MR-Guided Boost Irradiation for Patients with Pelvic Recurrence of Gynecological Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Pelvic recurrence of cervical or endometrial cancer may not be suitable to brachytherapy boost after chemoradiotherapy (CRT). The aim of this study was to evaluate the feasibility and appropriateness of using magnetic resonance guided radiotherapy (MRgRT) for pelvic boost irradiation. <h3>Materials/Methods</h3> Twelve consecutive patients with pelvic recurrence after hysterectomy receiving pelvic boost irradiation on MR-Linac 1.5T after CRT were included. Each patient underwent CT and MR simulation scans with arms above head using indexed patient positioning aids. A bladder catheter was used at each imaging scan to ensure that same bladder filling. Target and organs at risk (OARs) were contoured in a treatment planning software, and the treatment plan was generated using eight to eleven individual beam angles. A boost dose of 10 to 25 Gy was delivered in 5Gy/fraction. The plan was optimized to achieve the prescribed dose of minimum 90% of the target volume; the OARs constraints of the American ABS guidelines were followed; compromises were made if necessary. Patients followed the adapt to shape workflow consisting of a re-optimization of the plan on the MRI-based synthetic CT; planning CT contours propagated daily on MRI scans with a deformable registration were adjusted by the referring radiation oncologist. Considering the time elapsed for recontouring, plan optimization-calculation-approval to ensure the adequacy of the ongoing treatment, a second MRI was performed and rigidly registered with the MRI used for the adaptive plan to validate the isodoses obtained. If positional differences were observed, a further step was performed with a recalculation of the plan after adjustment to the isocenter. The accuracy of the target movement vs isodose lines was monitored during treatment delivery with real-time 3D MRI. The treatment parameters were acquired to assess the effectiveness of the workflow. <h3>Results</h3> Target coverage was beyond expectations with D95% varying between (4.9-5.5) Gy/fraction, keeping OARs below constraints. Patient characteristics, treatment parameters, target dosimetry and OARs acquired in 40 MR-gRT sessions of 12 patients are represented in the table below. <h3>Conclusion</h3> The use of the MRgRT boost approach for patients with gynecological cancer recurrence is feasible. The online daily adaptive plan allows the expected target coverage and low dose for OARs to be achieved. Close monitoring of bladder filling and repeated MRI along the treatment ensure repeatability and accuracy of sessions.