Predictive Ability of Rule of 3 in Parathyroid Cancer: Outcomes from a South Asian Cohort

Preoperative diagnosis of parathyroid cancer (PC) where possible allows for en-bloc resection of the tumour, which is associated with excellent prognosis. The rule of >3 (size of tumour larger than 3 cm; corrected calcium more than 3 mmol/L) as proposed by Schulte and Talat has a specificity of 95% in predicting malignancy in parathyroid neoplasms. We looked at the impact of rule of 3 in predicting malignancy and outcomes on intervention in a South Asian cohort.

Patients who underwent parathyroid surgery between 2010 and 2023 at two tertiary referral centres were assessed. Patients with PC were selected and their clinicopathological parameters, treatment modalities, and outcomes were analysed.

Thirteen of 336 (3.8%) patients with a mean age of 61.8 (±17.5) years were diagnosed with PC during the study period. The highest mean preoperative values were PTH (92.4 ± 66.27 pmol/L), highest corrected calcium (3.21 ± 0.28 mmol/L), and alkaline phosphatase (419 IU/mL). Nine patients underwent en-bloc excision while the other had focussed parathyroidectomy. Recurrences were recorded in 2 (28.5%) patients over a mean follow-up period of 69 (±48.6) months. One patient with lung metastasis underwent video-assisted thoracic surgery. There was no disease specific mortality in this cohort during the study period.

In our experience, the predictive rule of 3 has low sensitivity to suspect PC preoperatively, resulting in limited usefulness in clinical practice. Outcomes appear to be less favourable with higher recurrence rates in cases where less than en-bloc resection is performed.