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

Abstract

Background Preoperative diagnosis of parathyroid cancer (PTTC) 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 3cm; corrected calcium more 3mmol/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’s in predicting malignancy and outcomes on intervention in a South Asia cohort. Methods Patients who underwent parathyroid surgery between 2010 – 2023 at two tertiary referral centres were assessed. Patients with PTTC were selected and their clinicopathological parameters, treatment modalities, and outcomes were analyzed. Results 13 of 336 (3.8%) patients with a mean age of 61.8 (+/-17.5) years was 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 (VATS). There was no disease specific mortality in this cohort during the study period. Conclusions In our experience, the predictive rule of 3’s has low sensitivity to suspect parathyroid cancer preoperatively, resulting in limited usefulness in clinical practice. Outcomes appear to be less favourable with higher recurrence rates in cases where less than enbloc resection is performed.