Background
Prospective single-arm study establishing CTV margin recommendations for radiotherapy planning of non-melanoma skin cancer (NMSC: BCC and SCC). 159 lesions in 150 consecutive patients treated with surgical excision + intraoperative frozen sections at University of Toronto. Preoperatively, concentric 5-mm incremental marks were placed around the gross lesion border; microscopic tumor extent was calculated at clear margin resection.
Interventions and follow up
Arm A: No treatment arms — prospective margin mapping study
Arm B: N/A
Primary endpoint: Distance of microscopic tumor extension beyond gross lesion border
mFollow up: N/A (surgical margin mapping study)
Arm B: N/A
Primary endpoint: Distance of microscopic tumor extension beyond gross lesion border
mFollow up: N/A (surgical margin mapping study)
Results
Mean microscopic extension: 5.3 mm (range 1–15 mm) beyond gross lesion
CTV recommendation (BCC <2 cm): 10 mm margin
CTV recommendation (BCC ≥2 cm): 13 mm margin
CTV recommendation (SCC <2 cm): 11 mm margin
CTV recommendation (SCC ≥2 cm): 14 mm margin
95% coverage threshold: Margins sized to cover ≥95% of microscopic disease
CTV recommendation (BCC <2 cm): 10 mm margin
CTV recommendation (BCC ≥2 cm): 13 mm margin
CTV recommendation (SCC <2 cm): 11 mm margin
CTV recommendation (SCC ≥2 cm): 14 mm margin
95% coverage threshold: Margins sized to cover ≥95% of microscopic disease
Adverse events
Main adverse events: N/A (surgical margin study; RT toxicities addressed in dose/fractionation studies)
Conclusions
CTV margins of 10–14 mm (depending on histology and size) are required to cover 95% of microscopic disease extension beyond gross BCC/SCC. Larger tumors and SCC histology require larger margins. These data provide the evidence base for CTV expansion in RT planning for NMSC.
Key Limitations
Key Limitations: Single institution; predominantly head/neck location may not generalize to truncal or extremity NMSC; frozen section methodology may underestimate margins compared to permanent sections; does not address high-risk features (perineural invasion, poorly differentiated) separately; potential selection bias in surgically treated population.
Clinical Context
These margin recommendations (10–14 mm CTV) are incorporated into ASTRO guidelines and most institutional RT planning protocols for BCC/SCC. For RT planning, CTV = GTV + 10–14 mm, then PTV adds another 3–5 mm per institutional motion/setup protocol. The larger margins required for SCC vs BCC reflect its more infiltrative growth pattern.
References