WHAT IS ALREADY KNOWN ON THIS TOPIC

Based on two large prospective trials, sentinel node biopsy is the recommended treatment in unifocal, primary vulvar squamous cell carcinoma <4 cm. Previous subgroup analyses of tumors ≥4 cm and multifocal tumors revealed unacceptably high rates of false negative sentinel nodes and isolated groin recurrences.

WHAT THIS STUDY ADDS

This prospective, nationwide single arm interventional pilot study provides detection rates and negative predictive values for primary vulvar squamous cell carcinoma with tumors ≥4 cm or multifocal tumors comparable to those of unifocal tumors <4 cm. However, the results need to be confirmed by a larger prospective study, ideally a randomized controlled trial.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

The study results indicate that the extension of the indication of sentinel node biopsy to tumors ≥4 cm and to multifocal tumors may be feasible and oncologically safe. Prior to a change of clinical practice, confirmation by a prospective, multinational trial is warranted.

INTRODUCTION

Sentinel lymph node (SLN) biopsy has evolved to become a widespread technique, used for assessing the lymph node status in numerous cancer types, such as malignant melanoma, breast cancer, and endometrial cancer.1–3 Compared with traditional lymphadenectomy, the method has the advantages of less surgical morbidity and a more precise identification of metastatic dissemination.4–8

In vulvar squamous cell carcinoma, two prospective studies have confirmed the oncological safety of the technique.5 9 In 2008, the GROINSS-V-I study reported an inguinal recurrence rate of 2.3% after a negative SLN in 457 women with vulvar squamous cell carcinoma and tumors <4 cm in diameter.5 Women with multifocal tumors were excluded after the occurrence of two inguinal recurrences within the 19 included node-negative women.5

In 2012, the GOG-173 study reported a negative predictive value of 3.7% in 452 women with squamous cell vulvar carcinoma undergoing both an SLN biopsy and an inguinofemoral lymphadenectomy.9 However, in women with tumors 4–6 cm in diameter the authors found a detection rate of 92.0% and seven false-negative SLNs, resulting in a false-negative predictive value of 7.4%.

Based on these two studies, SLN biopsy in vulvar cancer became restricted to women with primary, unifocal vulvar squamous cell carcinoma <4 cm. An inguinofemoral lymphadenectomy is still recommended as the standard treatment in a large proportion of women with newly diagnosed and recurrent vulvar cancer. High complication rates and the risk of post-operative lymphedema might result either in a negative impact on the women’s quality of life or in the reluctance of surgeons to offer surgical groin staging, especially to elderly and frail patients.10–14 However, no further attempts have been made to extend the SLN technique to women with larger or multifocal tumors.

A small retrospective study investigated the SLN technique in 27 women with locally recurrent vulvar cancer15 and concluded that the technique seemed to be feasible and safe, even in local recurrent disease, but data from larger prospective studies are needed to ensure the safety and feasibility in clinical practice. Reducing the treatment-related morbidity of women with vulvar cancer by extending the indication for SLN biopsy is desirable. The aim of this study was to investigate the feasibility of SLN biopsy in tumors currently not eligible for the technique such as tumors ≥4 cm, multifocal tumors, and in first local recurrences.

METHODSStudy Design, Setting, Participants

This is a prospective, non-randomized nationwide multicenter interventional pilot study, conducted between December 2019 and December 2022. The study protocol has previously been published.16

Since 2017, vulvar cancer treatment has been centralized to four tertiary care hospitals in Sweden, and eligible women were identified at the weekly national digital multidisciplinary conference where all women with newly diagnosed or recurrent vulvar cancer were discussed. Patients were included if they had squamous cell vulvar carcinoma with a primary tumor ≥4 cm (group 1), multifocal disease (group 2), a first local recurrence without previous groin treatment or only SLN biopsy (group 3), or a first local recurrence with previous inguinofemoral lymphadenectomy and/or radiotherapy of the groins (group 4). Exclusion criteria were clinical or radiological signs of inguinal lymph node metastases, ongoing pregnancy, age <18 years, or an Eastern Cooperative Oncology Group performance status ≥3. Tumor size and multifocality were assessed clinically pre-operatively. Multifocality was defined as two macroscopically separate invasive tumor components, both confirmed by biopsy.

