GPP09 Presentation Time: 10:00 AM: Patient Specific Template-Based Hybrid Brachytherapy for Treatment of Gynecological Cancers

Abstract

<h3>Purpose</h3> Interstitial vaginal and pelvic brachytherapy (BT) is standard of care in locally advanced vaginal cancers and pelvic recurrence of gynecological malignancies. An in-house automated design and 3D printing workflow for pre-plannable patient-specific cylindrical templates (PSCTs) for IC/IS HDRBT was used for 7 patients treated at BC Cancer - Kelowna. Insertion positions and dosimetry are compared for preplan and final plans, and the practicality of preplanning for IC/IS HDRBT of the vagina is discussed. This work demonstrates that patient-specific treatment techniques for gynecological brachytherapy can be implemented within the clinic at a low cost. <h3>Materials and methods</h3> Seven patients with primary or recurrent vaginal malignancies were treated with IC/IS HDRBT facilitated with a PSCT which guided the position of IC/IS needles. Patients underwent pelvic T2-MRI imaging at least 7 days before their procedure with a dummy cylinder inserted. To satisfy pre-planning goals, virtual needles orientations and dwell times were optimized and adjusted as needed until the final plan is acceptable (Figure 1). Plans were exported and 3D modeled automatically using in-house written software. PSCTs were 3D printed of PEEK, and post-processed to have a smooth final finish, as shown in Figure 1. Each needle was pushed to its final depth under abdominal ultrasound guidance after the cylinder was inserted. Post-insertion CT scans were registered to preplan MRI scans and contoured, and final plans were optimized before treatment using digitized final needle positions. Patient final plan (Figure 1) dosimetry and needle insertion statistics were calculated and compared to pre-insertion plans. <h3>Results</h3> The treated median HRCTV D90, V100, V150, V200, Bladder, Rectum, Sigmoid and Small Bowel D2cm³, were 110.8%, 96.7%, 52.8%, 28.0%, 18.0 Gy, 15.4 Gy, 11.0 Gy, and 12.9 Gy respectively. The median differences of the final treated plan (post-plan) to the preplan were 1.1%, -0.5%, 13.1%, 4.1%, 1.0 Gy, 0.8 Gy, 0.5 Gy, and -0.8 Gy respectively. The median insertion displacement of dwell positions for straight needles, curved needles, and overall was 1.3mm, 4.0mm, and 2.3mm, respectively. <h3>Conclusion</h3> 3D printed patient-specific templates for the guidance of interstitial high dose rate brachytherapy of the vagina can be used to preplan and deliver highly conformal IC/IS Vaginal HDRBT treatments. The PSCT provides a significantly easier IC/IS insertion procedure as compared to using an IC cylinder + grid or freehand needles. Patients whose HRCTV geometry was in one location extending away from the cylinder rather than wrapping around the entire vagina had improved dosimetric indices because the PSCT is geometrically constrained to only be able to fit a maximum number of needles, thus covering a wide range of directions and depths is more difficult as compared to treating a more directed region. It is possible to combine a PSCT with an IS grid template to have the advantages of both techniques. Compared to commercial IC cylinders, the PSCT is preferable, since the preplan allows for patient customization and trade-offs optimization before insertion. Since IS dwell positions are achievable through PSCTs, they achieve preferable dose to target versus OAR trade-offs. Compared to techniques using IS Grid needles, the PSCT facilitates lower interstitial needle lengths, and preferable insertion locations in the vaginal mucosa rather than through the perineum. These templates can facilitate higher quality treatment plans as compared to state of the art techniques, particularly for deep, localized, and top-of-vault lesions, improving target coverage, reducing OAR dose, reducing interstitial needle length and trauma in non-tumor tissue and improving confidence in needle insertion without intraoperative MRI or CT.