Stereotactic body radiotherapy (SBRT) is an effective radiation therapy technique that has allowed for shorter treatment courses, as compared to conventionally dosed radiation therapy. As its name implies, SBRT relies on daily image guidance to ensure that each fraction targets a tumor, instead of healthy tissue. Magnetic resonance imaging (MRI) offers improved soft-tissue visualization, allowing for better tumor and normal tissue delineation. MR-guided RT (MRgRT) has traditionally been defined by the use of offline MRI to aid in defining the RT volumes during the initial planning stages in order to ensure accurate tumor targeting while sparing critical normal tissues. However, the ViewRay MRIdian and Elekta Unity have improved upon and revolutionized the MRgRT by creating a combined MRI and linear accelerator (MRL), allowing MRgRT to incorporate online MRI in RT. MRL-based MR-guided SBRT (MRgSBRT) represents a novel solution to deliver higher doses to larger volumes of gross disease, regardless of the proximity of at-risk organs due to the (1) superior soft-tissue visualization for patient positioning, (2) real-time continuous intrafraction assessment of internal structures, and (3) daily online adaptive replanning. Stereotactic MR-guided adaptive radiation therapy (SMART) has enabled the safe delivery of ablative doses to tumors adjacent to radiosensitive tissues throughout the body. Although it is still a relatively new RT technique, SMART has demonstrated significant opportunities to improve disease control and reduce toxicity. In this review, we included the current clinical applications and the active prospective trials related to SMART. We highlighted the most impactful clinical studies at various tumor sites. In addition, we explored how MRL-based multiparametric MRI could potentially synergize with SMART to significantly change the current treatment paradigm and to improve personalized cancer care.

Introduction: High-dose total body irradiation (TBI) is often part of myeloablative conditioning in acute leukemia. Modern volumetric modulated arc therapy (VMAT)-based plans employ arcs to the inferior-most portion of the body that can be simulated in a head-first position and use 2D planning for the inferior body which can result in heterogeneous doses. Here, we describe our institution's unique protocol for delivering high-dose TBI entirely with VMAT and retrospectively compare dosimetric outcomes with helical tomotherapy (HT) plans. Additionally, we describe our method of oropharyngeal mucosal sparing that was implemented after fatal mucositis occurred in two patients. Methods: Thirty-one patients were simulated and treated in head-first (HFS) and feet-first (FFS) orientations. Patients were treated with VMAT (n = 26) or HT (n = 5). In VMAT plans, to synchronize doses between the orientations, images were deformably registered and the HFS dose was transferred to the FFS plan and used as a background dose when optimizing plans. Six to eight isocenters with two arcs per isocenter were generated. HT was delivered with an established technique. Patients were treated to 13.2 Gy over eight twice daily fractions. Dosimetric outcomes and toxicities were retrospectively compared. Results: Prescription dose and organ at risk (OAR) constraints were met for all patients. Lower lung doses were achieved with VMAT relative to HT plans (7.4 vs 7.7 Gy, P = .009). Statistically significant improvement in mucositis was not achieved after adopting a mucosal-sparing technique, however lower doses to the oropharyngeal mucosal were achieved (6.9 vs 14.1 Gy, P = .009), and no further mucositis-related deaths occurred. Conclusions: This full-body VMAT method of TBI achieves dose goals, eliminates risk of heterogenous doses within the femur, and demonstrates that selective OAR sparing with the purpose of reducing TBI-related morbidity and mortality is possible at any institution with a VMAT-capable linear accelerator.

