Melanoma arising on the sole of the foot, characteristically in a so-called acral lentiginous growth pattern, is a rare clinical entity in Caucasians, accounting for only 2–8% of melanoma cases in white patients. However, in dark-skinned ethnic groups of African-American, Asian, or Hispanic descent, melanoma arises on the plantar surface of the foot in 35–90% of patients diagnosed with melanoma.
Although the metastatic potential (ability to spread) of these lesions is correlated significantly with the thickness of the primary melanoma, as it is for cutaneous melanomas arising elsewhere, these lesions are often diagnosed at later stages and therefore generally have a less favorable prognosis. The frequent delay in diagnosis of plantar lesions may in part be explained by their rarity as well as their unusual and infrequently examined anatomic location. In addition, the increased thickness of the epidermis of the plantar surface may soften and distort the characteristic clinical appearance of the melanoma. Even melanomas that appear to be flat may be revealed after adequate biopsy to be thick lesions. However, a major pitfall in the early diagnosis of this lesion is the failure to obtain a satisfactory biopsy specimen for histologic (microscopic anatomical) confirmation of malignancy. The extreme thickness of the plantar epidermis limits the utility of shave biopsy as a diagnostic method. In addition, the haphazard pigment pattern of these lesions also makes accurate diagnosis and assessment of lesion thickness by other techniques such as punch or even incisional biopsy also less likely to be successful.
The preferred method of biopsy for these difficult lesions is complete excisional biopsy. Definitive wound closure may be deferred until rapid histological diagnosis and margin inspection is complete. Once the diagnosis of melanoma is confirmed, the lesion is excised and staged according to guidelines established for other cutaneous primary melanomas of comparable thickness. Sentinel lymph node mapping and excision is recommended for those patients with melanomas >0.8 mm in Breslow thickness in the absence of clinically palpable regional nodes. Patients presenting with palpable nodal metastases and no other evidence of distant metastases undergo superficial inguinal (in the groin area) lymph node dissection. Dissection of the deep inguinal nodes is performed in patients with involvement of Cloquet’s node or extensive disease in the upper aspect of the femoral triangle.
Lesions confirmed to be melanomas on shave, punch, or incisional biopsy that approach or exceed 1.0 mm in thickness may be treated definitively as outlined above and do not require a preliminary excisional biopsy procedure.
Wound closure of the plantar surface requires special consideration. The exact location of the melanoma on the plantar surface, stage of disease, age, associated medical conditions, and lifestyle of the patient must be considered in the determination of wound closure. Defects on non-weight-bearing aspects of the plantar surface or those in patients with sedentary lifestyles, significant medical co-morbidities, or advanced metastatic disease may be most easily closed primarily or more commonly with split-thickness or full thickness skin grafts. Closure of defects on the weight-bearing surface of the plantar region in ambulatory patients is accomplished with a variety of flap reconstructive procedures. These include relatively straightforward cutaneous rotational or advancement flaps as well as more complex reconstructive procedures such as musculocutaneous free flaps with micro-vascular anastomosis (the connection of very small blood vessels performed under a surgical microscope). These latter procedures are usually performed with a plastic reconstructive surgeon who ideally has been involved in the care of the patient since the confirmation of the diagnosis of melanoma.