Interfraction volumetric changes/rotations in the prostate and proximal seminal vesicles (SVs) might compromise target coverage when tight margins are used for prostate stereotactic body radiotherapy (SBRT). We investigated on-board MRI images from MRI-guided SBRT to better understand this.
Twenty consecutive patients treated with MRI-guided prostate SBRT (40 Gy/5 fractions) enrolled on the MRI arm of a phase III randomized trial were included. A 2 mm isotropic margin was used for prostate and proximal SVPTV. Target volume, prostate dimensions, angles of the proximal SV on axial (angle α) and sagittal view (angle θ) were measured on a 0.35 T simulation MRI and five on-board pre-treatment MRIs. Dice coefficient of the targets and target dosimetry were calculated.
All patients experienced an isotropic increase in prostate volume during SBRT (p = 0.0016): 0.1%, 9.0%, 12.1%, 15.1%, and 14.2% (median) at fractions 1-5, respectively, regardless of baseline volume, which was significantly reduced with neoadjuvant ADT (p = 0.0042). There was minimal interfraction rotation of prostate, however, considerable variations in proximal SV angle α (median 21.5°) and angle θ (median 17.6°) were seen. Median V100% was 97.5% and 87.1% for prostate and proximal SV CTV, respectively. V95%≥95% was achieved in 94% of fractions for the prostate and in 59% for proximal SV.
Prostate volume consistently increased during SBRT. Interfraction prostatic rotation was minimal while rotation of the proximal SV was considerable. Prostate dosimetry was favorable, but online adaptive therapy may be indicated occasionally to account for prostatic swelling and in particular proximal SV rotations.

UNASSIGNED: Magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT) is one of the treatment options for radiorecurrent localized prostate cancer. However, due to the invasive nature of the treatment, not all patients are eligible. Magnetic resonance linear accelerator (MR-Linac) systems open up new treatment possibilities and could potentially replace FS-HDR-BT treatment. We conducted a planning study to investigate the feasibility of delivering a single 19 Gy dose to the recurrent lesion using a 1.5 Tesla MR-Linac system.
UNASSIGNED: Thirty patients who underwent FS-HDR-BT were included. The clinical target volume (CTV) encompassed the visible lesion plus a 5 mm margin. Treatment plans were created for a 1.5 Tesla MR-Linac system using a 1 mm planning target volume (PTV) margin. A dose of 19 Gy was prescribed to ≥ 95% of the PTV. In case this target could not be reached, i.e. when organs-at-risk (OAR) constraints were violated, a dose of ≥ 17 Gy to ≥ 90% of the PTV was accepted. MR-Linac plans were compared to clinical FS-HDR-BT plans.
UNASSIGNED: Target dose coverage was achieved in 14/30 (47%) FS-HDR-BT plans and 17/30 (57%) MR-Linac plans, with comparable median D95% and D90%. In FS-HDR-BT plans, a larger volume reached ≥ 150% of the prescribed dose. Urethra D10%, rectum D1cm3, and rectum D2cm3 were lower in the FS-HDR-BT plans, while bladder dose was comparable for both modalities.
UNASSIGNED: Single fraction treatment of recurrent prostate cancer lesions may be feasible using stereotactic body radiotherapy (SBRT) on a MR-Linac system.

OBJECTIVE: We aim to determine the feasibility and dosimetric benefits of a novel MRI-guided IMRT dose-adaption strategy for human papillomavirus positive (HPV+) oropharyngeal squamous cell carcinoma (OPC).
METHODS: Patients with locally advanced HPV+ OPC underwent pre-treatment and in-treatment MRIs every two weeks using RT immobilization setup. For each patient, two IMRT plans were created (i.e. standard and adaptive). The prescription dose for the standard plans was 2.12 Gy/fx for 33 fractions to the initial PTV. For adaptive plans, a new PTVadaptive was generated based on serial MRIs in case of detectable tumor shrinkage. Prescription dose to PTVadaptive was 2.12 Gy/fx to allow for maximum dose to the residual disease. Any previously involved volumes received minimally a floor dose of 50.16 Gy. Uninvolved elective nodal volumes were prescribed 50.16 Gy in 1.52 Gy/fx. Dosimetric parameters of organs at risk (OARs) were recorded for standard vs. adaptive plans. Normal tissue complication probability (NTCP) for toxicity endpoints was calculated using literature-derived multivariate logistic regression models.
RESULTS: Five patients were included in this pilot study, 3 men and 2 women. Median age was 58 years (range 45-69). Three tumors originated at the tonsillar fossa and two at the base of tongue. The average dose to 95% of initial PTV volume was 70.7 Gy (SD,0.3) for standard plans vs. 58.5 Gy (SD,2.0) for adaptive plans. The majority of OARs showed decrease in dosimetric parameters using adaptive plans vs. standard plans, particularly swallowing related structures. The average reduction in the probability of developing dysphagia ≥ grade2, feeding tube persistence at 6-month post-treatment and hypothyroidism at 1-year post-treatment was 11%, 4%, and 5%, respectively. The probability of xerostomia at 6-month was only reduced by 1% for adaptive plans vs. standard IMRT.
CONCLUSIONS: These in silico results showed that the proposed MRI-guided adaptive approach is technically feasible and advantageous in reducing dose to OARs, especially swallowing musculature.

Radiotherapy guidance based on magnetic resonance imaging (MRI) is currently becoming a clinical reality. Fast 2d cine MRI sequences are expected to increase the precision of radiation delivery by facilitating tumour delineation during treatment. This study compares four auto-contouring algorithms for the task of delineating the primary tumour in six locally advanced (LA) lung cancer patients.
Twenty-two cine MRI sequences were acquired using either a balanced steady-state free precession or a spoiled gradient echo imaging technique. Contours derived by the auto-contouring algorithms were compared against manual reference contours. A selection of eight image data sets was also used to assess the inter-observer delineation uncertainty.
Algorithmically derived contours agreed well with the manual reference contours (median Dice similarity index: ⩾0.91). Multi-template matching and deformable image registration performed significantly better than feature-driven registration and the pulse-coupled neural network (PCNN). Neither MRI sequence nor image orientation was a conclusive predictor for algorithmic performance. Motion significantly degraded the performance of the PCNN. The inter-observer variability was of the same order of magnitude as the algorithmic performance.
Auto-contouring of tumours on cine MRI is feasible in LA lung cancer patients. Despite large variations in implementation complexity, the different algorithms all have relatively similar performance.