guidelines vary wildly for a high-risk lesion. The British Association of Dermatology specifies 5 to 13 mm but does not really say whether you should be nearer the 5 mm or the 13 mm. By contrast, the Australian Cancer Network considers this somewhat excessive and only stipulates 3 to 5 mm. The European Forum of Dermatology suggests 5 to 10 mm, which sits nicely between these two (Table I.1). Unless the BCCs are different in Australia, that’s quite a disparity. But none of them describe how to actually draw this margin. Is the measurement taken from the outside of the very last bit of erythema and vasculature that you can see with your +2 glasses on? Or via reflectance confocal microscopy? Or is it a line drawn around the bulk of the obvious clinical tumor with a margin then added on to that? Even with the most sophisticated aids, it can be difficult to see tumor, and often we are looking at vasculature or other surrogate signs.

It’s all rather unsatisfactory, which is of course why it’s all so inconsistent in the literature. It’s a big heterogeneous group of tumors and a big heterogenous group of surgeons trying to hammer square pegs into round holes. And while the surgeons are busy reading guidelines and drawing lines, the pathologists are using their graticules (a microscope eyepiece with an inbuilt ruler) to measure microscopic distances. These measurements produce a pathology report with seemingly magically concrete numbers and affirmations about margin status. But when you think a little deeper, it’s clear that pathologic margin measurements can be even less precise than the clinical estimates.

What this really boils down to is math, and if you’re reading this book there’s a good chance you were either good at math or good at biology, but maybe not both. It’s possible you are not the former, but you are sure to understand that it’s all to do with confidence intervals. A confidence interval is a range of values that’s likely to include a value with a certain degree of confidence, often expressed as a percentage. In the skin surgery context, an increasing size of the excision margin results in a progressive decrease in the percentage of incomplete excisions. Importantly, even with very large excisions, a small percentage of tumors will involve the surgical margins.

Let’s imagine that you perform the mystical 5-mm surgical margin (presuming you know exactly where to start). This 5 mm is a surrogate for a lot of very complicated concepts hidden in the measurement such as the tumor biology, the growth patterns, the anatomic site of tumor, the tissue it’s growing in, the state of the immune system of the host, and so on. The margin in this sense, i.e., the strip of normal-looking tissue that’s removed in excess of the assessed tumor size, describes the confidence with which you will achieve complete tumor clearance. That strip of tissue may or may not contain tumor. As mentioned earlier, this depends on many factors, not least of which is how good the surgeon is at assessing clinically the extent of the tumor.

Margin guidelines are thus established with the understanding that a significant percentage of tumors will extend microscopically further than the recommendation. In essence, those who set margin guidelines are making a judgment call as to what percentage of incomplete tumor resections (outliers) is acceptable. According to the National Comprehensive Cancer Network® (NCCN), a 4-mm margin will remove a BCC 94% of the time. Stop to think about that. We wouldn’t accept it if planes landed safely only 94% of the time. Considering that there are 3.5 million BCCs a year in the United States, and let’s say that 75% are low risk, that’s at least 150,000 people a year with a positive margin. So, in that case, would taking 5 mm be better, or 6 mm, or 10 mm? What’s the impact on both morbidity and quality of life of taking larger margins? This is something that isn’t discussed much in the literature: The true cost of recurrence versus the cost of excess margin.

If you’ve ever visited New Zealand, you’d have noticed that there are volcanoes everywhere. Volcanoes in many other parts of the world have been worn down by erosion over the millennia and aren’t nearly as exciting as Mount Taranaki. Every flight from the South Island of New Zealand up to Auckland in the North Island flies over it. This impressive and, in some ways, the otherworldly natural phenomenon can lead you to think about margins (Fig. I.1).

In the mind’s eye, skin tumors are a lot like volcanoes, in that the clinical assessment determines where the edge of the slope meets the ground. Where does the volcano start and stop? From the side, Mt Taranaki looks very regular and smooth. But take a look at the satellite view from above (Fig. I.2).

The clinical disease is the snow-capped peak and the gentle slope of the classic volcano, but we can see subclinical areas of disease, at 11 o’clock. It’s pretty hard to draw a uniform margin around the whole thing. At 11 o’clock we’re getting pretty close to an important structure, the sea!

The clinical margin is the outline of the tumor we can see or guess.

The surgical margin is the excess we draw around our clinical margin.

The excision margin is what we actually remove, which correlates with the pathological margin, which is how far from the tumor our excision margin is (Fig. I.3).

We may use a dermatoscope to help define our clinical margins and aid us in the removal of the first excision level. Making dots on the skin as reference points and then going back and forth with the dermatoscope helps us to build up a picture of the tumor. But this is still guesswork, and a central theme of this book is to avoid guesswork as much as possible.

Therefore, what we need is a satellite image of the tumor-like Mt Taranaki. If we had a method to draw accurate clinical margins every time, we could thus achieve complete tumor clearance every time. Unfortunately, this is not something we have, and even the latest imaging technologies cannot show us the detail we need to achieve this goal. We, therefore, need to turn to something else, something that is an afterthought for most surgeons: the pathologist. With a map.

If we could survey the tumor in real time, map the outside like a cartographer, every nook and cranny, every lahar and lava flow, we could build up a real-time 2D image of the tumor. A sort of satellite image from above. But this is only half the story. We need a Geotechnical engineer to do core samples across the entire area to assess its depth, shape, and the way it interacts with the surrounding structures.

In this analogy, the cartographer and Geotechnical engineer are the pathologist, and the technique used is referred to as margin control surgery (MCS). Terminology can get a little confusing as different specialties have different names for similar procedures. But the essential point is that, central to the concept of MCS, the entire surgical margin is assessed histopathologically, and this assessment guides the size and shape of the excision to achieve the complete removal of a tumor. Clinical guesswork is eliminated.

Once you’ve used and become accustomed to this assistance in surgery, it’s like a lightbulb going on. And once you’ve seen the light, you won’t want to go back into the gloom.