Oral & Poster Presentations Archive

UK & Ireland Prostate Brachytherapy Conference 2022

Abstract number 5 

MRI-based planning in prostate low dose rate (LDR) brachytherapy – a retrospective volumetric and dosimetric comparison with the traditional 2-step transrectal ultrasound (TRUS) based technique 

Turner P*, Poder J**, Chin Y**, Beydoun N**, Malouf D **, Bucci J**
* Royal Surrey NHS Foundation Trust
** St George Hospital, Sydney, New South Wales 

PURPOSE: For low risk (LR) and favourable intermediate risk (fav-IR) prostate cancer, LDR brachytherapy is an accepted standard of care with excellent outcomes1. The 2-step Seattle approach2 has, in some centres, been replaced by 1-step procedures. These reduce patient visits and can allow for adaptive radiotherapy. In a single Australian institution, we used patients’ diagnostic MRI scans to generate a brachytherapy plan with a view to dispensing with the pre-implant TRUS. 

METHODS: A retrospective planning study was undertaken. 10 patients undergoing prostate LDR brachytherapy for LR or fav-IR prostate cancer were selected at random. Each had available pre-biopsy multiparametric MRI scans of prostate; the T2-series was imported into treatment planning software (TPS). Four independent radiation oncologists contoured CTV and OARs on each patient’s MRI scan. The contours of each clinician were combined to form an “average” MRI contour set for each patient on which the medical physics expert generated an LDR plan per institutional protocol (prescription = 145 Gy). Each patient then had 2-step LDR implant using I-125 seeds by Seattle method2. MRI plans were compared to final TRUS plans as implanted, across volumetric and dosimetric variables. Paired 2-sided T-tests were used to identify significant difference in means. 

RESULTS: Volumetric results – there were no significant differences in gland volume or maximum linear dimensions between MRI and TRUS contours. Dosimetric results – the MRI plans used significantly fewer seeds per plan: mean-MRI = 92.4 seeds, mean-TRUS = 96.7 seeds (p=0.014). MRI plans had significantly higher V100_rectum dose: mean-MRI = 0.33cc, mean-TRUS = 0.14cc (p=0.008); for both MRI and TRUS, these V100_rectum figures are still well inside GEC-ESTRO (=2cc)1 and institutional (=1cc) tolerances. There was no significant difference in the domains: number of needles, V100_prostate, V150_prostate, D90_prostate, V150_urethra. 

CONCLUSION: MRI-based planning may obviate the need for TRUS volume study, with no clinically significant dosimetric difference. 

References: 

1. Henry A, Pieters B, Siebert FA, Hoskin P. GEC-ESTRO ARCOP Prostate Brachytherapy Guidelines. Radiotherapy and Oncology. 2022 Feb; 167:244-251. 

2. Sylvester JE, Grimm PD, Eulau SM, Takamiya RK, Naidoo D. Permanent prostate brachytherapy preplanned technique: The modern Seattle method step-by-step and dosimetric outcomes. Brachytherapy. 2009 Apr;8(2):197-206.