Vulvar Cancer: An Introduction

Vulvar cancer represents 3% of all gynecologic cancers, affecting 2.4 per 100,000 women on average in Western Europe in 2020.¹ The most common type, vulvar squamous cell carcinoma (VSCC), accounts for 90% of vulvar cancer cases.² VSCC can develop via 2 different pathways, although these are not yet completely understood. The pathways are based on the presence of HPV: an HPV-associated pathway, accounting for 17.4% to 25% of VSCC, and an HPV-independent pathway, mainly related to the presence of lichen sclerosus.^3–5 Multiple studies found better a prognosis for HPV-associated vulvar cancers.^6–9 However, the possibility for less radical treatment approaches has not yet been investigated.

Early-stage VSCC is usually managed by a radical local excision of the primary tumor. For years it was recommended that one should aim for a margin of at least 8 mm, but evidence showing that this represents the optimal margin is insufficient. Although the optimal margins of the excision remain undefined, it is acceptable and often desirable to limit radicality to preserve midline structures, such as the clitoris, anus, and urethra.^10,11

If indicated, patients will undergo a sentinel node (SN) procedure for diagnostic purposes and determining further treatment. With this procedure, the first tumor-draining lymph nodes are identified, removed, and examined for metastases. Before the introduction of the SN procedure, inguinofemoral lymphadenectomy (IFL) was standard of care. With IFL, all lymph nodes in the groin were surgically removed. This procedure was efficient and the recurrence rate was low.¹² However, removing a large number of lymph nodes in the groin causes major treatment-related morbidity, such as lymphoedema and recurrent infections.¹³ The SN procedure replaced IFL as standard of care, after large studies showed its safety and efficacy.^14,15 Moreover, treatment-associated morbidity decreased tremendously with this less-radical approach.¹³

Groin treatment is influenced by lymph drainage patterns, which are dependent on the location of the tumor. The location of the tumor on the vulva can be classified as lateralized (>1 cm from the midline), near midline (≤1 cm from the midline but not crossing the midline), or true midline (crossing the midline) (Figure 1). Lateralized tumors show bilateral drainage less often than midline tumors (25% vs 88%, respectively). Tumors located in a near-midline position show bilateral drainage in 66% of the cases.¹⁶ If bilateral drainage is present, a bilateral SN procedure is indicated, and with unilateral drainage, a unilateral SN procedure is sufficient. In a true midline tumor, however, unilateral drainage cannot be accepted, and therefore an ipsilateral SN procedure and contralateral IFL should be performed.^16,17

Survival is good in patients with early-stage VSCC; patients with negative SNs have a 5- and 10-year disease-specific survival of 93.5% and 90.8%, respectively. However, if the patient has a metastasis in the SN, the 5- and 10-year disease-specific survival decreases to 75.5% and 64.5%, respectively.¹⁸

The SN Procedure

Today, patients are eligible for the SN procedure if they have a primary tumor <4 cm, with a depth of invasion (DoI) >1 mm and no suspicious lymph nodes at palpation and imaging.¹¹ If there is a suspicious lymph nodes at imaging, fine-needle aspiration cytology is indicated to rule out lymph node metastases. If the DoI is <1 mm, lymph node staging is not indicated, because the risk of lymph node metastases in these tumors is negligible.¹¹

The SN procedure consists of 3 steps: (1) mapping of the SN via tracer injection and performing lymphoscintigraphy (LSG), (2) surgical removal of the SN, and (3) workup by the pathologist.

SN Mapping

International guidelines recommend performing the SN procedure with at least the use of a radioactive tracer (usually a technetium-99m [Tc-99]–labeled nanocolloid).¹¹

Because the radioactive tracer will be taken up by the lymphatic system of the vulva and transported to the inguinofemoral region, the SNs can be visualized on an LSG prior to surgery. This facilitates the surgical procedure by providing information on the presence, localization, and number of SNs.^11,19 Early (30 minutes after injection) and late (2–4 hours after injection) imaging is usually performed. In 2023, Thissen et al²⁰ showed that late imaging showed additional SNs in 35% of the patients. Consequently, they stated that it is not safe to omit late scintigraphy. However, one can question whether these additional lymph nodes were clinically relevant, and the researchers did not investigate the rate of metastases in these additional SNs.

