Diagnosing And Treating A Pigmented Lesion On The Hallux

Naohiro Shibuya, DPM, MS, FACFAS

A 66-year old Caucasian male with a history of controlled diabetes, hypertension, hyperlipidemia, psoriasis, obstructive sleep apnea and post-traumatic stress disorder received a referral by dermatology to our surgery department. The referral was for surgical treatment of a suspicious lesion in the left hallux.

   The patient initially noticed the lesion approximately two months prior and it had been growing in size since then. He was worried about malignancy since his father died of malignant melanoma. He did not recall any trauma. He did not have any associated pain or discomfort. The patient was taking bupropion (Wellbutrin, GlaxoSmithKline), diclofenac (Voltaren, Novartis), fexofenadine (Allegra, Chattem), gabapentin (Neurontin, Pfizer), gemfibrozil (Lopid, Pfizer), metformin, metoprolol (Lopressor, Novartis) and pravastatin (Pravachol, Bristol-Myers Squibb) daily. He was allergic to codeine, cephalexin and fish oil. He denied any constitutional symptoms.

   The clinical examination revealed a hyperpigmented lesion of 2 x 3 mm on the medial aspect of the left hallux. The lesion was mildly raised and had three distinct hyperpigmented areas of different shades of red and purple. There was no satellite lesion. The lesion was asymmetrical and had an irregular border.

Key Questions To Consider

1. What questions should a physician ask the patient?
2. What is the differential diagnosis?
3. How common is melanoma in the foot?
4. What are the risk factors for melanoma?
5. What is the proper biopsy technique?

Answering The Key Diagnostic Questions

1. Have any of your family members had melanoma? What is your racial background? Have you had any puncture wound in your feet? Has the lesion grown lately?
2. The differential diagnosis includes: melanoma, dysplastic nevus, seborrheic keratosis, solar lentigines, dermatofibroma, pigmented basal cell carcinoma and verruca.
3. Melanoma in the feet is rare in light-skinned patients. However, in patients with pigmented skin, it is very common. Identification of risk factors is important.
4. Known foot specific risk factors include dark skin, a history of puncture trauma, a family history of melanoma and a high nevus count.
5. Excisional, full-thickness biopsy of 1 mm border is recommended for a smaller lesion and incisional (typically punch) biopsy is recommended for a large lesion.

A Guide To The Differential Diagnosis

Dysplastic nevus. It is also referred to as an atypical mole. Having multiple atypical moles is associated with development of melanoma. Clinically, this resembles melanoma. Biopsy rules out the malignant lesion.

   Seborrheic keratosis. This is a very common, raised or flat benign lesion, which often appears as black, gray, pale or purple in older patients. It can mimic melanoma or verruca. It is benign and no treatment is necessary. The lesion only affects the epidermis.

   Solar lentigo. It is also known as a liver spot, sun-induced freckle or senile lentigo. It is very common in older patients. The lesions are induced by ultraviolet light. Therefore, they are not common in the foot for those who live in the northern part of the United States. Lentigo simplex, however, is not caused by UV light and can develop during childhood. Both are benign.

   Dermatofibroma. This nodular skin lesion is one of the most common skin lesions that occur in the extremities of women. It is normally asymptomatic and benign. No treatment is necessary unless the condition is painful.

   Pigmented basal cell carcinoma. A basal cell carcinoma can be pigmented and mimic a melanoma. Pigmentation of a basal cell carcinoma is more common in Asians. It is rare in Caucasians.

   Verruca. There are many instances in which clinicians may misdiagnose a malignant lesion as verruca. A high index of suspicion is necessary when a verruca lesion appears to be atypical or does not respond to typical treatments.

   Melanoma. The patient went to surgery for an excisional biopsy. Since we were suspicious of melanoma, we decided to obtain a full-thickness specimen with at least 1 mm border around the lesion. Due to the location of the lesion, we anticipated that a circular open wound of approximately 5 mm in diameter would not close primarily without tension. Therefore, a complex skin closure was necessary. Since elliptical incisions with wedge resection would create tension in this tight area, we performed a double S-plasty.

