Beam Delivery Research Articles

Technical note: A method to evaluate the effect of scanning beam delivery error on 3D dose and its utilization on carbon ion radiotherapy for prostate cancer.

To establish a method for evaluating the effect of scanning ion beam delivery error on three-dimensional (3D) dose reconstructed on patients' CT based on log file. This study used the MATLAB program to reconstruct the 3D dose on the patient's CT based on the log file (Doselog ) for treatment delivery accuracy check. In addition, differences between the parameters in the log file and the treatment plan, such as the spot position, spot size, and particle number, were analyzed, as well as their effects on the dose distribution. The accuracy of the dose reconstruction algorithm was verified by comparing dose from TPS (DoseTPS ) and the dose recalculated based on the treatment plan (Doserec ). Twenty treatment plans of ten prostate cancer patients received carbon ion therapy, and their corresponding 160 log files were selected for analysis and treatment delivery accuracy check. The regions with dose higher than 10% of the maximum dose were selected and 2mm/2% criteria were used for global gamma analysis. Multiple linear regression was used to evaluate the relation between dose deviation and delivery errors. For the algorithm accuracy verification, the mean relative dose difference is 1.02%±0.12%. For prostate cancer patients treated in our facility using carbon ion radiotherapy, the average passing rate of the gamma analysis between the Doselog and the DoseTPS was 95.3%. The dose deviation caused by the difference in the spot position and the number of particles was smaller than that caused by the spot size deviation. This study established a 3D dose verification method based on log files to evaluate the accuracy of daily delivered treatment doses. In our facility, the daily delivered dose accuracy of carbon ion therapy for prostate cancer was mainly affected by the spot size deviation in terms of the machine delivery part.

Dose perturbations in proton pencil beam delivery investigated by dynamically deforming silicone-based radiochromic dosimeters

Objective. Proton therapy with pencil beam delivery enables dose distributions that conform tightly to the shape of a target. However, proton therapy dose delivery is sensitive to motion and deformation, which especially occur in the abdominal and thoracic regions. In this study, the dose perturbation caused by dynamic motion with and without gating during proton pencil beam deliveries were investigated using deformable three-dimensional (3D) silicone-based radiochromic dosimeters. Approach. A spread-out Bragg peak formed by four proton spots with different energies was delivered to two dosimeter batches. All dosimeters were cylindrical with a 50 mm diameter and length. The dosimeters were irradiated stationary while uncompressed and during dynamic compression by sinusoidal motion with peak-to-peak amplitudes of 20 mm in one end of the dosimeter and 10 mm in the other end. Motion experiments were made without gating and with gating near the uncompressed position. The entire experiment was video recorded and simulated in a Monte Carlo (MC) program. Main results. The 2%/2 mm gamma index analysis between the dose measurements and the MC dose simulations had pass rates of 86%–94% (first batch) and 98%–99% (second batch). Compared to the static delivery, the dose delivered during motion had gamma pass rates of 99%–100% when employing gating and 68%–87% without gating in the experiments whereas for the MC simulations it was 100% with gating and 66%–82% without gating. Significance. This study demonstrated the ability of using deformable 3D dosimeters to measure dose perturbations in proton pencil beam deliveries caused by dynamic motion and deformation.

Current Radiotherapy Considerations for Nasopharyngeal Carcinoma.

Radiotherapy is the primary treatment modality for nasopharyngeal carcinoma (NPC). Successful curative treatment requires optimal radiotherapy planning and precise beam delivery that maximizes locoregional control while minimizing treatment-related side effects. In this article, we highlight considerations in target delineation, radiation dose, and the adoption of technological advances with the aim of optimizing the benefits of radiotherapy in NPC patients.

Out-of-plane beam shaping with UV-written tilted Bragg gratings for beam delivery on quantum chips.

