Different histopathologic profiles and outcomes between sun-exposed BCC and non-sun-exposed BCC

Asian population is a low-risk group for basal cell carcinoma (BCC) and there is little data available in this setting. Sun-exposed BCC (SEBCC) may possess a different pathogenic mechanism from non-sun-exposed BCC (NSEBCC). To compare the histopathological profiles and outcomes between SEBCC and NSEBCC, and to assess the risk factors for tumor recurrences. Retrospective cohort study on 372 patients with pathologically diagnosed BCC from January 1, 1990 to August 31, 2017. Data were derived from a single medical center in Taiwan. SEBCC presented with higher Clark level and more high-risk factors for recurrence than NSEBCC. Nodular, micronodular, infiltrating/mixed infiltrating, basosquamous, and adenoid types were predominant in SEBCC, as superficial type in NSEBCC. Risk factors for recurrence included infiltrating/mixed-infiltrating subtypes and synchronous basosquamous cell carcinoma. No recurrence events were observed in NSEBCC. Our study showed an acceptable recurrence rate (4.2%) of the whole population after excision even under a smaller surgical margin width than suggested by current guidelines. SEBCC had a higher recurrence rate with a significantly different tumor characteristic from NSEBCC and a greater tumor depth than NSEBCC. A wider surgical margin in SEBCC than NSEBCC is suggested.

disease (CKD) (17.6% vs 7.1%, p < 0.001) and rheumatoid arthritis (RA) (5.9% vs 0%, p = 0.031) than with those in the SEBCC group. Details of the characteristics and comorbidities of the population are described in Table 1.
From the pathology reports, we identified 403 primary BCC lesions among 372 patients ( Table 2). The most commonly diagnosed sites were on the head and neck (88.1%), with the most common subsites being nose or surroundings of the nose (29.5%), followed by cheeks and zygomatic area (13.9%), then eyebrows or area surrounding the eye (12.4%). Of the remaining, 6.0% of the lesions were from the trunk, 4.5% from extremities, and 1.5% from the other sites.
Risk factors for multiple BCC. 22 (5.9%) patients were recorded with multiple BCCs, including 18 from the SEBCC group and 4 from the NSEBCC group. The associations between whether the lesions being sun-exposed or not, patient demographics, comorbidities, and the development of multiple BCCs using single and multivariate logistic regression analyses are shown in Table 3. Patients with dementia had a significantly higher association with the development of multiple BCCs (aOR=5.84, CI = 1.34-25.58, p = 0.019). Although patients with NSEBCC tended to have a higher association with developing multiple lesions than those with SEBCC, no significant differences were found. No significant associations regarding age, sex, diabetes mellitus, hypertension, chronic kidney disease, gout, and Arsenism were found to affect the development of multiple BCCs.
Histopathological profiles of BCC lesions. The  Although NSEBCC had a significantly wider surgical margin than that of SEBCC, no significant difference was found regarding the margin free rates. As for histopathologic types, nodular type was the most common type in both groups. Significant differences in the distribution of each subtypes were found between these two groups, with nodular, micronodular, infiltrating or mixed infiltrating, basosquamous, adenoid types being more predominant in SEBCC, while superficial types were more predominant in NSEBCC.
Risk factors for recurrent BCC. 17    www.nature.com/scientificreports www.nature.com/scientificreports/ was recorded. The associations between patient demographics, comorbidities, synchronous basosquamous cell carcinoma, and recurrence of BCC using Cox proportional hazards model are shown in Table 5. Patients with infiltrating or mixed-infiltrating subtype (aHR = 6.17, CI = 1.07-35.64, p = 0.042) and synchronous basosquamous cell carcinoma (aHR = 16.84, CI = 1.31-216.92, p = 0.030) had a significantly higher association with developing recurrent lesions of BCC. Age, sex, diabetes mellitus, hypertension, and end-stage renal disease had no significant association with developing recurrence after adjustment of covariates in the model. Survival curves regarding recurrence rate of BCC associated with synchronous basosquamous cell carcinoma and infiltrating or mixed-infiltrating type BCC are shown in Fig. 1.