Intervention

Women in groups 1, 2, and 3 underwent SLN biopsy and inguinofemoral lymphadenectomy. Women in group 4 underwent SLN biopsy if the pre-operative scintigraphy revealed an inguinofemoral SLN. In cases without mapping, no groin surgery was performed in group 4. The SLN procedure was standardized according to the GROINSS-V-I protocol, with the mandatory use of a radiotracer and optional use of blue dye.5 The pathological workup included routine hematoxylin and eosin (H&E) staining and so-called ultrastaging in cases without metastases on H&E staining. Briefly, the SLNs were further cut into three sections per millimeter and examined either with H&E or with cytokeratin immunostaining. According to the Cancer Staging Manual of the American Joint Committee on Cancer, micrometastases were defined as lymph node metastases >0.2 mm and ≤2 mm in diameter, and isolated tumor cells as tumor cell clusters ≤0.2 mm in diameter.17

Pre-operative imaging work-up comprised ultrasonography of the groins, MRI, or CT. Surgical treatment of the vulvar tumor and adjuvant treatment were performed according to the Swedish National Guidelines.18

Variables and Data Source

Clinical, pathological, and demographic characteristics of the participants comprised age, performance status, body mass index, smoking, comorbidity, International Federation of Gynecology and Obstetrics (FIGO) tumor stage,19 tumor characteristics, type of vulvar surgery, and data on adjuvant treatment. Furthermore, post-operative complications within 60 days were registered and classified according to Clavien-Dindo.20

Endpoints and Sample Size

The primary endpoint of the study was the detection rate and the negative predictive value for the SLN procedure, calculated separately for each subgroup. For group 4, only the detection rate could be calculated, as no additional inguinofemoral lymphadenectomy was performed. Secondary endpoints were the number of retrieved SLNs, the proportion of metastases diagnosed by ultrastaging, and the proportion of lymph nodes with micrometastases or isolated tumor cells.

Based on previous data on the prevalence of lymph node metastasis in vulvar cancer, a sample size of at least 100–150 negative groins would have been necessary to show a negative predictive value of at least 95% (lower boundary of 95% CI), with a risk for a type I error of 5% and for a type II error of 20%. In Sweden, about 160 patients are diagnosed with primary vulvar cancer each year, and we expected an inclusion of 10–20 women into each group per year. Thus, the necessary sample size did not seem to be achievable within a realistic timeframe by recruiting patients in Sweden only. This resulted in the design of this nationwide Swedish pilot study to evaluate whether a multinational approach would be worth pursuing. We aimed at including at least 20–30 patients into each subgroup.

The study was registered at ClinicalTrials.gov (ID: NCT04147780).

RESULTS

A total of 64 patients were enrolled, resulting in 120 operated groins and 99 successful SLN biopsies (Figure 1). Table 1 displays the demographic, clinical, and treatment characteristics of the cohort, and Table 2 provides details of the SLN procedure for groups 1 and 2. Most women were included in group 1 (36 patients, 56%) and group 2 (17 patients, 27%). Groups 3 and 4 comprised only four and seven patients and, due to the small numbers and heterogeneity, were consequently excluded from further analyses.

Figure 1

Flowchart study cohort. IFL, inguinofemoral lymph node dissection; RT, radiotherapy; SLN, sentinel lymph node.

Table 1

Demographic and clinical characteristics of women with vulvar cancer

Table 2

Details of the sentinel node procedure and groin surgery in groups 1 and 2

The detection rates of groups 1 and 2 reached 100% and 94.1% per patient, respectively (Table 3A). Twenty-five patients (47%) were diagnosed with node-positive disease. However, in eight of the patients (32%) the metastases comprised only isolated tumor cells or micrometastases, and in nine patients (36%), the metastases were detected only by ultrastaging. Sixteen of the 25 patients (64%) had no further non-SLN metastases in the lymphadenectomy specimen.

