PP02  Presentation Time: 10:09 AM: Prospective Assessment of Interstitial Needles with Transrectal Ultrasound in Cervical Cancer Brachytherapy

Purpose To prospectively evaluate the visibility (quantitatively and qualitatively) of interstitial needles in cervical cancer patients with combined intracavitary/interstitial (IC/IS) applications using transrectal ultrasound images (TRUS). Material and Methods This is a prospective single arm cohort study. Inclusion criteria were (1) biopsy proven cervical cancer, (2) treatment with MR-Image-guided adaptive brachytherapy (MR-IGABT) and (3) utilization of IC/IS. TRUS (bk5000, BK Medical) was performed during and after applicator insertion and each inserted needle was documented by an axial screenshot on the level of the largest diameter of the high-risk clinical target volume (HR-CTV) for analysis. For qualitative assessment, the visibility of each needle was rated on TRUS with the following scoring system: 0=no visibility; 1=poor discrimination, margin blurred; 2=fair discrimination, margin indistinct; 3=excellent discrimination, margin distinct. For quantitative assessment, the distance between the tandem and each needle was measured separately and compared to the respective measurement on MRI. The expected implant quality for optimal dose prescription based on TRUS imaging was rated with the following scoring system: 1=excellent (HR-CTV and organs at risk (OAR's) soft constraints were met), 2=sufficient (HR-CTV or OAR's soft constraints violated), 3=poor (HR-CTV and OAR's soft constraints violated), 4=insufficient (HR-CTV and/or OAR's hard constraints violated). Evaluation and measurements were performed by 3 radiation oncologists with more than 5 years of experience. Descriptive statistics were used for data analysis. Results Patient accrual began in 05/2022; 23 patients with local FIGO stage IB2-IVA with 41 applications and a total of 230 needles (186 straight through the ring, 34 oblique through the ring, 10 free-hand) were available. Overall, 228/230 needles (99.1%) were visible, mean visibility score ± standard deviation (SD) was 2.5 ± 0.7 for all visible needles. The two non-visible needles (both inserted free-hand) were masked by artefacts or outside the field of view. Maximum (max), minimum (min), mean ± SD distance of the visible needles to tandem was 35 mm, 4 mm, 17.8 mm ± 4.0 mm on MRI and 34 mm, 6 mm, 17.6 mm ± 3.7 mm on TRUS, respectively. Max and mean ± SD difference between MRI and TRUS was 8 mm, 1.1 mm ± 1.1 mm. A difference of more than 3 mm was found in 8/228 needles (3.5%). Expected implant quality based on TRUS imaging and actual implant quality after MRI-based planning are compared in table 1. Conclusions Almost all interstitial needles (99%) were visible on TRUS in this analysis. Needles inserted through the template positions were always detectable. Qualitatively, the majority of needles showed a distinct signal on TRUS. The quantitative difference compared to MRI was within 3mm in over 96%. The expected implant quality based on TRUS imaging was congruent with the actual MRI-based implant quality in the majority of cases. In this cohort, the real time use of TRUS for interstitial needle guidance resulted in high quality implants due to excellent visibility of needles during insertion, thereby rendering TRUS-guided brachytherapy a promising modality to pursue & develop for gynecological indications. To prospectively evaluate the visibility (quantitatively and qualitatively) of interstitial needles in cervical cancer patients with combined intracavitary/interstitial (IC/IS) applications using transrectal ultrasound images (TRUS). This is a prospective single arm cohort study. Inclusion criteria were (1) biopsy proven cervical cancer, (2) treatment with MR-Image-guided adaptive brachytherapy (MR-IGABT) and (3) utilization of IC/IS. TRUS (bk5000, BK Medical) was performed during and after applicator insertion and each inserted needle was documented by an axial screenshot on the level of the largest diameter of the high-risk clinical target volume (HR-CTV) for analysis. For qualitative assessment, the visibility of each needle was rated on TRUS with the following scoring system: 0=no visibility; 1=poor discrimination, margin blurred; 2=fair discrimination, margin indistinct; 3=excellent discrimination, margin distinct. For quantitative assessment, the distance between the tandem and each needle was measured separately and compared to the respective measurement on MRI. The expected implant quality for optimal dose prescription based on TRUS imaging was rated with the following scoring system: 1=excellent (HR-CTV and organs at risk (OAR's) soft constraints were met), 2=sufficient (HR-CTV or OAR's soft constraints violated), 3=poor (HR-CTV and OAR's soft constraints violated), 4=insufficient (HR-CTV and/or OAR's hard constraints violated). Evaluation and measurements were performed by 3 radiation oncologists with more than 5 years of experience. Descriptive statistics were used for data analysis. Patient accrual began in 05/2022; 23 patients with local FIGO stage IB2-IVA with 41 applications and a total of 230 needles (186 straight through the ring, 34 oblique through the ring, 10 free-hand) were available. Overall, 228/230 needles (99.1%) were visible, mean visibility score ± standard deviation (SD) was 2.5 ± 0.7 for all visible needles. The two non-visible needles (both inserted free-hand) were masked by artefacts or outside the field of view. Maximum (max), minimum (min), mean ± SD distance of the visible needles to tandem was 35 mm, 4 mm, 17.8 mm ± 4.0 mm on MRI and 34 mm, 6 mm, 17.6 mm ± 3.7 mm on TRUS, respectively. Max and mean ± SD difference between MRI and TRUS was 8 mm, 1.1 mm ± 1.1 mm. A difference of more than 3 mm was found in 8/228 needles (3.5%). Expected implant quality based on TRUS imaging and actual implant quality after MRI-based planning are compared in table 1. Almost all interstitial needles (99%) were visible on TRUS in this analysis. Needles inserted through the template positions were always detectable. Qualitatively, the majority of needles showed a distinct signal on TRUS. The quantitative difference compared to MRI was within 3mm in over 96%. The expected implant quality based on TRUS imaging was congruent with the actual MRI-based implant quality in the majority of cases. In this cohort, the real time use of TRUS for interstitial needle guidance resulted in high quality implants due to excellent visibility of needles during insertion, thereby rendering TRUS-guided brachytherapy a promising modality to pursue & develop for gynecological indications.