By Gary M. White, MD
This lesion grew over 37 years. Courtesy of Michael O. Murphy, MD.
Lentigo maligna (LM) is a variant of melanoma in situ that arises in sun-damaged skin in a fair-skinned, older patient. Lentigo maligna may progress to lentigo maligna melanoma (LMM)--true melanoma where invasion has occurred. For LMM, the prognosis is the same as other melanomas based upon depth. It comprises about 5-15% of all melanomas.
A pigmented patch on the sun-exposed areas of the head, neck, back, or arms of a Caucasian is typical. Darkly pigmented areas and irregular shapes are seen. Less commonly, amelanotic lesions may occur, which are red or pink and sometimes scaly. Slow growth over many years is the rule. A SPAK can look very similar.
A recent Dutch study showed the lifetime risk of progression of a LM to LMM (without treatment) is about 2% [JID 2016;1955]. Another study showed that the lifetime risk of such progression for a 45 year old is 4.7% and for a 65-year-old, 2.2% [BJD 1987;116;303].
A broad, thin shave biopsy encompassing all of the lesion is best. At a minimum, a biopsy encompassing all colors of the specimen is necessary. One study demonstrated that about 50% of LM are contiguous with lentigines or pigmented actinic keratoses. Dark color is associated with pigment incontinence, not malignancy. For these reasons, a small partial biopsy (e.g., punch) or just biopsying the darkest area has a high risk of giving a false negative result and should be avoided.
The optimal therapy is complete surgical excision. Usually, Wood's light with or without dermoscopy is used preoperatively to identify the clinical margin. Then an initial margin, of e.g., 5-10 mm, is used. Even among experts, there is no consensus on technique, e.g., standard surgery with 9 mm margins, Mohs surgery vs. staged excision repair/Slow-Mohs, Mohs, but send the last layer for permanent section, etc. Special stains (e.g., Mel-5, MART-1+) are very helpful in frozen-section-controlled surgery but are technically-demanding and time-consuming. For additional discussion of margins, see melanoma in situ.
When surgery is not possible, imiquimod or radiation therapy may be considered.
Imiquimod is a reasonable alternative for nonsurgical cases. It can produce a high and sustained remission rate with excellent cosmetic results. One study of 23 patients with LM of the face employed imiquimod 5% cream daily for 3 months, inducing skin inflammation for at least 10 weeks. Twenty out of 23 patients experienced clinical and histopathological clearance after a mean of 14 weeks of treatment [Acta Dermato-Venereologica 2015;95;83]. In another study [JAAD 2015;72;1047], the imiquimod 5% was given 3-5 times per week with increases up to daily in cases of less than adequate inflammation. A 2 cm margin around the clinical lesion or scar was utilized.
In a review of 347 cases, treatment with imiquimod resulted in a 76% histologic and 78% clinical clearance rate [JAAD 2015;05;022]. The clinical recurrence was 2.3% with a mean followup of 34.2 months. Treatment with >60 total applications, or with >5 applications per week was associated with a higher likelihood of histologic clearance. In another study [Gautschi et al JAAD Oct 2015], the recurrence rate was 18% with an average follow up of 5 years. The patients were treated with imiquimod 5% 1-2 daily till weeping erosions developed. In another study, an increased number of melanocytes histologically was correlated with a higher local recurrence rate after treatment with imiquimod [JAAD 2016;74;81].
XRT can be very successful in treating LM. In a review of published series [BJD 2014;170;52-58] there was a 5% recurrence rate. Progression to melanoma occurred in the 1.4 % of patients who tended to have poor outcomes. The authors recommend that the treatment area extend 1 cm beyond the visible lesion and that the radiation used penetrates at least 5 mm deep to treat cells extending down adnexal structures.
The CO2 laser has been reported as an alternative to nonsurgical cases [Arch Facial Plast Surg. 2011;13(6):398-403].
In one study, the mean time to local recurrence was approximately 6 years [JAAD 2016;74;1247]. Thus, studies evaluating therapy must have a long duration and in clinical practice, surgical sites should always be examined during followup.
These two have indistinct borders. Where do you take margins?
Lentigo maligna of the scalp. When doing a complete skin exam, always look in the scalp--especially if the patient is losing hair.
Rarely, MIS/lentigo maligna may be completely amelanotic.
Photodocumentation of the location, clinical extent and the exact biopsy site is helpful.