Influence of catheters on predicted tumour control probability and severity of acute genitourinary toxicity during high-dose-rate brachytherapy prostate boost

Purpose: High-dose-rate brachytherapy (HDR) boost is an effective method for dose escalation when treating prostate cancer. Optimal number and location of catheters play key role in radiation dose delivery. We studied the impact of catheters and associated trauma on the dose uncertainties and urethral toxicity. Methods and Materials: Between July 2008 to August 2009, 50 patients with prostate cancer were treated with 46 Gy of external irradiation of whole pelvis (2 Gy per fraction) and two HDR brachytherapy fractions (each 14 Gy) at the end of 10 fractions of external beam. All brachytherapy implants were planned using real-time, ultrasound-based planning system. Variables were prostate and urethral volumes, number of catheters and their mean distance from base of bladder and dose volume histogram parameters. All data were collected during first implant only. The toxicities were graded according to Radiation Therapy Oncology Group Toxicity Criteria. Statistical analysis was done on SPSS version 17.0. Results: The mean number of catheters implanted was 12.38 (8-19), and number of attempts per needle to achieve desired position was 1.6 (range = 0-5). Mean distance between the catheters tips to contrast filled bladder was 3.2 mm (1-8 mm) after the adjustment. Distances >5 mm showed lower doses to prostate and lower predicted tumour control probability (TCP) (p < 0.01). No correlation was found between numbers of catheters implanted, attempts per catheter and severity of acute genitourinary (GU) toxicity. Significant correlation was found between severity of acute GU toxicity and urethral V130, V150 (p < 0.001). Conclusion: Dose decline and subsequently lower TCP were seen for the greater distances between the needles and bladder. Acute GU toxicity increased with higher urethral, but severity of acute GU toxicity does not increase with increase in prostate/urethral volumes, number of catheters needles and attempts.