Higher scrotal uptake ratio of ^99mTc-MDP on bone scans in newly diagnosed prostate cancer: A reliable indicator of pelvic node metastasis

Objective Pelvic lymph node dissection (PLND) is the gold standard procedure for nodal staging in prostate cancer (PC) but less commonly used due to its invasiveness. More commonly computerized tomography (CT) and magnetic resonance imaging (MRI) are used although these have limited sensitivities and specificities. The aim of this study was to find out the correlation between higher scrotal uptake ratio (SUR) of ^99mTc-methylene diphosphonate (MDP) on bone scan and pelvic node metastasis in patients with PC at high risk for nodal metastasis. Methods This was a retrospective study which included 68 biopsy proven newly diagnosed PC patients who had bone scan from January 2008 till January 2012. MRI of the pelvis, prostate specific antigen (PSA) and Gleason's score were available in all patients. Whole body bone scan was performed in all patients and SUR was calculated by dividing mean counts over scrotum and soft tissue over lateral aspect of right thigh. PLND was carried out within 2-3 weeks of MRI study in these patients. Results Mean age of studied males was 71 ± 07 years with a mean PSA level of 65 ± 162 ng/ml. Prostate biopsy revealed adenocarcinoma in all patients with mean Gleason's score 7 ± 1. Mean SUR was 2.786 ± 0.496. MRI was positive for pelvic lymphadenopathy in 32/68 (47 %).PLND revealed evidence of nodal metastasis in 16/68 (24 %) patients. Receiver operating characteristic analysis revealed good diagnostic strength of SUR for nodal metastasis with a cut off value of >2.99 with an area under curve (AUC) 0.708 (95 %CI 0.533-0.847, p value<0.05) and a mean sensitivity of 68.75 % and mean specificity of 80 %. Diagnostic strength of MRI for nodal metastasis was found to be low (AUC 0.566, 95 %CI 0.047-0.657, non-significant p value). No significant correlation was found between SUR and PSA in nodes positive and nodes negative patients. Conclusion We conclude that in newly diagnosed PC patients, higher SUR on bone scan has a high diagnostic accuracy for pelvic node metastasis. Furthermore, a bone scan with a SUR <2.99 and negative for bone metastasis can stratify newly diagnosed PC patients as low risk.