Catheter reconstruction and dosimetric verification of MRI-only treatment planning (MRTP) for interstitial HDR brachytherapy using PETRA sequence


 Objective: The feasibility of MRI-only treatment planning (MRTP) for interstitial high-dose rate (HDR) brachytherapy (BT) was investigated for patients diagnosed with gynecologic cancer. Approach: A clinical MRTP workflow utilizing a ‘Pointwise Encoding Time Reduction with Radial Acquisition (PETRA)’ sequence was proposed. This is a clinically available MRI sequence optimized to improve interstitial catheter-tissue contrast. Interstitial needles outside the obturator region were reconstructed using MR images only. For catheters penetrating through the obturator, a library-based reconstruction was proposed. In this work, dwell coordinates from the clinical CT-based reconstruction were used as the surrogate for the library-based approach. For MR-only plan, dwell times were activated and assigned as in the clinical plans. The catheter reconstruction was assessed by comparing dwell position coordinates. The dosimetric comparisons between a clinical plan and MR-only plan were assessed for physical and EQD2 dose and volume parameters for D90, D50 and D98 for clinical target volume (CTV) and D2cc, D0.1cc and D5cc for OARs. Main Results: Catheter reconstruction was possible using the optimized PETRA sequence on MR images. An overall reconstruction difference of 1.7±0.5 mm, attributed to registration-based errors, was found compared to the CT-based reconstruction. The MRTP workflow has the potential to generate a treatment plan with an equivalent dosimetric quality compared to the conventional CT/MRI-based approach. For CTV D90, physical and EQD2 dose and volume parameter differences were 1.5±1.9% and 0.7±1.0 Gy, respectively. For D2cc OARs, DVH (EQD2) differences were -0.4±1.1% (-0.2±0.5 Gy), 0.5±2.8% (0.2±1.3 Gy) and -0.5±1.4% (-0.2±0.5 Gy) for rectum, bladder, and sigmoid, respectively. Significance: With the proposed MRTP approach, CT imaging may no longer be needed in HDR BT for interstitial gynecologic treatment. A proof-of-concept study was conducted to demonstrated that MRTP using PETRA is feasible, with comparable dosimetric results to the conventional CT/MRI-based approach.