Over recent years, SLNB recommendations in guidelines for cutaneous melanoma have changed considerably. Current and previous Dutch and international guidelines advise SLNB in melanoma stage IB or higher.14,20,21,22 Dutch guidelines from 2005 describe SLNB as promising and to reserve it for patients who want to be ‘informed as optimally as possible’.15 In 2007, the results of MSLT-1 were included, without rectification of advice from 2005.9 From 2012, SLNB has been advised in all patients with melanoma stage T1b or higher.16,17
We have shown that in The Netherlands, the use of SLNB for melanoma has increased, likely due to these evolving guidelines following the results of landmark studies. Enactment of SLNB increased from 39.1% in 2003 to 47.8% in 2014. SLNB guidelines were apparently not adequately adhered to in The Netherlands as only 39.7% of eligible tumors underwent SLNB. Although an obvious increasing trend has been observed since publication of the Dutch 2012 guidelines, even in more recent years, such as 2014, not even half of the eligible patients in fact underwent SLNB. When accounting for a possible delay to adoption of the 7th AJCC, SLNB enactment rose to 56.6% in 2014; however, there was an apparent 3-year delay from 2010 to 2013 due to patients with mitoses > 1/mm2 in whom SLNB was not performed. We found no studies on delays in the adoption of new guidelines in order to compare this finding.
We found female sex, older age, and melanoma in the head and neck region to be associated with non-enactment of SLNB. Huismans et al. assessed factors such as sex, age, socioeconomic status, BT, and hospital type influencing the use of SLNB in the north-eastern part of The Netherlands and found 42% of SLNB enactment in a total of 2413 patients with melanomas with a BT > 1 mm;23 however, compared with other nations, this percentage is low. Bilimoria et al.24 used US National Cancer Database (NCDB) data (n = 16,598) of stage I and II melanoma patients, between 2004 and 2005, and found a 48.7% enactment rate; Murtha et al.25 used Surveillance, Epidemiology, and End Results (SEER) data of 13,307 melanoma patients, between 2010 and 2012, with a 59.9% enactment rate; Moreno-Ramirez et al.26 analyzed 478 melanoma stage T1a–T4b patients in their center, with a 63.2% enactment rate; and Blakely et al.27 analyzed 865 melanoma patients, between 2005 and 2015, with a 93.2% enactment rate.
In The Netherlands, considerable regional practice variation of 22.5–56.6% was previously reported by Verstijnen et al.28 in patients with a BT > 1 mm. Interesting is the finding that guidelines for SLNB enactment are not adequately adhered to. Explanations for this non-adherence in general can vary greatly, ranging from lack of familiarity to low outcome expectancy or disagreement.29 For melanoma-specific adherence to guidelines, Kang and Wong and Varey et al.30,31 showed that for wide local excisions for melanoma, surgeons with a high melanoma caseload (> 30) were more likely to perform procedures concordant with the guidelines than those with a lower caseload. Another reason might be that Dutch guidelines have only advised on SLNB since 2005, and waited until 2012 to provide a recommendation, which is still not solid advice. Other than that, we do not have a plausible explanation, other than more defensive versus selective attitudes that may differ per country, with The Netherlands apparently being more selective and with a relatively low adherence rate of 39.7%. In line with this, Cormier et al.32 used US SEER data and showed almost 10% of stage IA melanomas are overtreated when it comes to lymph node therapy, probably reflecting a more defensive attitude. Another important finding is that for both SLNB indicated and non-indicated melanomas, female patients had significantly lower odds of receiving an SLNB, with an OR of 0.78 and 0.80, respectively. While Huismans et al. and Verstijnen et al. corroborate our findings, with ORs of 0.86 and 0.85, respectively, it is surprising that none of the previously mentioned studies have considered patient sex in their analyses. There are three possible sex-related explanations that may account for our lower OR; (1) female melanoma patients have other characteristics that we did not include in our multivariable model; (2) sex-specific decision making, e.g. when female patients more often decline SLNB, or medical information is perceived differently; or (3) clinician-specific sex bias in approaching and informing female patients. No studies have been conducted in melanoma patients to support any of these explanations, however there is some general evidence of physician sex-related differences in both decision making and approach to patients.33,34,35,36
Another finding supported by previous literature is that head and neck melanomas had the lowest percentage of SLNB enactment.23,24,25,29 This may be explained by the technical challenge associated with localization, and also as lymphatic drainage can occur to multiple or bilateral sites, with the sentinel lymph node itself being relatively small.37 Furthermore, our finding that older patients more often refrain from SLNB is also sustained by others.23,24,25,26,28 An explanation for this could be relevant comorbidities influencing prognosis in older patients, or a more conservative approach in view of a generally lower life expectancy.
Although we assessed multiple factors associated with SLNB use, we did not take into consideration socioeconomic status, race, and regional practice variation, which have been shown to influence SLNB use.23,24,25,28 Another limitation is that mitosis status was missing in 41.6% of T1 melanomas. As a mitotic rate ≥ 1/mm2 implies SLNB indication in the 7th AJCC, this might have influenced the number of eligible patients for SLNB. As opposed to Verstijnen et al. and Huismans et al., we included ulceration since its presence means SLNB indication for T1 melanoma.38 Other strengths of our study include our large sample size and generalizability due to the nationwide cohort.