Lack of consensus among Mohs surgeons diagnosing in situ carcinoma

While most Mohs surgeons agree on diagnostic differentiations between actinic keratosis and squamous cell carcinoma in situ, variability exists in the interpretation of in situ carcinoma on histological frozen sections, according to a study.

“Cutaneous [squamous cell carcinoma] (SCC) is a malignant neoplasm of epidermal keratinocytes that exists on a continuum between the mild atypia of early actinic keratosis (AK) and the disordered architecture, nuclear pleomorphism, and full thickness atypia that characterize squamous carcinoma in situ (SCCIS),” Tian Hao Zhu, MD, of the department of dermatology at the University of Virginia, and colleagues wrote. “While there exist interobserver differences, general concordance amongst Mohs surgeons in distinguishing AK from SCCIS is paramount when evaluating the need to take another Mohs stage.”

In this study, researchers assessed interrater concordance among Mohs surgeons in distinguishing AK from SCCIS when evaluating Mohs frozen sections and the histologic criteria used in making the distinction.

Members of the American College of Mohs Surgery responded to an electronic survey comprised of 17 digital cases and a corresponding questionnaire. A total of 67 responses were obtained from 55 Mohs surgeons, 6 dually trained Mohs surgeons and dermatopathologists and 6 fellows-in-training.

Of the 17 cases, 9 were originally diagnosed as SCCIS, 6 as AK, 1 as SCCIS with superficially invasive dermal component and 1 as AK with follicular extension.

Results showed a weak agreement between raters compared with the standard, with a Fleiss kappa of .26 overall, .34 for AK and .37 for SCCIS.

Within the SCCIS category, 61.9% of raters agreed with the diagnosis, 13.8% selected AK and 7.5% selected other diagnoses.

The AK category showed worsened agreement with only 56.7% of raters agreeing with the diagnosis while 23.1% selected SCCIS, 17.7% selected AK with focal SCCIS and 2.3% selected other diagnoses.

Concerning post-diagnosis management, the majority of raters agreed to take another Mohs stage for SCCIS (79.9%) and for AK with focal SCCIS (77.2%). On the other hand, 67.9% of raters elected not to take another Mohs stage for AK.

Raters were also asked to share the histologic features most considered in their diagnosis. Results showed that atypia involving full thickness of epidermis and abnormal maturation were in the top 3 features for both AK and SCCIS.

“While some variables such as size and tumor presence are more easily measured, histological differentiation can be quite subjective,” Zhu said. “Ongoing learning and consensus building among Mohs surgeons and trainees can aid in quality patient care, even if there may not be agreement on every case.”