What you are agreeing to before Mohs surgery

Before Mohs surgery you sign an informed consent form. It is written in legal language, it lists a series of complications, and most people read it for the first time on the morning of the operation, when they are already tense.

This page explains what it says, in plain language, and what the numbers actually mean.

Why the form reads more alarming than it is

An informed consent form is meant to list everything that could happen, however rare. That is its purpose. So it reads like a list of bad outcomes, while the reality is far quieter.

The largest study on the subject followed 20,821 Mohs procedures across 23 medical centres. An adverse event of any kind was recorded in 149 cases, that is 0.72 percent. A serious event was recorded in 4 cases, 0.02 percent, and no deaths were recorded. Of the events that did occur, most were infections that were treated, followed by wound-healing problems and bleeding.

So the list in the form is long, but the chance that any of it happens to you is small.

“The consent form is not there to frighten anyone. It is there to make sure the two of us are describing the same operation. If something in it is unclear, that is exactly the question to ask before signing rather than after.”

What the signature actually covers

Four things, all of which are also said out loud before anything begins:

The signature also covers the local anaesthetic. Most Mohs procedures are done under local anaesthesia alone, which is why there is no fasting and regular medication continues as usual.

How many stages it will take, and why that cannot be known in advance

Mohs surgery is built from stages. In each stage a layer is taken, examined in the laboratory next to the operating room, and only what the microscope shows decides whether another layer is needed.

In a prospective study following 1,686 procedures, the average number of stages ranged from 1.5 to 2.1, depending on the histological subtype of the tumour. Infiltrative subtypes needed more stages on average than nodular ones.

This is why nobody can tell you in advance how long you will be at the clinic. The wait between stages is not a delay, it is the examination itself. The method works precisely because of it.

Why the wound is larger than the spot you could see

This is the point most worth understanding in advance, and the form states it explicitly: the size of the excision cannot be estimated before surgery.

A skin cancer looks like a spot or a small sore, but it may continue beneath the surface past the visible border. This is called subclinical extension, and it is more common in certain subtypes. A study of tumours with extensive subclinical spread found it associated with infiltrative, morpheaform and sclerosing subtypes, and with recurrent tumours.

So the wound at the end of surgery is almost always larger than the spot you could see, sometimes considerably. This is not an error and not a complication. It is the situation that existed all along and has now been revealed.

The important point: Mohs removes the minimum necessary. A standard excision takes a fixed margin around the tumour, without knowing whether it is enough or more than needed. Mohs removes only as far as the microscope shows. The wound looks large, but it is smaller than the one another method would have left.

A scar is an outcome, not a complication

Every operation leaves a scar. That is in the form too, and it is worth reading this way: the question is not whether there will be a scar, but where it will be and how it will settle.

Three things worth knowing in advance:

What can be promised and what cannot

Nobody can promise zero complications, fast healing, or no scar. Anyone promising those is promising something outside their control.

What can be said: Mohs surgery gives the highest cure rates of any treatment for basal and squamous cell skin cancer. On five-year follow-up of primary basal cell carcinomas treated with Mohs, the recurrence rate was around one percent.

And two simple things can be promised: that you will know what is about to happen before it happens, and that if anything during the operation feels uncomfortable, you say so and we stop.

What to ask before signing

There is no question not worth asking before signing. That is precisely the moment the form exists for.

Further reading

Each hospital has its own wording, and both can be read in advance in Hebrew, English, Arabic and Russian: the Assuta wording, signed at HaShalom and Ramat HaHayal, and the Herzliya Medical Center wording. The practical instructions for the day of surgery are on the surgery preparation page.