In addition to the radioactive tracer, a blue dye can be used as an optical tracer to visualize the SN during surgery. In the operating room, the blue dye is injected at the same sites as the radioactive tracer, approximately 10 minutes prior to the surgical removal of the SN. Although the use of blue dye is not mandatory, the gold standard for SN mapping is a combination of blue dye and a radioactive tracer, because this has been shown to have the highest detection rate.^11,21 Furthermore, the technique is easier to learn when the nodes are visible, and therefore will also have a faster learning curve for less experienced gynecologic oncologists.

In order to supplement or even replace the previously described methods, multiple techniques are being developed and investigated for clinical implementation. For example, indocyanine green (ICG) is an optical tracer, which is fluorescent in light at wave lengths in the near infrared range. Use of ICG has been investigated and validated in a variety of cancers, such as early-stage breast cancer. Its use is easily reproducible, and has a steep learning curve.²² ICG is invisible to the naked eye, and therefore has the advantage of not coloring the surgical field. Furthermore, it has a penetration depth of 5 to 8 mm, which makes lymph nodes visible through overlying tissue, allowing for transcutaneous detection. On the other hand, this means the technique is only optimal in lean patients, with a maximal distance between femoral artery and skin of 24 mm. For obese patients, a skin incision may be needed to localize the SN, and the advantage of transcutaneous detection is lost.²³ Several studies investigated the accuracy of ICG in VSCC, either as a hybrid tracer, bound to human serum albumin (HSA), or alone. Theoretically, HSA improves the fluorescence of ICG, although differences have not yet been observed in practice.²⁴ Table 1 shows studies investigating ICG and its detection rates in VSCC. All clinical studies (n=10) until 2023 with full-text availability were included.^23,25–33 The detection rate varies, ranging from 76.8% to 100%. Until now, the use of ICG alone has not been considered for vulvar cancer guidelines, but only as an alternative for blue dye in combination with a radioactive tracer.¹¹ An optimal technique for use of ICG in the SN procedure has not yet been identified.

Table 1.

Studies on ICG for SN Detection in Vulvar Cancer

VIEW TABLE

Another novel tracer is superparamagnetic iron oxide (SPIO), which can be visualized on MRI prior to surgery, but can also be detected by a magnetometer probe during the procedure. Although SPIO is widely used in breast cancer, only one study has been published on this tracer in vulvar cancer.³⁴ In this study, 20 patients received SPIO and Tc-99, and a magnetometer probe was used to detect the SPIO. Sensitivity was 98.5% for the superparamagnetic technique and 93.8% for the radioactive tracer. Although the superparamagnetic technique seems to be feasible, studies on its safety in vulvar cancer are lacking. A review on the use of SPIO with ultra-small particles of 20 to 50 mm (USPIO) with MRI visualization in head and neck cancer has shown promising results for its use in detecting SNs.³⁵

Use of a single tracer without the need for a radioactive tracer has many advantages. First, the procedure is no longer dependent on the availability of medical isotopes. Additionally, planning of the procedure is more flexible. Moreover, the whole procedure can be performed under general anesthesia because early injection of a tracer is no longer necessary, making the procedure more patient-friendly. On the other hand, new techniques also require new equipment or machines, which are expensive because they are not frequently used. For these reasons, organizations might not be tempted to invest in these new techniques.

Excision of the SN

The SN procedure is performed preferably prior to the removal of the primary tumor. However, several studies have shown that scar injection (when the primary tumor has already been removed in a previous procedure) of the tracer might also be feasible to accurately visualize the SN.^36–38

The SNs are removed according to the findings on LSG. For the LSG to be acceptable, at least a unilateral SN should be found in lateralized and near-midline tumors. For real midline tumors, SNs should be found bilaterally. If this is not the case, this should be noted as a false-negative result and IFL should be performed on that side.^16,17 Once all identified SNs are removed, they are separately labeled and sent to the pathology department for routine workup.

