ESTRO 2023 - Abstract Book

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ESTRO 2023

grading about sexual impairment. PROs may be a benefit, owing to fully assess the cost-effectiveness of LDR-BCT and other curative treatments of PCa. Prospective evaluation of our promising findings on wider series is granted.

PO-2193 Organ positional differences between planning US and implant procedure in two-step LDR brachytherapy

O. Houlihan 1,2 , S. Esteve 3 , O. McLaughlin 3,2 , G. Workman 3 , M. Byrne 3 , K. Prise 2 , A. Hounsell 3 , D. Mitchell 1 , S. Jain 1,2

1 Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Clinical Oncology, Belfast, United Kingdom; 2 Queen’s University Belfast, Patrick G. Johnston Centre for Cancer Research, Belfast, United Kingdom; 3 Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Radiotherapy Medical Physics, Belfast, United Kingdom Purpose or Objective To quantify dosimetric differences for the prostate, urethra and rectum between the planning transrectal ultrasound (TRUS) and the implant procedure in two-step LDR brachytherapy. Materials and Methods A TRUS was performed immediately prior to radioactive seed implantation for 25 patients undergoing two-step LDR prostate brachytherapy. The brachytherapy plan, generated from a planning TRUS performed 3-4 weeks prior to the implant procedure was imported onto this pre-implant TRUS. The prostate, urethra and rectum were contoured and the doses to these organs were compared to the dose received on the original planning TRUS. Institutional dose constraints were prostate V150% (volume receiving at least 150% of the prescription dose) < 60% and V200% < 20%, urethra D5% (minimum dose to the most irradiated 5%) < 150% and D30% < 130% and rectum D1cc (minimum dose to the most irradiated 1cc) < 100% and D0.1cc < 138%. Target coverage for prostate was D90% > 121% and V100% > 99.5%. Due to the small of the urethra, the dose to the percentage volume parameter is sensitive to the volume contoured (e.g. if extra slices are contoured cranially or caudally due to slice resolution). To mitigate this limitation, the absolute urethral volumes (in cm3) corresponding to 5% and 30% were initially extracted from the planning TRUS DVH. These absolute volumes were then interpolated on the DVH for the pre-implant TRUS. This method was independent of urethral volume contoured and therefore allowed direct comparison of these parameters. Results The median urethra D5% was greater on the pre-implant TRUS relative to the planning TRUS by 2.1% ((D5% pre-implant–D5% original)/(D5% original)*100) and the median urethra D30% was greater by 1.4%. The median rectum D1cc was lower by 4.9% and D0.1cc by 4.1% on pre-implant TRUS relative to the planning TRUS. The median prostate dosimetric parameters were also lower on the pre-implant TRUS compared to the planning TRUS with median differences relative to the planning TRUS for D90% of 2.8%, V100% of 0.7%, V150% of 4.7% and V200% of 5.3% (Table 1). Using the DVH absolute volume interpolation method the number of cases exceeding dose constraints for urethra D5% increased from one on the original planning US to nine on the pre-implant TRUS, for urethra D30% increased from three on the original TRUS to fifteen on the pre-implant TRUS and for rectum D1cc and D0.1cc increased from zero on the original TRUS to one on the pre-implant TRUS. The urethral doses on the clinical plans were all within tolerance.

Conclusion Median relative differences between the pre-implant TRUS and planning TRUS were low at ≤ 5.3% which is reassuring for centres performing two-step LDR prostate brachytherapy. The increased number of cases exceeding dose constraints for the urethra on pre-implant US may be a result of positional differences caused by relaxation of the urethra due to spinal anaesthetic used during the implant procedure and should be considered intra-operatively by the brachytherapy clinician.

PO-2194 uLDR-BT in treating patients with unfavorable intermediate-risk group prostate cancer.

A. Kluska 1 , A. Chiche ł 1 , W. Burchardt 1 , M. W ł odarczyk 2 , A. Chyrek 1

1 Greater Poland Cancer Center, Brachytherapy Department, Pozna ń , Poland; 2 Greater Poland Cancer Center, 3rd Department of Radiotherapy, Pozna ń , Poland Purpose or Objective Ultra-low dose rate brachytherapy (uLDR-BT) is effective and widely used as monotherapy for low-risk and favorable intermediate-risk prostate cancer. Treatment with sole uLDR-BT for unfavorable intermediate risk factor (IUR) group prostate cancer patients is not recommended by guidelines due to the lack of solid evidence of its effectiveness. However, in older trials regarding the efficacy of this method in the IUR group, numerous patients were treated with good results. The purpose of this work was to retrospectively assess the effectiveness of uLDR-BT in the IUR group treated in our department. Materials and Methods We performed a retrospective analysis of 34 IUR prostate cancer patients treated with uLDR-BT between 2015-2018. The local staging was assessed with either MRI or TRUS. The treatment was performed using I-125 seeds, and the total dose prescribed to the clinical target volume (prostate with margin) was 145 Gy. Androgen deprivation therapy (ADT, neoadjuvant or concomitant) was given in 25 cases. All patients had a follow-up in our ambulatory every three months