PO40

Abstract

Purpose Tumor coverage with conventional MRI compatible combined intracavitary/interstitial (IC/IS) applicators is scarce in some patients with locally advanced gynecological malignancies. In these cases, it is recommended to add a larger interstitial component using transperineal templates (P-ISBT). Our department has been performing this type of implant since 2005 using MUPIT applicator and CT-based planning. In 2013 we switched to MRI-based planning and a compatible applicator had to be developed. It combines an IC component (intrauterine tandem) with a perineal template and Titanium needles. It is an attempt to combine the technical advantages of the MUPIT and of the MRI, while preserving the stability, geometry, and robustness of the implant. In contrast with the CT, MRI provides an excellent visibility of soft tissue, allowing a better delineation of residual tumor at the time of BT, resulting in more accurate and generally smaller treatment volumes. The aim of this work is to present the impact and benefice of MRI implementation in P-ISBT. For this purpose, the two groups of patients (pre- and post-2013) were compared in terms of CTV volume and late toxicity. Materials and Methods From 2005 to 2022, 169 patients diagnosed with primary/recurrent gynecological tumors were treated with P-ISBT. 80 patients, without dosimetric data (planned in a retired TPS) were excluded, leaving 89 patients for analysis. Patients were treated with either MUPIT or MRI-based applicator. Implants were performed by the same team of radiation oncologists, and following the same delineation and prescription protocols. Dose prescription was 24 Gy in 6 fractions for CT-based plans, and 25.5 Gy in 6 fractions for MRI-based plans. Fractions were administered twice daily. Dosimetric planning is also homogeneous within the two patient groups plan optimization was performed through the help of geometrical optimization, followed by a fine-tuning manual optimization, in order to avoid inner over-dose volumes.The CTV volumes of both groups of patients have been compared. Similarly, to demonstrate homogeneity in dosimetric planning, CTV overdose volumes V120%, V150% and V200% were compared. Finally, toxicity outcomes were analyzed using CTCAE v5.0. SPSS Statistics was used for analysis. Results 24 patients treated with MUPIT were compared to 65 patients treated with MRI- applicator. Mean CTV volumes were compared in Table 1 for patients with primary cervical cancer and in other cases (vaginal primary or recurrent), showing a halved volume in favor of MRI.Overdose volumes were compared for different CTV volume categories (image 1). The results for the 3 indices are fully equivalent for the different volume ranges As for late toxicities: G1-2 rectal toxicity was 37.5% in MUPIT vs. 7.7% in MRI-based applicator (P=0.0006); G3 rectal toxicity was 12.5% vs. 6.2% respectively (ns). G1-2 urinary toxicity was 8.3% in MUPIT vs. 6.2% in MRI-based applicator (ns); G3 urinary toxicity was 8.3% vs. 1.5% respectively (ns). G3 vaginal toxicity was 12.5% in MUPIT vs. 6.2% in MRI-based applicator (ns). No G4 toxicities were reported. Conclusions MRI implementation in P-ISBT offers an improvement in brachytherapy plans, allowing for better volume definition, thus resulting in smaller CTV volumes and a significant better outcome in global toxicity. Tumor coverage with conventional MRI compatible combined intracavitary/interstitial (IC/IS) applicators is scarce in some patients with locally advanced gynecological malignancies. In these cases, it is recommended to add a larger interstitial component using transperineal templates (P-ISBT). Our department has been performing this type of implant since 2005 using MUPIT applicator and CT-based planning. In 2013 we switched to MRI-based planning and a compatible applicator had to be developed. It combines an IC component (intrauterine tandem) with a perineal template and Titanium needles. It is an attempt to combine the technical advantages of the MUPIT and of the MRI, while preserving the stability, geometry, and robustness of the implant. In contrast with the CT, MRI provides an excellent visibility of soft tissue, allowing a better delineation of residual tumor at the time of BT, resulting in more accurate and generally smaller treatment volumes. The aim of this work is to present the impact and benefice of MRI implementation in P-ISBT. For this purpose, the two groups of patients (pre- and post-2013) were compared in terms of CTV volume and late toxicity. From 2005 to 2022, 169 patients diagnosed with primary/recurrent gynecological tumors were treated with P-ISBT. 80 patients, without dosimetric data (planned in a retired TPS) were excluded, leaving 89 patients for analysis. Patients were treated with either MUPIT or MRI-based applicator. Implants were performed by the same team of radiation oncologists, and following the same delineation and prescription protocols. Dose prescription was 24 Gy in 6 fractions for CT-based plans, and 25.5 Gy in 6 fractions for MRI-based plans. Fractions were administered twice daily. Dosimetric planning is also homogeneous within the two patient groups plan optimization was performed through the help of geometrical optimization, followed by a fine-tuning manual optimization, in order to avoid inner over-dose volumes.The CTV volumes of both groups of patients have been compared. Similarly, to demonstrate homogeneity in dosimetric planning, CTV overdose volumes V120%, V150% and V200% were compared. Finally, toxicity outcomes were analyzed using CTCAE v5.0. SPSS Statistics was used for analysis. 24 patients treated with MUPIT were compared to 65 patients treated with MRI- applicator. Mean CTV volumes were compared in Table 1 for patients with primary cervical cancer and in other cases (vaginal primary or recurrent), showing a halved volume in favor of MRI.Overdose volumes were compared for different CTV volume categories (image 1). The results for the 3 indices are fully equivalent for the different volume ranges As for late toxicities: G1-2 rectal toxicity was 37.5% in MUPIT vs. 7.7% in MRI-based applicator (P=0.0006); G3 rectal toxicity was 12.5% vs. 6.2% respectively (ns). G1-2 urinary toxicity was 8.3% in MUPIT vs. 6.2% in MRI-based applicator (ns); G3 urinary toxicity was 8.3% vs. 1.5% respectively (ns). G3 vaginal toxicity was 12.5% in MUPIT vs. 6.2% in MRI-based applicator (ns). No G4 toxicities were reported. MRI implementation in P-ISBT offers an improvement in brachytherapy plans, allowing for better volume definition, thus resulting in smaller CTV volumes and a significant better outcome in global toxicity.