We theoretically and numerically investigate the performance of tilted Bragg gratings in planar waveguides, fabricated by direct UV writing in photosensitive silica, to couple light out of a chip. An analytic expression is derived for the coupling efficiency and validated numerically by finite element simulations. Using the analytic result, we can design gratings to generate output beams in free space of any specific shape and calculate their overall power coupling efficiency. Our simulations indicate that for currently achievable grating index contrasts devices of millimeter length are most suitable for this technology.

Patterns of utilization and clinical adoption of 0.35 Tesla MR-guided radiation therapy in the United States – Understanding the transition to adaptive, ultra-hypofractionated treatments

Purpose/ObjectiveMagnetic resonance-guided radiation therapy (MRgRT) utilization is rapidly expanding worldwide, driven by advanced capabilities including continuous intrafraction visualization, automatic triggered beam delivery, and on-table adaptive replanning (oART). Our objective was to describe patterns of 0.35Tesla(T)-MRgRT (MRIdian) utilization in the United States (US) among early adopters of this novel technology. Materials/MethodsAnonymized administrative data from all US MRIdian treatment systems were extracted for patients completing treatment from 2014 to 2020. Detailed treatment information was available for all MRIdian linear accelerator (linac) systems and some cobalt systems. ResultsSeventeen systems at 16 centers delivered 5736 courses and 36,389 fractions (fraction details unavailable for 1223 cobalt courses), of which 21.1% were adapted. Ultra-hypofractionation (UHfx) (1–5 fractions) was used in 70.3% of all courses. At least one adaptive fraction was used for 38.5% of courses (average 1.7 adapted fractions/course), with higher oART use in UHfx dose schedules (47.7% of courses, average 1.9 adapted fractions per course). The most commonly treated organ sites were pancreas (20.7%), liver (16.5%), prostate (12.5%), breast (11.5%), and lung (9.4%). Temporal trends show a compounded annual growth rate (CAGR) of 59.6% in treatment courses delivered, with a dramatic increase in use of UHfx to 84.9% of courses in 2020 and similar increase in use of oART to 51.0% of courses. ConclusionsThis is the first comprehensive study reporting patterns of utilization among early adopters of MRIdian in the US. Intrafraction MR image-guidance, advanced motion management, and increasing adoption of adaptive radiation therapy has led to a substantial transition to ultra-hypofractionated regimens. 0.35 T-MRgRT has been predominantly used to treat abdominal and pelvic tumors with increasing use of on-table adaptive replanning, which represents a paradigm shift in radiation therapy.

Development and validation of an optimal GATE model for proton pencil-beam scanning delivery

ObjectiveTo develop and validate a versatile Monte Carlo (MC)-based dose calculation engine to support MC-based dose verification of treatment planning systems (TPSs) and quality assurance (QA) workflows in proton therapy. MethodsThe GATE MC toolkit was used to simulate a fixed horizontal active scan-based proton beam delivery (SIEMENS IONTRIS). Within the nozzle, two primary and secondary dose monitors have been designed to enable the comparison of the accuracy of dose estimation from MC simulations with respect to physical QA measurements. The developed beam model was validated against a series of commissioning measurements using pinpoint chambers and 2D array ionization chambers (IC) in terms of lateral profiles and depth dose distributions. Furthermore, beam delivery module and treatment planning has been validated against the literature deploying various clinical test cases of the AAPM TG‐119 (c-shape phantom) and a prostate patient. ResultsMC simulations showed excellent agreement with measurements in the lateral depth-dose parameters and spread-out Bragg peak (SOBP) characteristics within a maximum relative error of 0.95 mm in range, 1.83% in entrance to peak ratio, 0.27% in mean point-to-point dose difference, and 0.32% in peak location. The mean relative absolute difference between MC simulations and measurements in terms of absorbed dose in the SOBP region was 0.93% ± 0.88%. Clinical phantom studies showed a good agreement compared to research TPS (relative error for TG-119 planning target volume PTV-D95 ∼ 1.8%; and for prostate PTV-D95 ∼ −0.6%). ConclusionWe successfully developed a MC model for the pencil beam scanning system, which appears reliable for dose verification of the TPS in combination with QA information, prior to patient treatment.