Table 3

Sentinel lymph node detection rates (A) and negative predictive values (B) for groups 1 and 2

In total, metastases were found in 13% (22/173) of the SLNs and in 2.1% (13/615) of the non-SLNs. Four metastatic SLNs showed extranodal growth. The metastatic lesions in these SLNs were between 2 and 18 mm in diameter. A detailed distribution of metastases is depicted in Figure 2. No false-negative SLNs were found, resulting in a negative predictive value of 100% (Table 3B). However, in four of the 15 groins without successful SLN mapping (27%), metastatic lymph nodes were detected in the lymphadenectomy specimen. These four patients were 55–80 years of age and had p53-mutated tumors 5–50 mm in size. One tumor was multifocal, and in three of the cases (75%) the lymph node metastases showed extranodal growth.

Figure 2

Distribution of lymph node metastases in the subgroups. (A) Tumors ≥4 cm, n=36 (58/69 groins with successful sentinel mapping). (B) Multifocal tumors, n=17 (29/34 groins with successful sentinel mapping).

Within 60 days post-operatively, 45% of patients developed one or more complications related to the groin surgery, and 28% developed complications related to the vulvar surgery (Table 4).

Table 4

Post-operative complications within 60 days in groups 1 and 2

DISCUSSIONSummary of Main Results

In this prospective pilot study, we found detection rates and negative predictive values of SLN biopsy for women with primary squamous cell vulvar cancer ≥4 cm and multifocal tumors comparable to the results for smaller, unifocal tumors. Moreover, we found no false-negative SLN in the whole cohort. In every third patient, the metastasis comprised only micrometastases or isolated tumor cells and could only be detected by ultrastaging. Due to the heterogeneity and small numbers in groups 3 and 4, no conclusions can be drawn concerning recurrent disease.

Results in the Context of Published Literature

Three prospective cohorts investigating SLN biopsy in vulvar cancer reported detection rates of 88–97% per patient.5 9 21 In a meta-analysis from 2020, di Donna et al calculated a pooled detection rate of 93% per groin.22 Although somewhat lower per groin, we achieved comparable detection rates in groups 1 and 2. We could only identify two prospective studies investigating tumors ≥4 cm.9 21 The GOG-173 study reported for this subgroup an unacceptable high false-negative predictive value of 7.4% with a wide confidence interval. However, the circumstances in which that study was conducted were radically different from ours today. The 10-year accrual period with 47 participating institutions resulted in an average of one patient per year and center, at a time when SLN biopsy in vulvar cancer was not regarded as a routine treatment and no proof of surgical proficiency was required. Furthermore, no pre-operative imaging was required, and during the first 2 years of the study a radiotracer was not mandatory for detection.9 In 2017, Garganese et al reported on 12 patients with tumors >4 cm undergoing an SLN biopsy followed by inguinofemoral lymphadenectomy. No patients in this cohort had a false-negative SLN.21

As the outcome negative predictive value is a function of the sensitivity of the test and prevalence of the condition, a higher prevalence of lymph node metastases in larger tumors may be reflected by somewhat lower negative predictive values compared with smaller tumors with a lower probability of lymph node metastases. However, by optimizing the technique, excluding patients with radiological signs of metastases, and with solid surgical proficiency and optimal tracer use, our study showed that even for larger tumors the results seemed to achieve satisfactory oncological safety.

The accrual of 17 multifocal tumors in the present study was lower than anticipated and reflects the rarity of the condition. Because of unsatisfactory oncological safety, the GROINSS-V-I study stopped the inclusion of multifocal tumors after two early inguinal recurrences between the hitherto 19 included patients.5 Garganese et al included nine women with multifocal tumors in their study, with no false-negative SLN.21 The early exclusion of this subgroup from the GROINSS-V-I study might have been premature, and in our opinion, further evaluation seems justified, albeit the relatively small number of eligible patients and a probable higher risk for lymph node metastases than in unifocal tumors.