Histopathology

Lymph nodes are fixated in formalin and embedded in paraffin. Then, the SN will be investigated by routine hematoxylin-eosin (HE) staining on one slide per 2 mm of lymph node tissue. If these slides do not contain tumor cells, ultrastaging is performed. With ultrastaging, the entire lymph node will be cut into serial sections and stained with HE and cytokeratin AE1/3.¹¹ Both the European Society of Gynaecological Oncology (ESGO) and the Groningen Iternational Study on Sentinel Nodes in Vulvar Cancer (GROINSS-V) study protocol advise to include at least 3 slides per millimeter.^11,39 Despite the fact that standardized protocols have been described, many variations between institutes exist. Alternatively, the procedure can be started with up-front ultrastaging to reduce turnaround times. It may be worthwhile to investigate which procedure renders optimal results.

Frozen sectioning is normally not performed, presumably due to the high rate of false-negatives and to prevent loss of diagnostic tissue. Literature describing frozen section analysis, such as a study by Swift et al,⁴⁰ has shown a sensitivity of 89.7% and specificity of 99.5%. Brunner et al⁴¹ showed similar results, with a sensitivity of 88.5% and specificity of 100%. However, an additional analysis of GROINSS-V I by Oonk et al⁴² showed a much lower sensitivity of 48% in 315 patients who underwent frozen sectioning. Furthermore, low-volume metastases were even less accurately detected. Theoretically, low-volume metastases can be missed with frozen sectioning because these are smaller than 2 mm, and lymph nodes are usually not cut every 2 mm. Smaller nodes are usually cut in half. Although frozen sectioning of the SN offers the possibility that the surgical procedure could be performed in one session, and therefore is regarded as a patient-friendly option, it also has logistic drawbacks, such as prolonged duration of the surgical procedure. For this reason, a 2-step approach is considered equivalent.

The OSNA technique, in which the whole lymph node can be assessed by detecting mRNA cytokeratin 19, has not yet been described for vulvar cancer.⁴³ In breast cancer, multiple studies have investigated the accuracy of OSNA, and sensitivity is high.^44–46 Because OSNA accurately detects macrometastases (>2 mm), it appears to be a promising technique.⁴⁶ However, Tiernan et al⁴⁷ argue it is not reliable for detecting micrometastases (≤2 mm) because on histopathologic examination, 21% of patients needed to be reclassified from macrometastasis to micrometastasis. For breast cancer, this will result in overtreatment when using OSNA alone and is therefore not recommended outside of a clinical research setting. Moreover, a criterion for the use of OSNA is that the tumor expresses mRNA cytokeratin 19. In head and neck cancer, sensitivity of OSNA is high, ranging from 82.4% to 90%.^48–50 However, expression of cytokeratin 19 in head and neck squamous cell carcinoma is approximately 60% to 80%, and even lower in early-stage head and neck tumors.^51,52 The rate of expression of mRNA cytokeratin 19 in VSCC has yet to be determined.

SN Metastases

If a metastasis is found, it can either be a macrometastasis, a micrometastasis, or isolated tumor cells (ITCs). A previous study from GROINSS-V showed that a patient with a micrometastasis (defined as a metastasis ≤2 mm) has a better prognosis than a patient with a macrometastasis.⁴² Nevertheless, the 2021 FIGO (International Federation of Gynecology and Obstetrics) classification uses a cutoff value of 5 mm for lymph node metastases between stage IIIA and IIIB.⁵³ GROINSS-V previously found comparable prognosis among patients with metastases between 2 and 5 mm and those with a metastases >5 mm, questioning the clinical relevance of a 5-mm cutoff value. Furthermore, the 2021 FIGO staging states that ITCs are not categorized as positive lymph nodes. Indeed, in breast cancer, detection of micrometastases and ITCs is less important because the vast majority of the patients will undergo adjuvant locoregional radiotherapy and/or systemic therapy. In vulvar cancer, however, adjuvant therapy is not standard after SN biopsy. An analysis of GROINSS-V I data showed that 4.2% of patients with only ITCs in the SN had additional metastases found at lymphadenectomy.⁴² In GROINSS-V II, patients with ITCs or metastases ≤2 mm in their SN (n=56) had no groin recurrences when they underwent inguinofemoral radiotherapy (n=45), and 1 groin recurrence was observed in the 11 patients who did not undergo any additional treatment.³⁹ Therefore, all patients with SN metastases, including those with ITCs, should undergo additional groin treatment.

Broadening Indications

To this day, the SN procedure is performed in a well-selected group of patients with vulvar cancer. Several studies aim to investigate the possibility of broadening the indication for the SN procedure, for example in tumors ≥4 cm, multifocal tumors, or patients with locally recurrent disease.

