Although there are many possible ways to treat skin cancer, most skin cancers are effectively treated by complete excision followed by standard histologic evaluation to ensure clear margins. The bread loaf technique describes a common method of processing specimens in which multiple slices of tissue are taken perpendicular to the major axis of an excision and submitted for microscopic analysis. Although sections may only be approximately four microns thick (0.000004 meters), this method is associated with high cure rates for basal cell and squamous cell carcinoma. Some authors have stated that this technique assesses less than 1\% of the margins. We critically reviewed this assumption. While we confirm that the bread loaf technique often directly visualizes 1\% or less of the peripheral and deep margins when considering only the width of sections compared to the entire length of an excisional specimen of the excision, much useful additional information is gained as soon as clear sections are identified towards the tips of a typical excisional specimen. For tumors that tend to grow in a nodular or spherical arrangement such as nodular basal cell carcinoma or squamous cell carcinoma of keratoacanthomatous type, we show that a variable but significant portion of the margin may be considered sampled by proxy when slice faces are clear. We highlight the importance of understanding the principles involved in tissue sectioning in order to allow clinicians to make informed decisions on behalf of patients.

EGFR immunohistochemistry showed weak to strong expression in all cSCCs, colocalizing with the fluorescence signal. Low EGFR expression was found in the basal cell carcinoma. The keratoacanthomas showed no EGFR expression, as reported previously. 7 This proof-of-concept study demonstrates that FMI using the fluorescent tracer cetuximab-800CW can differentiate between tumour and adjacent tissue with high contrast. FMI detected all tumour-positive margins intraoperatively within seconds. FMI could be valuable for patients with a large or complex cSCC, where obtaining 100% intraoperative margin control is anticipated to be critical. However, it seems less useful for keratoacanthoma-like cSCC or in patients with excessive actinic damage. Future studies should determine the clinical value of FMI in surgery of high-risk cSCCs, ideally with new imaging methodologies that deliver improved depth information. 8Acknowledgments: We thank all patients who participated in this study. We thank Marloes van Kester for her help in designing the study, and Alet Leus and Aniek Lamberts for their help in recruiting patients. We thank the lab technicians from the Department of Pathology for their help in tissue processing.