Marieke van Son

166 CHAPTER 9 Figure 2 – Diagnostic 3T mp-MRI revealing first recurrence lesion. Transversal plane of the diagnostic 3T multiparametric-magnetic resonance imaging (mp-MRI) revealing a suspect lesion in the right dorsal peripheral zone next to the prostate midline and in the right seminal vesicle. Delineations of the bladder (purple), urethra (yellow), rectum (green), prostate (red), and gross tumor volume (GTV, blue) are shown. In this case, the clinical target volume (CTV) was considered equal to the GTV because there was mainly seminal vesicle invasion. (a) T2-weighted image, (b) ADC image, (c) K-trans image Focal salvage treatment Prior to treatment, the gross tumor volume (GTV), clinical target volume (CTV), defined as GTV with a five-millimeter margin, and the organ at risk (OAR) (prostate, bladder, rectum, and urethra) were delineated based on a 3T mp-MRI and 68Ga-PSMA PET-CT scan. For the CTV to planning target volume (PTV), the margin was 0 millimeter as the source and dose distribution along the tumor in brachytherapy. With the patient in the lithotomy position and under spinal anesthesia, seven MR-compatible catheters were placed in and around the right peripheral zone and seminal vesicle via the perineum (Figure 3). Catheter insertion was guided by fused diagnostic MRI delineations and intraoperative transrectal ultrasound. To evaluate catheter positions with respect to the tumor and OAR, an additional 1.5T MRI scan was made and delineations were adjusted to account for swelling. Next, a simulation of dose distribution was made by the treatment planning system. To ensure the safe delivery of the radiation dose, an additional 1.5T MRI scan was made just before the radiation treatment to check for catheter displacements. The dose to 95% of the GTV (D95) was 20.3Gy (aim: >19Gy). The minimum dose to the most exposed 1 cc of rectum or bladder (D1cc) was limited to 10Gy and the dose to 10% of the urethra (D10) was 5.2Gy. We used constraints of D1cc <12Gy for the rectum and bladder and D10 <17.7Gy for the urethra [7]. Later, all catheters were removed, leaving no source of radioactivity behind in the patient. There were no perioperative complications and the patient was discharged the same day.