Transition from 3D conformal to modern modulated craniospinal radiotherapy

Background: Achieving optimal tumour coverage during craniospinal irradiation (CSI) is a challenge. Whilst several critical organs are at risk of radiation-induced toxicity, if target volume structures like the cribriform plate receive less than 95% of the prescribed dose, tumours could recur.Aim: This single-institution study seeks to establish the most effective craniospinal radiotherapy amongst 3D conformal radiation therapy (CRT), intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) by comparing dosimetry across target volumes, organs at risk (OARs) and total irradiated volume. Time taken for contouring, generation and evaluation of treatment plans, quality assurance and treatment beam delivery is assessed.Setting: The demographics of patients comprised of six children and one adult who underwent 3D CRT craniospinal radiotherapy at a Western Cape hospital.Methods: Approval from the Human Research Ethics Committee was obtained. The 3D CRT plan consisted of two parallel opposing lateral fields at the cranial isocentre and a single posterior field at the spinal isocentre. Both the IMRT and VMAT plans comprised three isocentres, one cranial and two spinal, with a total of 15 fields.Results: Volumetric modulated arc therapy was the most conformal (CI = 0.48) and IMRT the most homogeneous (HI = 0.06). Although the VMAT low-dose bath (58.1%) was highest at 2 Gy, OARs were least exposed with VMAT. The total time taken for VMAT was the shortest.Conclusion: Volumetric modulated arc therapy was recommended as the most effective CSI technique owing to its superior conformality, least OARs exposure and fastest planning times. Clinical investigation into possible late adverse effects arising from the VMAT low-dose bath should be conducted.Contribution: This study will establish which of the three radiation therapy (RT) techniques is most effective in the treatment of craniospinal tumours, as well as, which technique offers the fastest turn around time.

Image-mode performance characterisation of a positron emission tomography subsystem designed for Biology-guided radiotherapy (BgRT).

In this study, we characterise the imaging-mode performance of the positron emission tomography (PET) subsystem of the RefleXion X1 machine using the NEMA NU-2 2018 standard. The X1 machine consists of two symmetrically opposing 900 arcs of PET detectors incorporated into the architecture of a ring-gantry linear accelerator rotating up to 60 RPM. PET emissions from a tumour are detected by the PET detectors and used to guide the delivery of radiation beam. Imaging performance of the PET subsystem on X1 machine was evaluated based on sensitivity of the PET detectors, spatial resolution, count-loss performance, image quality, and daily system performance check. PET subsystem sensitivity was measured as 0.183 and 0.161 cps/kBq at the center and off-center positions, respectively. Spatial resolution: average FWHM values of 4.3, 5.1, and 6.7 mm for the point sources at 1, 10, and 20 cm off center, respectively were recorded. For count loss, max NECR: 2.63 kcps, max true coincidence rate: 5.56 kcps, and scatter fraction: 39.8%. The 10 mm sphere was not visible. Image-quality contrast values were: 29.6%, 64.9%, 66.5%, 81.8%, 81.2%, and background variability: 14.8%, 12.4%, 10.3%, 8.8%, 8.3%, for the 13, 17, 22, 28, 37 mm sphere sizes, respectively. When operating in an imaging mode, the spatial resolution and image contrast of the X1 PET subsystem were comparable to those of typical diagnostic imaging systems for large spheres, while the sensitivity and count rate were lower due to the significantly smaller PET detector area in the X1 system. Clinical efficacy when used in BgRT remains to be validated. This is the first performance evaluation of the PET subsystem on the novel BgRT machine. The dual arcs rotating PET subsystem on RefleXion X1 machine performance is comparable to those of the typical diagnostic PET system based on the spatial resolution and image contrast for larger spheres.

Miniaturized structured illumination microscopy using two 3-axis MEMS micromirrors.

