Radiologic Assessment of the Genitourinary Diaphragm Using MRI with Endorectal Coil in Men with Localized Prostate Cancer ASTRO Annual Meeting 2014, #3792 September 17, 2014 Neil K. Taunk MD MS, Oguz Akin MD, Xin Pei PhD, Michael Zelefsky MD

Assessing the Genitourinary diaphragm Identification of the prostate apex is critical in radiotherapy planning ‒ Overestimation can lead to unnecessary dose to the penile bulb ‒ Underestimation leads to undertreatment of the prostate apex Genitourinary diaphragm (GUD) is the distance between the prostate apex and the penile bulb – Also known as the membranous urethra length (MUL) – Discordant measurements with expert guidelines, retrograde urethrography, bony landmarks, ultrasound localization, CT-based planning MRI prostate improves identification of prostate anatomy* – Reduces systematic overestimation of prostate size at the apex – Endorectal coil improves T2-weighted spatial resolution and apex delineation Assessment of the GUD using MRI with endorectal coil – Characterize the length of the GUD/MUL – Evaluate whether patient specific characteristics are related to GUD length

MR-based assessment MRI prostate improves identification of prostate anatomy* – Reduces systematic overestimation of prostate size at the apex – Improves delineation of the prostate-rectal interface Endorectal coil improves T2-weighted spatial resolution – Primarily used to examine for extracapsular extension and SVI, for more accurate staging – Also improves delineation of the prostate apex Assessment of the GUD using MRI with endorectal coil – Characterize the length of the GUD/MUL – Evaluate whether patient specific characteristics are related to GUD length *Roach IJROBP 1996; McLaughlin IJROBP 2010

Patients and Methods Patients with clinically localized prostate cancer – GS 6 or 7, cT1c-cT2b, PSA <20 – Disease and demographic information collected Imaging modality – 3T or 1.5T MRI – Endorectal coil – Axial, sagittal, and coronal T2-weighted images GUD length measured on sagittal and coronal T2-weight images – Patients excluded if significant TURP defect or significant artifact the prostate apex Assessment – Two independent raters measured GUD length Treatment – All patients were treated with definitive dose-escalated external beam radiotherapy ( Gy) +/- androgen deprivation therapy +/- pelvic lymph node treatment

Sagittal and Coronal T2-Weighted Imaging 75 yo M with cT1c GS (3+4)=7 prostate adenocarcinoma

Results: Interobserver Agreement and Demographics Two independent observers measured each GUD – Random sampling of patients resulted in excellent concordance (Spearman ρ = 0.834; p<0.001) – >85% interobserver agreement within 2mm No significant association with age, T-stage, Gleason score, NCCN risk group, height, weight (p=NS)

Results: GUD Characteristics Characteristic Median15mm Mean15.3mm Std Dev4mm Min3mm Max28mm

Conclusions Mean and median GUD length is 15mm Largest series to date of GUD length assessment in men Target volume delineation of the prostate is critical in radiotherapy planning Prostate MRI with endorectal coil is an excellent modality to delineate the prostate apex and GUD length

Acknowledgements MSKCC Department of Radiation Oncology ‒ Michael Zelefsky MD ‒ Xin (Isaac) Pei PhD MSKCC Department of Radiology – Oguz Akin MD