Skin biopsy

Skin biopsy is a biopsy technique in which a skin lesion is removed and sent to the pathologist to render a microscopic diagnosis. It is commonly performed by dermatologists. Skin biopsies are also done by family physicians, internists, surgeons, and other specialties. However, performed incorrectly, and without appropriate clinical information, a pathologist's interpretation of a skin biopsy can be severely limited. There are four main types of skin biopsies: shave biopsy, punch biopsy, excisional biopsy, and incisional biopsy. The choice of the different skin biopsies is dependent on the suspected diagnosis of the skin lesion. Like most biopsies, patient consent and anesthesia (usually lidocaine injected into the skin) are prerequisites.

Different types of skin biopsies:

1. Shave biopsy:  This is done with either a small scalpel blade, a curved razor blade, or a broken piece of "safety" razor. The technique is very much user skill dependent, as some surgeons can remove a small fragment of skin with minimal blemish using any one of the above tools, while other have great difficulty securing the devices. Ideally, the razor will shave only a small fragment of protruding tumor and leaving the skin relatively flat after the procedure. Hemostasis is obtained using light electrocautery, Monsel solution, or aluminum chloride. This is the ideal method of diagnosis for basal cell cancer. It can be used to diagnose squamous cell carcinoma and melanoma-in-situ, however, the doctor's understanding of the growth of these last two cancers should be considered before one uses the shave method. The punch or incisional method is better for the latter two cancers as false negative is less likely to occur (i.e. calling a squamous cell cancer an actinic keratosis or keratinous debris). Hemostasis for the shave technique can be difficult if one relied on electrocautery alone. A small "shave" biopsy often ends up being a large burn defect when the surgeon tries to control the bleeding with electrocautery alone. Pressure dressing or chemical astringent can help in hemostasis in patients taking anticoagulants.

2. Punch biopsy:  This is done with a round shaped knife ranging in size from 1mm to 8 mm. Some punch biopsies are shaped like an ellipse, although one can accomplish the same desired shape with a standard scalpel. The 1 mm and 1.5 mm punch are ideal for locations where cosmetic appearance is difficult to accomplish with the shave method. Minimal bleeding is noted with the 1 mm punch, and often the wound is left to heal without stitching for the smaller punch biopsies. Disadvantage of the 1 mm punch is that the tissue obtained is almost impossible to see at times due to small size, and the 1.5 mm biopsy is preferred in most cases. The common punch size use to diagnose most inflammatory skin condition is the 3.5 or 4 mm punch. Ideally, the punch biopsy include the full thickness skin and subcutanous fat in the diagnosis of skin diseases. The punch biopsy is preferred over the shave biopsy for the diagnosis of squamous cell carcinoma and for melanomas. One or two sutures are required to close most punch biopsies with the exception of the smallest punches. Two "dog ear" defects can result in punch biopsies much larger than 5 mm, thus an incisional biopsy is preferred on larger lesions.

3. Incisional biopsy:    When a cut is made through the entire dermis down to the subcutanous fat. A punch biopsy is essentially an incisional biopsy, except it is round rather than elliptical as in most incisional biopsies done with a scalpel. Incisional biopsies can include the whole lesion (excisional), part of a lesion, or part of the affected skin plus part of the normal skin (to show the interface between normal and abnormal skin). Incisional biopsy often yield better diagnosis for deep pannicular skin diseases and more subcutanous tissue can be obtained than a punch biopsy. Long and thin deep incisional biopsy are excellent on the lower extremities as they allow a large amount of tissue to be harvested with minimal tension on the surgical wound. Advantage of the incisional biopsy over the punch method is that hemostasis can be done more easily due to better visualization. Dog ear defects are rarely seen in incisional biopsies with length at least twice as long as the width.

4. Excisional biopsy:  This is essentially the same as incisional biopsy, except the entire lesion or tumor is included. This is the ideal method of diagnosis of small melanomas (when performed as an excision). Ideally, an entire melanoma should be submitted for diagnosis if it can be done safely and cosmetically. This "excisional" biopsy is often done with a narrow margin to make sure the deepest thickness of the melanoma is given before prognosis is decided. However, as many melanoma-in-situs are large and on the face, a physician often chose to do multiple small punch biopsies before committing to a large excision for diagnostic purpose alone. Many prefer the small punch method for initial diagnostic value before resorting to the excisional biopsy. An initial small punch biopsy of a melanoma might say "severe cellular atypia, recommend wider excision". At this point, the clinician can be confident that an excisional biopsy can be performed without risking committing a "false positive" clinical diagnosis.

5. Curettage biopsy: This can be done on the surface of tumors or on small epidermal lesions with minimal to no topical anesthetic using a round curette blade. Diagnosis of basal cell cancer can be made with some limitation, as morphology of the tumor is often disrupted. The pathologist must be informed about the type of anesthetic used, as topical anesthetic can cause artifact in the epidermal cells.

6. Fine needle aspirate: This is done with the rapid stabbing motion of the hand guiding a needle tipped syringe and the rapid sucking motion applied to the syringe. It is a method used to diagnose tumor deep in the skin or lymphnodes under the skin. The cellular aspirate is mounted on a glass slide and immediate diagnosis can be made with proper staining or submitted to a laboratory for final diagnosis. A fine needle aspirate can be done with simply a large bore needle and a small syringe (1 cc) that can generate rapid changes in suction pressure. Fine needle aspirate can be used to distinguish a cystic lesion from a lipoma. Both the surgeon and the pathologist must be familiar with the method of procuring, fixing, and reading of the slide. Many center have dedicated team used in the harvest of fine needle aspirate.

A pathology report is highly dependent on the quality of the biopsy that is submitted. It is not unusual to miss the diagnosis of a skin tumor or a skin biopsy due to a poorly performed or inappropriately performed skin biopsy. The clinical information provided to the pathologist will also affect the final diagnosis. An example would be a rapidly growing dome shaped tumor of the sun exposed skin. Despite doing a large wedge incision, a pathologist might call the biopsy keratin debris with characteristics of actinic keratosis. But provided with an accurate clinical information, he/she might consider the diagnosis of a well differentiated squamous cell carcinoma or keratoacanthoma. It is not infrequent for two, three or more biopsies to be performed by different doctors for the same skin condition, before the correct diagnosis is made on the final biopsy. The method, depth, and quality of clinical data will all affect the yield of a skin biopsy. For this reason, doctors specializing in skin diseases are invaluable in the diagnosis of skin cancers and difficult skin diseases. Specific stains (PAS, DIF, etc), and certain type of sectioning (vertical and horizontal) are often requested by an astute physician to make sure that the pathologist will have all the necessary information to make a good histological diagnosis.