Mohs surgery provides complete margin evaluation of extirpated tumors, which can identify and remove subclinical extension of tumors and results in higher cure rates. Tumors can be excised with narrow margins, keeping the resulting wound as small as possible and expanding it only where there is residual tumor. The frozen section processing results in rapid turnaround time for slides allowing multiple stages to be performed at one appointment thus eliminating the need for multiple procedures on different days to excise residual tumor. The procedures are overall low risk, generally done using only local anesthesia with very low rates of adverse events, such as infection or bleeding.^7,8

As the incidence of skin cancers increases, so does the health care expenditure associated with treatment of these tumors.⁹ The cost of Mohs surgery (which includes the surgery and surgical pathology) is comparable to standard surgical excision done in an outpatient setting with permanent section processing, when considering the additional costs associated with pathology and re-excision for inadequate margin control.^10,11 Mohs is less expensive than excisions done in an ambulatory surgical center or hospital-based operating room and is more economical than excisions done using frozen tissue section margin control.¹⁰

However, Mohs does not provide wide margin excision around a tumor. Thus when it is considered important, such as with a high-risk tumor with perineural invasion, wider margins may be taken with Mohs or after Mohs slides are clear. In addition, tissue analysis during Mohs focuses on margin evaluation; if there is any question about the diagnosis or other features of the tumor that may affect patient management, the central tumor can be processed separately for further diagnostic information. For example, when treating melanoma in situ, it is imperative that the central tumor be processed to evaluate for invasive disease. Finally, although the entire tumor removal and reconstruction can be done in one day, given the processing time involved, patients must be able to tolerate a procedure that can take several hours. However, since the majority of Mohs cases are done under local anesthesia in an office setting, patients can be mobile and participate in reading or other activities that make the waiting period more tolerable.

Mohs surgery offers the dual benefits of 100% margin evaluation while being tissue sparing. However, it is adaptable and may need to be modified to treat tumors that have “skip areas,” difficult histology on frozen sections, deep invasion to bone or other structures not amenable to frozen sectioning, and a greater likelihood of recurrence or metastasis. These modifications may include taking a wide-margin first Mohs layer, taking an extra rim beyond the negative Mohs margins to be examined with permanent sections, and the use of rapid immunohistochemical stains.^12,13 When tumors are large and invasive, Mohs may be used to gain circumferential cutaneous margin control, and another member of the surgical team may clear the deep tissue, such as bone or other vital tissue margins. For potentially metastatic tumors, the multidisciplinary team may perform sentinel lymph node biopsy in coordination with treatment of the primary tumor with Mohs, consolidating and optimizing surgical management for the patient. Thus Mohs surgery plays a vital role in the multidisciplinary management of patients with complex tumors.

BCCs are the most common periorbital skin cancers, and occur most frequently on the lower eyelid, followed by the medial canthus, with a lower percentage occurring on the upper eyelid and lateral canthus.^2,17,18 There are a wide range of histologic subtypes of BCCs, and many tumors display mixed subtypes.¹⁹ These subtypes vary in their clinical presentation and histologic growth pattern, and an understanding of these subtypes aids in diagnostic acumen and appropriate surgical planning. Nodular BCC is the most common periorbital subtype that classically presents as a pearly papule or nodule with branching telangiectasias. These tend to have more indolent growth patterns; however, they can result in significant tissue destruction, especially when they are large or neglected (▶ Fig. 3.3, ▶ Fig. 3.4, ▶ Fig. 3.5). Other subtypes, including micronodular, sclerosing, morpheaform, infiltrative, and basosquamous BCCs typically behave more aggressively. Some of these aggressive tumors can be subtle clinically yet have significant subclinical extension and invasion. These tumors may present with eyelid contracture, indurated plaques, or madarosis (▶ Fig. 3.6, ▶ Fig. 3.7, ▶ Fig. 3.8). In contrast, superficial BCCs often present as scaly pink patches, whereas pigmented BCCs can be clinically misdiagnosed as nevi (▶ Fig. 3.9). Given the range of clinical presentations and the ability of BCCs to mimic benign processes, clinicians should have a low threshold to biopsy periorbital lesions as needed.

BCCs are generally indolent tumors; however, they can be more aggressive and have the potential to develop deep invasion, recurrence, and rarely, metastasis. Factors that contribute to aggressive BCCs include histologic subtype as discussed above, occurrence on the central face, perineural invasion, large tumors, long-standing tumors, and those that are incompletely excised.²⁰ Standard surgical excision results in incomplete tumor excision in approximately 16 to 25% of periorbital BCCs.^21,22 Given the risk of extensive invasion, including invasion into the orbit and rarely intracranially and the complexity of re-excising a persistent or recurrent tumor, the goal of initial treatment for periocular BCCs should be complete tumor removal with appropriate margin evaluation. Mohs surgery provides the highest cure rate of all treatment options for periorbital BCCs, and is the treatment of choice for these tumors.^23,24 Cure rates using Mohs for primary periorbital BCCs have been reported from 98 to 100%, and 92 to 94% for recurrent tumors (▶ Table 3.1).^4,25,26 When Mohs is not available or feasible, excision with en face frozen section margin control can be an effective treatment option; however, this process can lead to gaps in tumor margin evaluation and requires close collaboration between the surgeon and the pathologist to optimize tumor mapping.¹⁸ In contrast, as the Mohs technique evaluates margins in one plane of section, there is less risk of false-negative results with Mohs surgery.