Dermatology & Skin Cancer RACGP Exam: Clinical Guide

Dermatological presentations represent a highly tested clinical domain in both the RACGP Applied Knowledge Test (AKT) and Key Feature Problem (KFP) examinations. Because skin cancer and inflammatory rashes are highly prevalent in Australian primary care, examiners heavily blueprint questions focusing on precise diagnosis, biopsy selection, surgical margins, and topical pharmacotherapy.

Many candidates lose marks in this domain due to a lack of familiarity with Cancer Council Australia skin cancer guidelines, particularly regarding the exact excision margins required for invasive melanoma. Candidates also frequently make errors in the KFP when selecting appropriate biopsy techniques or prescribing topical corticosteroids of incorrect potency.

This clinical revision guide details high-yield dermatology and skin cancer presentations, maps them directly to current Australian guidelines, and highlights the examiner-reported pitfalls you must avoid to pass.

Melanoma Excision Margins: Cancer Council Standards

In Australian general practice, the management of suspected melanoma is a highly structured process. You must memorise the exact clinical excision margins required for both initial biopsy and subsequent wide local excision (WLE) based on histological Breslow thickness:

1. Initial Diagnostic Excision

• Clinical Margin: 2mm clinical margin of normal-appearing skin, with a brief cuff of subcutaneous fat.
• Rule: Never perform a wide local excision (e.g., 1cm or 2cm margins) as the initial diagnostic biopsy. Excision margins are determined solely by the histological Breslow thickness confirmed on the initial complete excision.

2. Wide Local Excision Margins

Once the diagnosis of melanoma is confirmed histologically, a definitive wide local excision must be performed. The clinical margins of normal skin surrounding the scar are determined as follows:

• Melanoma in situ: 5mm clinical margin.
• Invasive Melanoma less than 1.0mm Breslow thickness (T1): 10mm (1cm) clinical margin.
• Invasive Melanoma 1.0mm to 2.0mm Breslow thickness (T2): 10mm to 20mm (1cm to 2cm) clinical margin, depending on anatomical location and cosmetic factors.
• Invasive Melanoma greater than 2.0mm Breslow thickness (T3/T4): 20mm (2cm) clinical margin.

Diagnostic vs Treatment Biopsies: High-Yield Rules

Selecting the correct biopsy technique is a common clinical decision point in KFP questions. You must understand which biopsy is appropriate for specific lesions:

Suspected Melanoma

• Preferred Technique: Complete excisional biopsy with 2mm margins.
• When to perform partial biopsy (incisional, punch, or shave): Only if the lesion is extremely large, located on a sensitive anatomical site (e.g., eyelid, nose, lip), or where complete excision would cause severe cosmetic or functional impairment.
• Exam Trap: Under exam pressure, candidates often select "punch biopsy" as the routine investigation for a standard 6mm suspicious pigmented lesion on the back. This is a clinical error because partial biopsies carry a high risk of misdiagnosing or understaging melanoma due to sampling error.

Suspected Non-Melanoma Skin Cancer (BCC or SCC)

• Preferred Technique: Complete excisional biopsy or partial biopsy (punch or shave) is acceptable depending on lesion size, location, and diagnostic certainty.
• Punch Biopsy: Ideal for confirming the diagnosis of a large suspected squamous cell carcinoma (SCC) or basal cell carcinoma (BCC) before definitive surgical or non-surgical treatment.
• Shave Biopsy: Highly useful for superficial or nodular BCCs in low-risk sites, but must not be used if invasive SCC is suspected, as dermal invasion depth cannot be determined.

Topical Corticosteroid Potency Ladder

A high-yield pharmacology topic in the AKT is the topical corticosteroid potency ladder. Candidates must know the specific potencies of common topical steroids and apply them appropriately based on patient age, anatomical site, and disease severity.

