Absolute and Relative Washout Rates Associated With Parathyroid Adenoma

Background Parathyroid adenoma is a benign parathyroid gland tumor that causes excessive parathyroid hormone production, leading to primary hyperparathyroidism. High serum calcium levels characterize it. Accurate diagnosis and localization of adenomas are crucial for effective surgical management. Computed tomography is a fundamental imaging technique used to identify and characterize parathyroid adenomas. This study aimed to provide a comprehensive overview of the absolute and relative contrast washout rates of parathyroid adenoma and the thyroid gland, and compare enhancement patterns to establish the absolute and relative washout rates of parathyroid adenoma. Materials & methods This retrospective study analyzes the CT findings of 33 patients with histopathologically proven parathyroid adenomas. All patients with 4D CT scans have been included with no exclusion criteria. The mean attenuation was measured in Hounsfield units for the parathyroid adenoma and thyroid gland in the non-enhancing, arterial, venous, and delayed phases, depending on the region of interest. All statistical analyses were performed using SPSS (IBM Corp., Armonk, NY, USA). Student's t-test was used to evaluate the differences in measurements between the parathyroid adenoma and thyroid tissue. One-way ANOVA was used to evaluate the difference in calculations between the parathyroid adenoma and thyroid tissue. P-values <0.001 were considered statistically significant. Results The most common location of parathyroid adenomas is inferior to the thyroid gland. The average pre-contrast attenuation of the parathyroid adenoma is 61.8 ± 15.5 HU compared to 105.5 ± 15.2 HU of the thyroid gland. The arterial attenuation of the parathyroid adenoma is 170.3 ± 40.7 HU, relatively comparable to the thyroid gland arterial attenuation, which is 188.0 ± 9.6 HU. The venous and delayed-phase attenuation of the parathyroid adenoma were 146.8 ± 37.5 and 96.8 ± 26.7 HU, respectively, and 178.8 ± 20.2 HU and 149.3 ± 15.2 HU for the thyroid gland, respectively. The calculated absolute and relative arterial washout rates for the parathyroid adenoma were 69.4 ± 13.4% and 43.2 ± 8.0%, respectively, as compared to 46.4 ± 9.9% and 20.6 ± 6.7% for the thyroid gland. The calculated absolute and relative venous washout rates for the parathyroid adenoma were 58.0 ± 21.4% and 33.0 ± 13.7%, respectively, as compared to 37.2 ± 17.2% and 15.9 ± 9.6% for the thyroid gland. Conclusions Parathyroid adenoma demonstrated a significantly higher washout rate than the thyroid gland tissue. Absolute arterial washout ≥69% and relative arterial washout ≥43% indicate parathyroid adenoma. Moreover, absolute venous washout ≥58% and relative venous washout ≥33% can be considered diagnostic factors for parathyroid adenoma. Further, pre-contrast attenuation of <60 Hounsfield units has a substantial predictive value for parathyroid adenoma in addition to the described washout rate. Increased awareness of the washout rate can increase the success rate of four-dimensional computed tomography interpretation.