Table 3 Model assumptions

General Pathway—assumptions

Patients found with a false-negative SLN biopsy (blue dye and/or 99mTc) who subsequently develop metastasis receive both IFL and radiotherapy.

Patients are followed-up every 2 months following a negative SLN biopsy result (and therefore do not receive IFL) and every 3 months following an IFL.

There are no occasions in which radiotherapy might be administered to a patient who has not previously received an IFL (apart from following a recurrence).

Complications following a SLN biopsy (blue dye/99mTc) and then an IFL implemented during the same procedure will be the same as those experienced following an IFL.

Complications following all types of SLN biopsy (for example, blue dye/99mTc) will be the same.

Recurrence—assumptions

Recurrence will only occur either in the groin or the vulva (local), distant recurrence will not be considered. This is because any distant recurrences, while rare are likely to occur following either a local or groin recurrence and rarely occur without either.

An additional primary excision will be required in the case of a local recurrence.

In the case of groin recurrence, the treatment is IFL+RT if it has not been administered already, chemotherapy will be administered if it has.

Mortality following recurrence within the 2-year time horizon is always due to vulval cancer, with these patients receiving palliative care as a result of their condition. Although it is acknowledged that the findings show that the death rate among vulval cancer patients due to vulval cancer or other causes is 50/50 following treatment, the risk of death following a recurrence is high enough for this assumption to be made.

Further modelling—assumptions

For the purpose of costing follow-up, all deaths from vulvar cancer and all other causes occur at 12 months following screening.

All parameters in this model are independent of age, with the exception of the all cause death rate. This assumption is made due to the paucity of age specific data in this field.

Patients experience long-term complications independently of whether they experience short-term complications. This assumption is made due to the paucity of data in the literature describing what proportion of patients experience both short-term and long-term complications.

Short-term and long-term morbidity have no impact on the mortality of the patients. This assumption is made due to paucity of data, however, its impact is investigated through sensitivity analysis.

All patients in the patient cohort are aged 65, the impact of this assumption is investigated through sensitivity analysis by examining patients aged 55 and 75, respectively.