   The specimen came back as a “benign vascular lesion, consistent with cavernous hemangioma.” There was no evidence of malignancy or a melanocytic lesion.

A Closer Look At Melanoma In The Foot

According to the U.S. Central Cancer Registry, the incidence of invasive melanoma in the United States is 59,695 per year.1 Of those, only 0.02 percent happen in the foot.1 Other population studies agree that the incidence and prevalence of melanoma in feet are very small.2-5 However, some smaller studies, conducted in a population with majority of the people having pigmented skin, show a much higher proportion of melanoma in the foot.6-8 It is therefore important to have a higher index of suspicion when a patient has darker skin tone.

   While typical risk factors for melanoma for rest of the body include sun/UV exposure and light skin, these are not risk factors for foot melanoma. Rather, pigmented skin and a history of foot trauma are the major risk factors for foot melanoma. It has been documented that people with a history of puncture wounds to the foot are five to 40 times as likely to develop foot melanoma in comparison to those without a history of trauma.9 Besides these risk factors, a family history of melanoma and a high nevus count are associated with development of foot melanoma. Unlike the rest of the body, the acral lentiginous type of melanoma is more common in the foot than the superficial spreading type.

Key Insights On Prevention And Treatment

Physicians often use the ABCD screening tool to screen melanoma in the rest of the body. Asymmetry, irregular Border, Color and Diameter of more than 6 mm are good indicators for melanoma. The “E” criteria, which stands for “Evolving over time,” has been added in recent years to improve the specificity of diagnosis.10 Having more than one of these five criteria provides up to 97 percent sensitivity while having all five criteria indicates almost 100 percent specificity for melanoma.10 While these criteria are not foot and ankle specific, they are useful for patients to self-screen.

   Screening recommendations of melanoma, however, are not consistent among different organizations. The American Academy of Dermatology recommends frequent self-screening and once a year professional screening.11 The Institute of Medicine simply recommends to “be alert” without any specific guideline.12 On the other hand, the United States Preventive Services Task Force states that there is no evidence to support the need for periodic checkups.13

   Some of the screening guidelines are based on few randomized clinical trials comparing populations with or without periodic melanoma screening. Though the results showed that more melanomas were detected in those who were routinely screened, there was no difference in survival or mortality between the groups.14 Therefore, the need for routine screening was under question.

   However, in general, melanomas found in the foot have a worse prognosis than melanomas in the rest of the body simply because they can advance undetected for a longer period of time. Therefore, I caution against generalizability of those randomized clinical trials to the foot melanoma.

   A biopsy is recommended when one is suspicious of melanoma. When lesions are small, an excisional biopsy with 1 mm of normal appearing border is recommended. When the lesion is large, a punch biopsy is recommended. The purpose of biopsy is not only to rule out malignancy but also to predict prognosis if the lesion is malignant. It is known that thickness of melanoma is highly correlated with the prognosis. Therefore, a full-thickness sample is necessary for the excisional or incisional biopsy to provide the depth of the lesion. A shave biopsy is usually not recommended.

   For more definitive treatment and excision of the lesion after an incisional biopsy if the lesion is malignant, one should excise the lesion with the recommended border size. The size of the border is dependent on the depth of the melanoma measured in the initial biopsy. If the thickness of the melanoma is less than 2 mm, excision of at least 1 cm of normal appearing border is recommended. If the lesion is more than 2 mm deep, a border of more than 2 cm is needed. In the foot and ankle, this often means some degree of amputation.

   Identifying depth and excising a lesion according to the guideline is important. However, clinicians should not overlook consulting an appropriate specialist, especially on high melanoma grade cases. Oncologists will normally follow the patient with a PET scan and/or sentinel biopsy depending on the melanoma grade.

   Dr. Shibuya is an Associate Professor of Surgery at the Texas A&M Health Science Center College of Medicine. He is the Acting Chief of the Section of Podiatry with the Central Texas VA Health Care System. He is on the staff with the Scott and White Health Care System. Dr. Shibuya is a Fellow of the American College of Foot and Ankle Surgeons.

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   For further reading, see “When Should You Biopsy?” in the June 2013 issue of Podiatry Today.

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