Discussion
In this study, we analyzed the histopathological profiles and outcomes of BCC between sun-exposed and non-sun-exposed areas.
Our study showed that SEBCC had a greater depth and presented with more high-risk features of recurrence than did NSEBCC. The tendency of recurrence in SEBCC may be contributed to a predominant ratio in nodular, morpheaform, basosquamous, micronodular, and infiltrating or mixed infiltrating subtypes. Previous studies reported that anatomic location may favor the development of particular BCC subtypes [15][16][17][18][19] , and certain pathological subtypes were prone to having recurrences [20][21][22] .
In this study, the risk factors associated with tumor recurrence were synchronous basosquamous cell carcinoma and infiltrating or mixed-infiltrating subtype. Basosquamous cell carcinoma is an uncommon subtype that behaves with a high tendency of local recurrence or metastasis. Changing to basosquamous carcinoma in an initially ordinary BCC has also been reported 23 . Armstrong et al. found that incomplete and close excision margins, infiltrating and micronodular subtypes, and previous excision were strong risk factors for facial BCC recurrence 24 . In our study, no significant differences in the margin free rate between the two groups were found. Current guidelines suggested an excision with 4-mm clinical margins should be made in well-circumscribed, low-risk BCC lesions less than 2 cm in diameter 20,25 . Our study showed a relatively low recurrence rate after  www.nature.com/scientificreports www.nature.com/scientificreports/ excision comparing to previous study 26 , even under a smaller surgical margins in average (2.01 mm in SEBCC, 3.77 mm in NSEBCC).
From our results, we demonstrated that NSEBCC had a significantly larger mean dimension measured and a wider surgical margin than that of SEBCC. This suggests that surgeons always cut a wider surgical margin in NSEBCC due to the large size of the tumors. As nodular, micronodular, infiltrating or mixed infiltrating, basosquamous, and adenoid types were more predominant in SEBCC, superficial types were more predominant in NSEBCC. A total recurrence rate of 4.2% was noted in all patients with no recurrence in patients with NSEBCC. Further subgroup analysis was conducted regarding recurrence rate of the SEBCC and NSEBCC group. SEBCC had a higher recurrence rate than NSEBCC in subgroup which the surgical margin of tumor was less than or equal with 3 mm. No recurrence event was found in subgroup which the surgical margin of tumor was larger than 3 mm. In SEBCC, no significant difference of recurrence rate was found between groups that had surgical margin less than or above 3 mm. This supports our hypothesis that the tendency of recurrence in SEBCC might have to do with its histopathological nature or pathogenic mechanism, rather than its differences in surgical margin width in this study. Altogether, we conclude that unlike SEBCC, NSEBCC might not need a wider surgical margin as current guideline suggested and that a wider safe margin for SEBCC than NSEBCC is indicated.
The existence of NSEBCC may imply a different pathogenesis other than UV light exposure. Our study showed that patients with NSEBCC had a significantly higher association with RA and CKD. Tseng et al. found an increased association with NMSC in RA patients, especially in those using disease-modifying anti-rheumatic drugs 27 . Wang et al. reported that patients with CKD are at a higher risk of developing NMSC 28 , and previous studies implied that accumulation of uremic toxins, oxidative stress, and systemic inflammation in CKD patients may act as important roles in developing malignancy 29,30 . In our study, the different distribution observed in comorbidities between the two groups may suggest a relatively systemic pathogenesis involved in NSEBCC. However, these results should be carefully interpreted given that there was only a limited number of patients in these subgroups and the design of study was a retrospective cohort study rather than a case-control study.  www.nature.com/scientificreports www.nature.com/scientificreports/ Our study observed an association between immunosuppression status with the development of NSEBCC. Although previous study showed that immunosuppression act as a risk factor of developing non-melanoma skin cancer 31 , we did not observe a significantly higher risk for BCC recurrence among patients with immunosuppression. This discrepancy may result from the following reasons. First, our study design was different from previous studies, as our study aims at comparing patients with SEBCC and NSEBCC, rather than comparing the incidence rate of BCC between immunocompetent and immunosuppressive patients. Since NSEBCC had a lower recurrence rate than SEBCC, this may affect the recurrence rate observed in immunosuppressed patients. Second, the patient's demographics were different among each study, as our study focusing on Asian population.
Our study showed that patients with dementia had a higher association with developing multiple BCC tumors. A. J. Schmidt et al. reported that NMSC was associated with reductions in risks of dementia 32 . We assumed that the seemingly contrary results may be explained by delayed visits to the hospital or even negligence due to decreased self-awareness on skin condition in patients with cognitive impairment. Selection bias from the surgeons cannot be excluded as well. Further studies to elucidate the causation between dementia and multiple BCC should be done.
This study was conducted at a single medical center with retrospective chart review. To our knowledge, no previous studies have been published, comparing histopathological profiles, risks of multiple tumor and recurrence between SEBCC and NSEBCC in an Asian population. However, this study still has a number of limitations. First, the exact duration and intensity of UV light exposure were impossible to quantify by chart review and the results should be interpreted with caution due to other unmeasured factors attributing to developing multiple BCC or recurrence, such as occupation and lifestyle. Second, lesions that had not yet recurred or recurred after the study period would not have been included in the analysis. Since this was a single medical center study, loss of follow up or referral of patients to local hospitals and clinics may cause potential bias. Third, not all BCC lesions had a comprehensive description of histopathological features in the pathological report. Finally, due to the limited patient numbers and demographic profiles, the results should be carefully applied to other ethnic populations. Future investigation should aim at comparing the optimal surgical margin width in both SEBCC and NSEBCC group under a prospective study design with a larger number of patients and in different ethnic groups. All patients in this study were admitted to ward for examinations and surgical excision after receiving biopsy in outpatient department. Patients admitted without pathology to prove BCC with or without recurrence were excluded. Patients were also excluded if they had other skin lesions or tumors, or lost medical records. Both electronic and paper charts within this period of time were reviewed. Study design. Patients were divided into two groups: those with SEBCC, and those with NSEBCC. Sun-exposed body parts included head and neck, forearms, wrists, and lower legs. In order to compare the differences between the two groups, patients with both SEBCC and NSEBCC were excluded from this study. Figure 2 shows the patient selection flowchart.