Currently, local recurrences are managed by performing a radical local excision of the recurrent tumor and a unilateral or bilateral IFL in patients who did not undergo lymphadenectomy at primary treatment.¹¹ In a retrospective evaluation of a series of 27 patients who had undergone repeat SN procedure, van Doorn et al⁵⁴ reported that the procedure was successful in 78% of the patients, although it appeared to be more challenging for the gynecologic oncologist. The ongoing V2SLN study by van Doorn et al⁵⁵ is prospectively evaluating the repeat SN procedure.

Considering larger tumors, Levenback et al¹⁵ evaluated outcomes of the SN procedure in patients with tumors 4 to 6 cm. They found that the false-negative predictive value was 7.4%, compared with 2.0% for patients with tumors <4 cm. In 2019, Nica et al⁵⁶ investigated whether the SN procedure could be used in patients with tumors >4 cm. Finding that 9% of the patients (1/11) had a groin recurrence after negative SNs by the SN procedure, they concluded that performing the SN procedure in patients with larger tumors might not be safe. However, the cohort was very small.

Garganese et al⁵⁷ investigated the safety of the SN procedure in patients with clinically negative nodes but who, according to guidelines, were not fit for the SN procedure. All patients underwent a preoperative PET/CT to confirm negative nodes. The included patients had larger tumors (>4 cm), multifocal tumors, complete tumor excision, unilateral nodal involvement, or a local recurrence. In all groups (n=47) the SN biopsy false-negative rate was 0%. Larger prospective studies are needed to confirm these results.

Furthermore, Zach et al⁵⁸ are currently conducting a prospective multicenter study on this subject, investigating the feasibility of the SN procedure in patients with larger tumors, multifocal tumors, and local recurrences.

Treatment of SN Metastases

Further treatment of nodes in the groin is determined based on the outcomes of the SN workup. In the case of a negative SN, no further treatment is required. On the other hand, if the SN is found to be metastatic, further groin treatment should be performed, independent of the size of the SN metastases.⁴² Additional groin treatment can be limited to the groin where the metastatic SN(s) was found, because the risk on contralateral metastases appears to be low in patients with unilateral SN involvement.¹⁶

IFL is the standard of care when a macrometastasis is present. In 2021, the results of GROINSS-V II showed that radiotherapy is a safe alternative in the case of a micrometastatic SN, with a groin recurrence rate of only 1.6%. Additionally, treatment-related morbidity was less frequent compared with IFL.³⁹ In GROINSS-V II, the groin recurrence rate for patients with a macrometastasis in the SN was high, therefore standard of care (IFL) was continued.

Currently, GROINSS-V III is investigating whether patients with a macrometastatic SN can be treated with chemoradiation instead of radiotherapy to prevent groin recurrences, but also to reduce treatment-related morbidity for these patients.⁵⁹ NCCN already recommends use of concurrent chemotherapy with radiotherapy for the treatment of SN metastases.¹⁰ However, this recommendation is based on studies mainly investigating patients with advanced vulvar cancer. To date, chemotherapy and chemoradiation have insufficiently been studied as treatments for SN metastases in early-stage vulvar cancer.

Future Perspectives

The SN procedure in early-stage vulvar cancer is a safe and efficient option for assessing SN status. Even though it is a widely used technique, optimization of the procedure is still possible and is the subject of current research in this field. Several prospective studies are currently being conducted, investigating whether eligibility for the SN procedure can be extended to more categories of patients. This would reduce treatment-related morbidity in these patient groups as well. Although large studies recently showed progress in the treatment of patients with a micrometastatic SN, ample room remains for further improvement.

Furthermore, in the search for a more patient-friendly SN procedure, larger prospective studies are needed with new tracers for SN detection to provide data on safety in terms of false-negative rates of the SN procedure. Lastly, there is a need to define the optimal pathologic workup for SNs, with consideration toward minimizing the workload for the pathologist without decreasing the detection rate of metastasis.

Conclusions

The SN procedure is an established procedure in treating early-stage vulvar cancer, although optimization of the technique, pathologic workup, indications, and treatment in the case of metastatic disease are the subject of ongoing research.