Corticosteroid Potency Summary Table

Safe Prescribing Rules (Avoid the Traps)

• Face and Intertriginous Areas: Always prescribe mild potency corticosteroids for the face, eyelids, axillae, and groin. Utilizing potent or ultra-potent steroids in these areas carries a high risk of skin atrophy, striae, telangiectasia, and glaucoma/cataracts (if used near the eyes).
• Finger Tip Units (FTUs): Inform patients how to apply topical steroids using the Finger Tip Unit method (one FTU is the amount of cream squeezed from a standard tube from the distal crease to the tip of an adult index finger, sufficient to cover an area equal to two adult flat palms).
• Children: Limit the use of potent corticosteroids in children to short-term applications (e.g., 5 to 7 days) and avoid ultra-potent corticosteroids entirely unless directed by a specialist.

High-Yield Differential Diagnoses: GP Rashes

AKT scenarios often present clinical descriptions or images of rashes, requiring you to select the correct diagnosis or initial management:

1. Eczema (Atopic Dermatitis) vs Psoriasis

• Atopic Dermatitis: Pruritic, poorly demarcated, erythematous papules and plaques, occurring primarily on flexural surfaces (antecubital and popliteal fossae) in adults. Initial management focuses on emollients and topical corticosteroids.
• Psoriasis: Well-demarcated erythematous plaques covered with silvery-white scales, occurring primarily on extensor surfaces (elbows, knees, scalp). Auspitz sign (pinpoint bleeding after scraping scales) is classic. Managed with topical potent steroids combined with calcipotriol.

2. Tinea Incognito

• Definition: A dermatophyte (fungal) infection that has had its classic appearance modified due to the inappropriate application of topical corticosteroids.
• Clinical Picture: The typical active, scaling, raised border of tinea is lost, resulting in a poorly defined, erythematous, minimally scaling plaque.
• Exam Rule: If a patient presents with a worsening rash after using topical steroids, suspect tinea incognito. Cease the topical steroid immediately and perform skin scrapings for microscopy and fungal culture before starting topical terbinafine or clotrimazole.

Top 5 Examiner-Reported Dermatology Errors

Official RACGP public exam reports detail specific recurring mistakes that candidates make in skin-related questions:

1. 1
   Ordering Immediate Wide Local Excision for Melanoma

   Under exam pressure, candidates often select "wide local excision with 1cm margins" as the initial diagnostic step for a suspicious pigmented mole. The correct initial step is always a complete excisional biopsy with 2mm margins. Wide local excision is only performed after histological confirmation of melanoma, with margins determined by Breslow thickness.

2. 2
   Prescribing Potent Steroids for Facial Rashes

   Candidates frequently select a potent steroid cream (e.g., betamethasone dipropionate 0.05%) for facial seborrheic dermatitis or atopic dermatitis. Prescribing potent corticosteroids on the face is a severe clinical error due to the rapid development of skin atrophy, perioral dermatitis, and systemic absorption. Mild hydrocortisone is the correct clinical choice.

3. 3
   Inadequately Specifying Excision Margins in KFP Answers

   When asked to outline the surgical management of a confirmed 0.8mm invasive melanoma, candidates often write "excision of the lesion" or "excision with margins." KFP marking grids are highly specific; you must write the exact margin required ("10mm wide local excision" or "1cm margin") to secure marks.

4. 4
   Failing to Cease Steroids Before Fungal Scrapings

   When presented with a patient with suspected tinea incognito, candidates often order skin scrapings while instructing the patient to continue their hydrocortisone cream. Topical steroids must be ceased for at least 1 to 2 weeks before taking skin scrapings, as the anti-inflammatory effect suppresses fungal replication and leads to a high rate of false-negative culture results.

5. 5
   Confusing Squamous Cell Carcinoma (SCC) and Basal Cell Carcinoma (BCC) Urgency

   Candidates under exam pressure often treat BCC and SCC with the same clinical urgency. While BCC is slow-growing and rarely metastasizes, SCC carries a significant risk of local invasion and lymphatic metastasis, particularly on high-risk sites (lip, ear, scalp). SCC must be excised or referred with higher clinical priority.

Frequently Asked Questions