To determine baseline accuracy and reproducibility of T 1 and T 2 relaxation times over 12 months on a dedicated radiotherapy MRI scanner. Methods: An International Society of Magnetic Resonance in Medicine/National Institute of Standards and Technology (ISMRM/NIST) System Phantom was scanned monthly on a 3T MRI scanner for 1 year. T 1 was measured using inversion recovery (T 1 -IR) and variable flip angle (T 1 -VFA) sequences and T 2 was measured using a multi-echo spin echo (T 2 -SE) sequence. For each vial in the phantom, accuracy errors (%bias) were determined by the relative differences in measured T 1 and T 2 times compared to reference values. Reproducibility was measured by the coefficient of variation (CV) of T 1 and T 2 measurements across monthly scans. Accuracy and reproducibility were mainly assessed on vials with relaxation times expected to be in physiological ranges at 3T. Results: A strong linear correlation between measured and reference relaxation times was found for all sequences tested (R 2 > 0.997). Baseline bias (and CV[%]) for T 1 -IR, T 1 -VFA and T 2 -SE sequences were +2.0% (2.1), +6.5% (4.2), and +8.5% (1.9), respectively. Conclusions: The accuracy and reproducibility of T 1 and T 2 on the scanner were considered sufficient for the sequences tested. No longitudinal trends of variation were deduced, suggesting less frequent measurements are required following the establishment of baselines.

The responses of two silicon on insulator (SOI) 3-D microdosimeters developed by the Centre for Medical Radiation Physics were investigated with a range of different low energy ions, with high linear energy transfer (LET). The two microdosimeters n-SOI and p-SOI were able to measure the LET of different ions including 7 Li, 12 C, 16 O, and 48 Ti with ranges below 350 μm in silicon. No plasma effects were seen in the SOI microdosimeters when irradiated with the high LET ions. A Monte Carlo simulation using Geant4 was compared to the experimental measurements, whereby some discrepancies were observed for heavier ions at lower energies. This discrepancy can be partly attributed to uncertainties in the thickness of the energy degraders and overlayers of the devices. The microdosimetric measurements of low energy 16 O ions were obtained and compared to a therapeutic 16 O ion beam. The radiation hardness of the two devices was studied using the ion beam induced charge collection technique. Both types of the microdosimeters when biased had no essential changes in charge collection efficiency in the sensitive volume after irradiation with low energy ions.

We present the first case in the literature of a 78‐year‐old woman with recurrent cardiac sarcoma adjacent to a bioprosthetic mitral valve treated with magnetic resonance linear accelerator (MR‐Linac) guided adaptive stereotactic ablative body radiotherapy (SABR). The patient was treated using a 1.5 T Unity MR‐Linac system (Elekta AB, Stockholm, Sweden). The mean gross tumour volume (GTV) size was 17.9 cm3 (range 16.6–18.9 cm3) based on daily contours and the mean dose received by the GTV was 41.4 Gy (range 40.9–41.6 Gy) in five fractions. All fractions were completed as planned and the patient tolerated the treatment well with no acute toxicity reported. Follow‐up appointments at 2 and 5 months after the last treatment showed stable disease and good symptomatic relief. Results of transthoracic echocardiogram after radiotherapy showed that the mitral valve prosthesis was normally seated with regular functionality. This study provides evidence that MR‐Linac guided adaptive SABR is a safe and viable option for the treatment of recurrent cardiac sarcoma with mitral valve bioprosthesis.