Methods
Data were derived from chart review and pathology reports, including sex, age at diagnosis, relevant medical comorbidities, anatomic tumor site, surgical data, staging, and histopathological profile. Multiple BCC, recurrence, and metastasis were also recorded. Multiple BCC is defined as more than one tumor diagnosed at different anatomic sites synchronously or asynchronously. Recurring lesions were observed from the time period of January 1990 until the patient's latest visit to the hospital and are defined as a tumor re-occurring in the primary tumor site after excision.
Histopathological profile. Both the width and depth of the lesions were documented according to the pathology report. Pathological type, Clark level, tumor staging, surgical margin status, and high-risk features of recurrence were also obtained from the pathology report. The staging of BCC is defined according to the American Joint Committee on Cancer (AJCC) 7 th edition of cutaneous squamous cell carcinoma (cSCC) extrapolated in BCCs.
Statistical analysis. Differences in the distribution of demographic data, comorbidities and BCC characteristics were compared using the t-test for continuous variables, and the Pearson χ 2 test or Fisher's exact test for categorical variables. Logistic regression models were used to calculate crude and adjusted odds ratios (aOR) with 95% confidence intervals (CI) for the development of multiple BCC lesions. No significant interactions were found among variables that were selected for the multivariate logistic regression model. Cox proportional hazards model was used to estimate hazards ratios (HR) and 95% CIs for the recurrence of BCC lesions. SPSS statistical software, version 19.0 for Windows, was used for all data analysis. All methods were performed in accordance with the relevant guidelines and regulations. This retrospective study adhered to the tenets of the Declaration of Helsinki and was approved by the Institutional Review Board of Kaohsiung Veterans General Hospital (approved number: VGHKS19-CT7-02). A waiver of informed consent was granted by the approving Institutional Review Board.

Conclusion
In conclusion, our study found that SEBCC had a higher recurrence rate with a significantly different tumor characteristic from NSEBCC and a greater tumor depth than NSEBCC. As the superficial type is predominant in NSEBCC and no recurrence in NSEBCC patients was found in this study, we suggested NSEBCC might not need (2020) 10:7387 | https://doi.org/10.1038/s41598-020-64391-9 www.nature.com/scientificreports www.nature.com/scientificreports/ a wider surgical margin as current guideline suggested and a wider safe margin for SEBCC than NSEBCC is indicated. The risk factors for BCC recurrence included infiltrating or mixed-infiltrating subtype and the existence of synchronous basosquamous cell carcinoma. Patients with NSEBCC had a significantly higher association with chronic kidney disease and rheumatoid arthritis than with those in the SEBCC group.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.