All patients in the Garganese et al study went through a pre-operative positron emission tomography-CT scan to detect even smaller metastases; consequently, a lower percentage of women than in our cohort showed metastatic growth.21 Excluding patients with grossly metastatic disease from the procedure is paramount to avoid false-negative lymph nodes due to tumor cells obstructing lymph channels.23–25 On the other hand, small metastases may not pose a risk of establishing false negativity. According to the results of the recently published GROINSS-V-II study, patients with metastatic SLN can be treated by radiotherapy instead of an inguinofemoral lymphadenectomy. Oonk et al reported low inguinal recurrence rates for women with micrometastases or isolated tumor cells in the SLNs, treated by radiotherapy instead of inguinofemoral lymphadenectomy, with significantly reduced treatment-related morbidity.26 However, the GROINSS-V-II study was restricted to unifocal tumors <4 cm. If similar results could be assumed for micrometastasis or isolated tumor cells in larger or multifocal tumors, even node-positive women with low-volume metastases could benefit from the lower morbidity of an SLN biopsy combined with post-operative radiotherapy. Taken together with the 28 node-negative patients in groups 1 and 2, more than two-thirds of the women could have been successfully treated without the morbidity of a full inguinofemoral lymphadenectomy. This is further supported by the fact that 64% of the patients in this cohort had no further non-SLN metastases in their lymphadenectomy specimen. Moreover, the detection of about a third of all metastases by ultrastaging underlines the increased precision and accuracy of surgical staging by the SLN technique. Only 2% of all non-SLN bore metastases.

The number of patients recruited in groups 3 and 4 was far below what had been anticipated. Common reasons for non-eligibility were pre-operative suspicious lymph nodes, inoperable vulvar disease, severe comorbidities, or late treatment side-effects impeding further groin surgery. Patients with local recurrent disease and no previous groin surgery commonly underwent SLN biopsy only. Hence, no conclusions regarding oncological safety can be drawn. Furthermore, given the low inclusion rate, we would be reluctant to propose a larger study for this subgroup. A prospective, national multicenter study was recently launched in the Netherlands, aiming to investigate the feasibility and safety of sentinel node biopsy in the first local recurrences of vulvar carcinoma.27 The study aims at a sample size of at least 150 node-negative patients to demonstrate oncological safety. First results from that study might shed more light on the feasibility of a further attempt to start a larger, international study.

Every fourth groin with failed mapping in our study showed metastatic disease, in most cases with extranodal extension. In these cases, tumor cells might have obstructed the lymphatic vessel to the metastatic SLN.23–25 Thus, particularly in non-mapping groins, an inguinofemoral lymphadenectomy seemed to be mandatory to exclude undetected metastasis.

Strengths and Weaknesses

A strength of this study is the prospective, nationwide design and the well-established cooperation between all four participating centers, resulting in the inclusion of 77% of all eligible patients. Pre-operative workup, imaging, and treatment are synchronized and clearly defined in national guidelines. Thus, high internal validity, and in similarly centralized healthcare systems high external validity, can be assumed.

The study has limitations. One limitation might be the rarity of the disease and consequently the low number of eligible patients, even in a nationwide context. The study was designed without a control group and without quality control for imaging or surgery. Furthermore, the protocol did not provide standards for pre-operative imaging. The use of indocyanine green in combination with a radiotracer might have enhanced the detection rates of SLNs. The results should only be applied to vulvar squamous cell carcinoma, and may not be generalized to other healthcare systems.

Implications for Practice and Future Research

Based on the results, further efforts to launch a multinational trial confirming our results seem to be justified and highly warranted. The extension of the SLN technique could spare a large proportion of patients with tumors ≥4 cm and multifocal tumors the morbidity from undergoing an inguinofemoral lymph node dissection. The introduction of indocyanine green as a new, non-radioactive tracer for SLN biopsy in vulvar cancer may lead to further refinements of the technique. In locally recurrent disease, sufficient evidence on the oncological safety of sentinel node biopsy is still lacking and might be difficult to obtain.

CONCLUSION

In a centralized healthcare system with high proficiency, SLN biopsy might be a safe treatment option for women with primary vulvar squamous cell carcinoma with tumors ≥4 cm and multifocal tumors. Detection rates and negative predictive values seem to be comparable to those of smaller, unifocal tumors. For patients with a first local recurrence, no conclusions can be drawn about the safety of the procedure.

Data availability statement

Data are available upon reasonable request. In accordance with the journal’s guidelines, we will provide our data for independent analysis by a selected team by the Editorial Team for the purposes of additional data analysis or for the reproducibility of this study in other centers if such is requested.

Ethics statementsPatient consent for publication

Not applicable.

Ethics approval

This study involves human participants and was approved by the Swedish Ethical Review Authority (diary number 2019-04647). Participants gave informed consent to participate in the study before taking part.

Acknowledgments

The authors wish to thank Mats Fredrikson, Linköping University, for statistical support.