Benjamin Hindson
Canterbury Regional Cancer and Haematology Service
Radiation oncologist

Victoria Beenstock
Canterbury Regional Cancer and Haematology Service

Hayley Wooding
Radiation therapist
Canterbury Regional Cancer and Haematology Service

Background and Purpose. For prostate high-dose-rate brachytherapy (HDRBT), accounting for catheter displacement prior to treatment delivery is essential to ensure adequate target coverage, whilst minimising the risk of increased dose to caudal structures such as the membranous urethra.

Method. The Christchurch Oncology service commenced a HDRBT programme in December 2016. Needles are inserted under TRUS guidance. Fiducial markers are placed at the time of catheter insertion. Following the insertion, patients are then CT planned (PCT). Treatment is delivered later the same day and a verification CT (VCT) is done just prior to this. The prostate is contoured as the CTV. The prostatic urethra and the membranous urethra 1.5cm caudal to the CTV are contoured. For the catheter displacement measurement the two CTs are fused using the fiducial markers and catheter displacement movement calculated from a fiducial marker. Corrective action is applied using an origin adjustment technique. The dosimetric impact of not applying corrective action was analysed based on individual and average catheter displacement.

Results. 10 patients were used for analysis. Mean time between PCT and VCT was 208min. Seven of the 10 patients required corrective action. The mean caudal displacement was 5.3 mm (-2.0 – 15.0 mm). Appling an average or individual catheter shift was shown to be very similar. Without corrective action the mean CTV D90 for the VCT decreased to 86.5 % (individual) and 86.2 % (average) from a PCT value of 101.9 %. The mean D10 for membranous urethra for the VCT increased to 93.5 % (individual) and 94.9 % (average) from a PCT value of 70.1 %.

Modelled caudal displacement of >4 mm lead to a CTV D90 reduction by 9.5%; a mean prostatic urethra D10 increase by 3% and a mean membranous urethra D10 increase of 16%.

Conclusions. Caudal displacement is common. Using a verification CT allows for correction. It appears applying an average shift to all catheters with a 5 mm threshold is a safe and practical method of ensuring accurate HDRBT delivery. Uncorrected shifts ≥5 mm may increase the membranous urethra dose and cause unacceptable decreases in CTV coverage.

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