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  • The Mohs surgeon must become expert at interpreting tangential sections and distinguishing normal structures from tumor.

  • This may be challenging in certain situations depending on slide quality, section quality, and tumor subtype.

image Beginner Tips

  • An experienced technician is vital in the preparation of high-quality slides for evaluation.

  • Aggregates of BCC may be distinguished from adnexal structures by peripheral palisading and retraction artifact.

  • Examining multiple step sections may be helpful.

  • Look closely at lymphocytic aggregates, as they often surround tumor nests.

image Expert Tips

  • Histopathologic interpretation of certain tumor subtypes, such as infiltrative BCC, may be challenging.

  • These tumors should be differentiated from syringomas, desmoplastic trichoepitheliomas, and microcystic adnexal carcinomas.

image Don’t Forget!

  • The differential diagnosis for SCCIS (and occasionally SCC) includes actinic keratosis, extramammary Paget’s disease, inflamed seborrheic keratosis, verruca, pseudoepitheliomatous hyperplasia, and normal tangential sectioning of the epidermis.

  • With moderate-to-poorly differentiated SCCs, perineural invasion is of concern.

image Pitfalls and Cautions

  • SCC may appear as single-cell infiltrates, which can be difficult to distinguish on frozen sections without the aid of immunohistochemical stains.

  • There may be areas of focal necrosis, and often there is a prominent inflammatory lymphocytic (or lymphohistiocytic) infiltrate surrounding the malignant cells.

image Patient Education Points

  • Mohs surgery is unique in that one physician acts as both surgeon and pathologist.

  • Educating patients regarding the complexity of the process, and the logistics behind tissue processing, may ameliorate stress regarding wait times between stages.

image Billing Pearls

  • If a lesion is biopsied for the first time on the day of Mohs surgery and a frozen section interpretation is made on the tissue, CPT code 88331 should be utilized with modifier 59.

  • Billing for immunohistochemical stains is in addition to standard Mohs layer billing, and is generally billed on a per-specimen basis with code 88342 for the first antibody followed by 88341 for each additional antibody. If multiple separately identifiable antibodies are applied to the slide, use one unit of 88344.


Intraoperative histopathology is the cornerstone of Mohs micrographic surgery. The fundamental challenge for the Mohs surgeon is the accurate histologic identification and differentiation of benign versus malignant processes, as the goal of Mohs surgery is both to eradicate the cancer while concomitantly maximally conserving surrounding healthy tissue.

Often, features such as dense inflammation, chronic actinic damage with background solar elastosis, scar tissue, transected follicles, and other normal or benign cutaneous structures make differentiating benign versus malignant structures very challenging.


The skin consists of three distinct layers: the epidermis, the dermis, which is further divided into the superficial papillary and deeper reticular layers, and the subcutaneous fat.

The epidermis generally consists of four layers, with the exception of acral skin, which ...

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