▪Clinically Significant Prostate Cancer Local Recurrence After Radiation Therapy Occurs at the Site of Primary Tumor: Magnetic Resonance Imaging and Step-Section Pathology Evidence

Purpose: To determine whether prostate cancer local recurrence after radiation therapy (RT) occurs at the site of primary tumor by retrospectively comparing the tumor location on pre-RT and post-RT magnetic resonance imaging (MRI) and using step-section pathology after salvage radical prostatectomy (SRP) as the reference standard. Methods and Materials: Nine patients with localized prostate cancer were treated with intensity modulated RT (69–86.4 Gy), and had pre-RT and post-RT prostate MRI, biopsy-proven local recurrence, and SRP. The location and volume of lesions on pre-RT and post-RT MRI were correlated with step-section pathology findings. Tumor foci >0.2 cm3 and/or resulting in extraprostatic disease on pathology were considered clinically significant. Results: All nine significant tumor foci (one in each patient; volume range, 0.22–8.63 cm3) were detected both on pre-RT and post-RT MRI and displayed strikingly similar appearances on pre-RT and post-RT MRI and step-section pathology. Two clinically insignificant tumor foci (≤0.06 cm3) were not detected on imaging. The ratios between tumor volumes on pathology and on post-RT MRI ranged from 0.52 to 2.80. Conclusions: Our study provides a direct visual confirmation that clinically significant post-RT local recurrence occurs at the site of primary tumor. Our results are in agreement with reported clinical and pathologic results and support the current practice of boosting the radiation dose within the primary tumor using imaging guidance. They also suggest that monitoring of primary tumor with pre-RT and post-RT MRI could lead to early detection of local recurrence amenable to salvage treatment. [Copyright &y& Elsevier]