Ventilation imaging using 4D CT is a convenient and low-cost functional imaging methodology which might be of value in radiotherapy treatment planning to spare functional lung volumes. Deformable image registration (DIR) is needed to calculate ventilation imaging from 4D CT. This study investigates the dependence of calculated ventilation on DIR methods and ventilation algorithms. DIR of the normal end expiration and normal end inspiration phases of the 4D CT images was used to correlate the voxels between the two respiratory phases. Three different DIR algorithms, optical flow (OF), diffeomorphic demons (DD), and diffeomorphic morphons (DM) were retrospectively applied to ten esophagus and ten lung cancer cases with 4D CT image sets that encompassed the entire lung volume. The three ventilation extraction methods were used based on either the Jacobian, the change in volume of the voxel, or directly calculated from Hounsfield units. The ventilation calculation algorithms used are the Jacobian, ?\mathrm{V} , and HU method. They were compared using the Dice similarity coefficient (DSC) index and Bland-Altman plots. Dependence of ventilation images on the DIR was greater for the ?\mathrm{V} and the Jacobian methods than for the HU method. The DSC index for 20% of low-ventilation volume for ?\mathrm{V} was 0.33?0.03\left(1\text{SD}\right) between OF and DM, 0.44?0.05 between OF and DD, and 0.51?0.04 between DM and DD. The similarity comparisons for Jacobian were 0.32?0.03,0.44?0.05 , and 0.51?0.04 , respectively, and for HU they were 0.53?0.03,0.56?0.03 , and 0.76?0.04 , respectively. Dependence of extracted ventilation on the ventilation algorithm used showed good agreement between the ?\mathrm{V} and Jacobian methods, but differed significantly for the HU method. DSC index for using OF as DIR was 0.86?0.01 between ?\mathrm{V} and Jacobian, 0.28?0.04 between ?\mathrm{V} and HU, and 0.28?0.04 between Jacobian and HU, respectively. When using DM or DD as DIR, similar values were obtained when comparing the different ventilation calculation methods. The similarity values for the 20% high-ventilation volume were close to those found for the 20% low-ventilation volume. The results obtained with DSC index were confirmed when using the Bland-Altman plots for comparing the ventilation images. Our data suggest that ventilation calculated from 4D CT depends on the DIR algorithm employed. Similarities between ?\mathrm{V} and Jacobian are higher than between ?\mathrm{V} and HU, and Jacobian and HU. PACS number: 87.57.nj

Background: The use of stereotactic body radiation therapy (SBRT) is widely utilized for treatment of localized prostate cancer. Magnetic-resonance-guided radiotherapy (MRgRT) was introduced in 2014 and has recently been implemented in SBRT for prostate cancer as it provides an opportunity for smaller margins and adaptive daily planning. Currently, the only publications of MRgRT for prostate SBRT describe European clinical experiences which utilized adaptive planning. However, adaptive planning adds significantly to the time required for daily treatment. Objectives: Since prostate SBRT has demonstrated acceptable toxicity for several years, we did not consider daily adaptation critical to the process of prostate SBRT. After Institutional Review Board approval, we analyzed and now report our experience using MRgRT without adaptation. Methods: Between 25 September 2019 and 21 December 2020, 35 consecutive patients were treated with MRgRT prostate SBRT at our center. Patients treated with MRgRT included favorable intermediate risk (43%) and unfavorable intermediate risk (54%), and only one patient had low-risk prostate cancer. Nine patients (25%) received adjuvant leuprolide for a median of 4.5 months (range 4–6 m). Our clinical pathway allows for a maximum prostate gland volume of 60 cc; median prostate volume of this cohort was 35.0 cc (range 17–58.4 cc). Median pre-treatment PSA was 6.30 (range 2.55–16.77). Each patient was treated with 36.25 Gy delivered in five fractions over 2 weeks with urethral sparing to a maximal dose of 35 Gy. Target volumes included the prostate gland and proximal seminal vesicles with a 3 mm margin. Results: Median follow-up as of 26 May 2021 was 11.97 months (range 4.37–19.80). First follow-up data are available for all patients, with a median of 1.10 month from completion of treatment (0.63–3.40). The median PSA at first visit was 2.75 (range 0.02–9.00) with a median AUA symptom score of 9 (range 1–24). Second follow-up data are available for 34 patients at a median of 4.45 months (range 2.57–8.90). At second follow-up, the median PSA was 1.60 (range 0.02–5.40) with a median AUA symptom score of 6 (range 1–33). Seventeen patients had third follow-up data with a median of 9.77 months (range 4.70–12.33) after SBRT. The median PSA was 1.13 (range 0.02–4.73) with an AUA score of 9 (2–22) at the third follow-up. We observed a statistically significant decrease in PSA between pre-treatment and at first follow-up (p < 0.005). The most common toxicity was grade 2 urethritis, managed in all cases by tamsulosin. One patient developed grade 2 tenesmus relieved by topical steroids. No cases of grade ≥ 3 toxicity were seen in our patient population. Conclusions: By avoiding the extra time required for plan adaptation, MRgRT without daily adaptation allows for successful prostate SBRT with manageable toxicity. We continue to reserve our limited adaptive treatment slots for preoperative pancreatic and ultra-central lung SBRT patients, which require time-intensive respiratory gating and adaptive planning.