Mohs surgery Study Guide

📖 Core Concepts Mohs micrographic surgery – a same‑day, microscopically controlled excision where each removed layer is examined immediately (frozen‑section) to confirm clear margins before taking more tissue. Complete margin assessment – 100 % circumferential peripheral and deep margins are evaluated, unlike sampling methods that look at only a fraction. Tissue‑sparing principle – because cancer is removed only where it exists, the remaining healthy tissue is maximally preserved, which is crucial in cosmetically or functionally sensitive sites. Cost‑effectiveness – combines excision and histopathology in one visit, often cheaper than separate surgical removal + conventional pathology. 📌 Must Remember Cure / recurrence rates Primary BCC: 5‑yr recurrence 1–3.3 % → cure 96.7–99 %. Recurrent BCC: recurrence 5.2–5.6 % → cure 94.4–94.8 %. High‑risk facial BCC (primary): recurrence 2.1 % → cure ≈ 98 %. Cutaneous SCC: cure 97–99 % (primary). Melanoma in situ / thin invasive melanoma: cure 97–99 % (4–5 yr) and ≈ 99.86 % respectively. Indications – high‑risk BCC/SCC in face, ears, scalp, neck, genitalia, hands/feet; recurrent/aggressive tumors; early melanoma (lentigo maligna, thin invasive) when tissue preservation matters; rare tumors requiring complete margin control (e.g., DFSP, Merkel‑cell carcinoma). Contraindications – small, low‑risk, well‑defined tumors in non‑critical sites; cases where tissue is sent to an outside pathologist (standard excision); patients unable to tolerate local anesthesia or with uncontrolled bleeding disorders (relative). Comparison of cure rates Mohs ≈ 96–99 % vs. standard excision 90–95 % (small tumors) down to ≈ 70 % for large/narrow‑margin lesions. Radiation therapy ≈ 90–95 % for ≤2 cm lesions, 85–90 % for larger lesions. Technical nuance – serial sectioning every ≈ 100 µm dramatically improves detection compared with only two sections. 🔄 Key Processes Local anesthesia → patient prepared. Precise tissue removal – surgeon maps the tumor’s outline on the skin. Specimen mapping & orientation – tissue is flattened, inked, and labeled to retain anatomical orientation. Frozen‑section preparation – tissue is frozen, cut into thin layers, and stained. Microscopic margin review – pathologist checks 100 % of peripheral and deep margins for cancer cells. Decision point Clear margins: wound closure/reconstruction. Positive margins: surgeon removes an additional “layer” precisely where cancer persists; return to step 3. Repeat until all margins are negative, then finalize reconstruction. 🔍 Key Comparisons Mohs vs. Standard Excision Margin control: 100 % vs. 4–6 mm empirical margin (sampling only). Cure rates: ≈ 96–99 % vs. 90–95 % (drops to 70 % for large/poorly placed lesions). Mohs vs. Radiation Therapy Tissue preservation: maximal with Mohs; radiation may cause dermatitis, pigmentation changes. Cure for ≤2 cm: similar (≈ 90–95 %) but Mohs offers immediate histologic confirmation. Mohs vs. Bread‑Loaf Sectioning Margin sampling: 100 % (Mohs) vs. limited “bread‑loaf” slices (≈ 1–2 % of margin). False‑negative risk: higher with bread‑loaf. ⚠️ Common Misunderstandings “Mohs is only for basal‑cell carcinoma.” – Also indicated for SCC, early melanoma, and many rare skin cancers. “If a pathologist is involved, it’s still Mohs.” – True Mohs requires the surgeon’s own immediate frozen‑section interpretation; otherwise it’s standard excision. “Mohs guarantees 100 % cure.” – Cure rates are high but not absolute; recurrence can still occur, especially with incomplete epidermal sampling. “Mohs is always more expensive.” – When factoring the combined cost of excision + separate pathology, Mohs is often cost‑effective. 🧠 Mental Models / Intuition “Puzzle piece” model: Imagine the tumor as a puzzle; each layer you shave off is a piece that is examined. You keep removing only the pieces that still have “puzzle pieces” (cancer) until no pieces remain. “Margin as a fence”: The 100 % margin assessment is like a fence that completely surrounds a property—no gap is left unchecked. “Layer‑by‑layer excavation”: Think of the process as digging down in thin slices, checking each slice for hidden “treasure” (cancer) before deciding how much more to dig. 🚩 Exceptions & Edge Cases Relative contraindications – severe bleeding disorders, uncontrolled anticoagulation, significant immunosuppression, or need for extensive reconstruction beyond Mohs scope. Small, low‑risk lesions in non‑critical areas are better suited to standard excision. Large lesions (>2 cm) on the face may still be considered for Mohs, but radiation or staged excision can be alternatives depending on patient factors. Melanoma in situ requires special immunohistochemical stains; without them, margin assessment can be unreliable. 📍 When to Use Which Choose Mohs when: Tumor is high‑risk (size, histology, recurrent) AND located in a cosmetically or functionally critical site. Tissue preservation is a priority (e.g., eyelids, nose, ears, genitalia). Precise margin control is essential (rare aggressive tumors, melanoma in situ). Choose Standard Excision when: Lesion is small, low‑risk, well‑circumscribed, and in a non‑critical area. Patient cannot tolerate the intra‑operative waiting time or local anesthesia. Choose Radiation when: Patient is a poor surgical candidate, or lesion size >2 cm in a location where surgery would cause unacceptable morbidity. 👀 Patterns to Recognize High recurrence → non‑complete margin assessment (e.g., bread‑loaf, inadequate peripheral sampling). Immunohistochemical staining appears in prompts about melanoma in situ or pigmented lesions. Serial section interval (100 µm) is often mentioned when discussing improved detection vs. limited sections. Cure rate disparity between Mohs (≈ 97 %+) and standard excision (drops below 80 % for large or narrow‑margin tumors). 🗂️ Exam Traps “Four‑mm margin equals Mohs.” – Wrong; Mohs does not rely on a fixed numeric margin. “Mohs is contraindicated in immunosuppressed patients.” – Only a relative contraindication; many immunosuppressed patients still receive Mohs. “Bread‑loaf sectioning assesses 100 % of the margin.” – False; it samples a tiny fraction, leading to higher false‑negatives. “All facial skin cancers require Mohs.” – Incorrect; low‑risk, well‑defined lesions on the face can be safely managed with standard excision. “Mohs always costs more than radiation.” – Misleading; when you account for combined surgery + pathology, Mohs is frequently more cost‑effective. --- Use this guide to quickly recall the why, when, and how of Mohs micrographic surgery—and to dodge common pitfalls on your exam.