Background: Magnetic resonance-guided adaptive radiotherapy (MRgART) treatment sessions at MR-Linac are time-consuming and changes in organs at risk volumes can impact the treatment dosimetry. This study aims to evaluate the feasibility to control bladder filling during the rectum MRgART online session and its effectiveness on plan dosimetry. Methods: A total of 109 online adaptive sessions of 24 rectum cancer patients treated at Unity 1.5 T MR-Linac with a short course radiotherapy (25 Gy, 5 Gy × 5) for whom the adaptive plan was optimized and recalculated online based on the daily magnetic resonance imaging (MRI) were analysed. Patients were fitted with a bladder catheter to control bladder filling; the bladder is emptied and then partially filled with a known amount of saline at the beginning and end of the online session. A first MRI ( $${\mathrm{MRI}}_{\mathrm{i}}$$MRI i ) acquired at the beginning of the session was used for plan adaptation and the second ( $${\mathrm{MRI}}_{\mathrm{f}}$$MRI f ) was acquired while approving the adapted plan and rigidly registered with the first to ensure the appropriateness of the isodoses on the ongoing delivery treatment. For each fraction, the time interval between the two MRIs and potential bladder changes were assessed with independent metrics, and the impact on the plan dosimetry was evaluated by comparing target and organs at risk dose volume histogram cut-off points of the plan adapted on $${\mathrm{MRI}}_{\mathrm{i}}$$MRI i and recalculated on $${\mathrm{MRI}}_{\mathrm{f}}$$MRI f . Results: Median bladder volume variations, DSC, and HD of 8.17%, 0.922, and 2.92 mm were registered within a median time of 38 min between $${\mathrm{MRI}}_{\mathrm{i}}$$MRI i and $${\mathrm{MRI}}_{\mathrm{f}}$$MRI f ; dosimetric differences < 0.65% were registered for target coverage, and < 0.5% for bladder, small bowel and femoral heads constraints, with a p value > 0.05. Conclusion: The use of a bladder filling control procedure can help ensure the dosimetric accuracy of the online adapted treatment delivered.

The accurate reconstruction of the in vivo dose is a critical step in radiation therapy. X-ray-induced acoustic imaging is a promising technology for in vivo dose reconstruction, as it enables the nonradiative and noninvasive monitoring of radiation dose. However, current X-ray acoustic imaging methods suffer from several limitations, including high signal-to-noise ratio, poor imaging quality and massive loss of structural information. To address these limitations, we propose a dose image reconstruction method based on tensor sparse dictionary learning. Specifically, we combine tensor coding with compressed sensing data, extend two-dimensional dictionary learning to three-dimensional by using tensor product, and then utilize the spatial information of X-ray acoustic signal more efficiently. To reduce the artifacts of reconstruction images caused by spare sampling, we design the alternate iterative solution of the tensor sparse coefficient and tensor dictionary. In addition, we build the X-ray-induced acoustic dose images reconstruction system, simulate the X-ray acoustic signals based on patients’ information from Sichuan Cancer Hospital, and then create the simulated datasets. Compared to some typical state-of-the art imaging methods, the experimental results demonstrate that our method can significantly improve the quality of reconstructed images and the accuracy of dose distribution.

In magnetic resonance- (MR-) based adaptive workflows for an MR-linac, the treatment plan is optimized and recalculated online using the daily MR images. The Unity MR-linac is supplied with a patient positioning device (ppd) using pelvic and abdomen thermoplastic masks attached to a board with high-density components. This study highlights the dosimetric effect of using this in such workflows when there are relative patient-ppd displacements, as these are not visualized on MR imaging and the treatment planning system assumes the patient is fixed relative to the ppd. The online adapted plans of two example rectum cancer patients treated at a Unity MR-linac were perturbed by introducing relative patient-ppd displacements, and the effect was evaluated on plan dosimetry. Forty-eight perturbed clinical adapted plans were recalculated, based on online MR-based synthetic computed tomography, and compared with the original plans, using dose-volume histogram parameters and gamma analysis. The target volume covered by the prescribed dose ( $${\text{D}}_{\text{p}\text{r}\text{e}}$$D pre ) and by at least 107% of $${\text{D}}_{\text{p}\text{r}\text{e}}$$D pre varied up to − 1.87% and + 3.67%, respectively for 0.5 cm displacements, and to − 3.18% and + 4.96% for 2 cm displacements; whilst 2%–2 mm gamma analysis showed a median value of 92.9%. The use of a patient positioning system with high-density components in a Unity MR-based online adaptive treatment workflow can introduce unrecognized errors in plan dosimetry and it is recommended not to use such a device for such treatments, without modifying the device and the workflow, followed by careful clinical evaluation, or alternatively to use other immobilization methods.

Objective: This study evaluates the efficacy and safety of a modified sandwich (mSAND) neoadjuvant regimen combining 1.5T magnetic resonance guided linear accelerator (MR-Linac) guided short-course radiotherapy (SCRT; 25 Gy in 5 fractions) with sequential chemotherapy in patients with locally advanced rectal cancer (LARC).Methods: All patients received an mSAND neoadjuvant regimen. Tumor response was reassessed 2 weeks after neoadjuvant chemoradiotherapy (NCRT), followed by total mesorectal excision (TME) and six cycles of adjuvant mFOLFOX6. Primary endpoints were the rates of clinical complete response (cCR), pathological complete response (pCR), and objective response (ORR), secondary endpoints included 2-year overall survival (OS), locoregional recurrence-free survival (LRFS), progression-free survival (PFS), health-related quality of life (QoL), and treatment-related adverse events (TRAEs).Results: In this single-arm, phase II trial, 62 LARC patients were enrolled from December 2022 to October 2025, with a median follow-up of 15.5 months. The cCR rate was 22.6% ORR 61.3%, and pCR 25.8% (intention-to-treat). Following the mSAND regimen, both radiologic tumor regression and a significant decline in tumor biomarkers were observed (p < 0.05). At 2 years, LRFS, PFS, and OS rates were 79.9%, 78.3%, and 89.7%, respectively. Treatment was well-tolerated, with <2% Grade ≥3 gastrointestinal toxicity during NCRT and 6.5% Grade ≥3 hematologic toxicity during adjuvant chemotherapy. Quality-of-life scores (EORTC QLQ-C30) improved post-treatment steadily, reaching 87.8 at completion.Conclusion: The mSAND regimen demonstrated encouraging rates of cCR, pCR, and survival, with an acceptable safety profile.