A skin biopsy removes a small section of a suspicious growth or lesion for examination under a microscope. This procedure is the only definitive way to diagnose skin cancers (basal cell carcinoma, squamous cell carcinoma, and melanoma) and various other skin conditions. When a mole (nevus) changes size, shape, or color, a biopsy is typically ordered to determine its nature. The most common question relates to the “shave” technique and whether it removes the entire lesion. Understanding the mechanics of the shave biopsy is important for appreciating its role in skin diagnostics.
Understanding the Shave Biopsy Procedure
The shave biopsy samples tissue from the most superficial layers of the skin. This procedure uses a sharp instrument, such as a flexible blade, to remove a tissue specimen parallel to the skin’s surface. The cut is typically limited to the epidermis and the upper portion of the dermis, essentially skimming the lesion off the skin. This method is often chosen for raised lesions, such as seborrheic keratoses or small, non-melanoma skin cancers. The technique is fast, minimally invasive, and usually does not require sutures, which reduces procedure time and potential scarring.
Shave Biopsy and Completeness of Removal
A shave biopsy generally does not remove the entire mole, especially if the mole extends deeply into the skin’s layers. Most moles are collections of pigment cells (melanocytes) residing at the junction between the epidermis and dermis or within the dermis itself. Since the shave technique is a superficial sampling method, it often only captures the top portion of a deeper mole.
While a shave may remove a superficial lesion entirely, its primary function is diagnostic sampling rather than complete therapeutic removal. For any lesion suspected of being a melanoma, the depth limitation of the shave method is a concern. If the mole is deeper than the cut, a portion of the lesion, including the deep margin, remains in the skin. Studies show that a substantial percentage of shave biopsies result in positive deep margins, meaning the deepest edge of the lesion was cut through.
The remaining tissue from an incompletely removed mole is referred to as residual nevus or residual melanoma if the diagnosis is malignant. This retained tissue can sometimes regrow, leading to a recurrent mole that is microscopically difficult to distinguish from a new cancer. If a shave biopsy returns a diagnosis of a dysplastic (atypical) or malignant lesion, a second procedure is often required to ensure the complete removal of any remaining cells.
Diagnostic Challenges of Incomplete Sampling
The most significant consequence of incomplete sampling relates directly to the accurate staging of melanoma. Melanoma prognosis and treatment depend heavily on the Breslow depth, which is the precise measurement of the tumor’s thickness from the top of the skin to its deepest point. If a shave biopsy cuts through the deepest part of the melanoma, the pathologist cannot measure the true, full thickness of the tumor. This transection results in an underestimation of the Breslow depth, which can lead to the incorrect staging of the cancer.
A partial sample might suggest a thin, low-risk melanoma, when the remaining, deeper portion would place the cancer into a higher-risk category. This potential for inaccurate staging means that even after a positive diagnosis, a patient will likely require a subsequent wide local excision to remove the remaining tissue and allow for a definitive depth measurement. For thinner melanomas (less than one millimeter thick), the shave technique has been shown to underestimate the final Breslow depth in a small percentage of cases, sometimes requiring further surgery. The presence of a positive deep margin complicates management, as it results in the loss of complete pathological information necessary for precise treatment planning.
Other Biopsy Techniques and Their Purpose
When a lesion is highly suspicious for melanoma, or when a full-thickness sample is required, other biopsy techniques are employed to overcome the depth limitation of the shave method.
Punch Biopsy
The punch biopsy utilizes a circular cutting tool to remove a narrow, cylindrical core of tissue. This technique penetrates through the entire thickness of the skin, including the epidermis, dermis, and sometimes the superficial fat. This provides a complete cross-section of the lesion for analysis.
Excisional Biopsy
For lesions where the goal is both diagnosis and definitive removal with clear margins, an excisional biopsy is the preferred method. This procedure involves using a scalpel to remove the entire lesion, along with a small, surrounding border of normal-appearing skin. By taking a full-thickness, elliptical sample that extends into the subcutaneous fat, the excisional biopsy ensures the pathologist can accurately measure the full depth and determine if the margins are clear of disease.