We present the development and performance characterisation of a novel structured illumination microscope (SIM) in which the grating pattern is generated using two optical beams controlled via 2 micro-electro-mechanical system (MEMS) three-axis scanning micromirrors. The implementation of MEMS micromirrors to accurately and repeatably control angular, radial and phase positioning delivers flexible control of the fluorescence excitation illumination, with achromatic beam delivery through the same optical path, reduced spatial footprint and cost-efficient integration being further benefits. Our SIM architecture enables the direct implementation of multi-color imaging in a compact and adaptable package. The two-dimensional SIM system approach is enabled by a pair of 2 mm aperture electrostatically actuated three-axis micromirrors having static angular tilt motion along the x- and y-axes and static piston motion along the z-axis. This allows precise angular, radial and phase positioning of two optical beams, generating a fully controllable spatial interference pattern at the focal plane by adjusting the positions of the beam in the back-aperture of a microscope objective. This MEMS-SIM system was applied to fluorescent bead samples and cell specimens, and was able to obtain a variable lateral resolution improvement between 1.3 and 1.8 times the diffraction limited resolution.

Failure Mode and Effects Analysis for Experimental Use of FLASH on a Clinical Accelerator

PurposeThe use of a linear accelerator (LINAC) in ultrahigh-dose-rate (UHDR) mode can provide a conduit for wider access to UHDR FLASH effects, sparing normal tissue, but care needs to be taken in the use of such systems to ensure errors are minimized. The failure mode and effects analysis was carried out in a team that has been involved in converting a LINAC between clinical use and UHDR experimental mode for more than 1 year after the proposed methods of TG100. Methods and MaterialsA team of 9 professionals with extensive experience were polled to outline the process map and workflow for analysis, and developed fault trees for potential errors, as well as failure modes that would result. The team scored the categories of severity magnitude, occurrence likelihood, and detectability potential in a scale of 1 to 10, so that a risk priority number (RPN = severity×occurrence×detectability) could be assessed for each. ResultsA total of 46 potential failure modes were identified, including 5 with an RPN >100. These failure modes involved (1) patient set up, (2) gating mechanisms in delivery, and (3) detector in the beam stop mechanism. The identified methods to mitigate errors included the (1) use of a checklist post conversion, (2) use of robust radiation detectors, (3) automation of quality assurance and beam consistency checks, and (4) implementation of surface guidance during beam delivery. ConclusionsThe failure mode and effects analysis process was considered critically important in this setting of a new use of a LINAC, and the expert team developed a higher level of confidence in the ability to safely move UHDR LINAC use toward expanded research access.

Investigation of the impact of machine operating parameters on beam delivery time and its correlation with treatment plan characteristics for synchrotron-based proton pencil beam spot scanning system.

To investigate the beam delivery time (BDT) reduction due to the improvement of machine parameters for Hitachi synchrotron-based proton PBS system. BDTs for representative treatment plans were calculated to quantitatively estimate the BDT improvement from our 2015 system at Mayo Clinic in Arizona to our system to be implemented in 2025 at Mayo Clinic in Florida, and to a hypothetical future system. To specifically assess how each incremental improvement in the operating parameters reduced the total BDT, for each plan, we simulated the BDT 10,368 times with various settings of the nine different operating parameters. The effect of each operating parameter on BDT reduction and its correlation with treatment plan characteristics were analyzed. The optimal number of multiple energy extraction (MEE) layers per spill for different systems was also investigated. The median (range) decrease in BDT was 60% (56%-70%) from the 2015 to the 2025 system. The following incremental improvement in parameters of the 2015 system for the 2025 system played an important role in this decreased BDT: beam intensity (8 to 20 MU/s), recapture efficiency (50% to 80%), number of MEE layers per spill (4 to 8), scanning magnet preparation and verification time (1.9 to 0.95 msec), and MEE layer switch time (200 to 100 msec). Reducing the total spill change time and scanning magnet preparation and verification time from those of the 2025 system further reduced BDT in the hypothetical future system. 8 MEE layers per spill is optimal for a system with 50% recapture efficiency; 16 MEE layers per spill is optimal for a system with 80% recapture efficiency; and more than 16 MEE layers per spill is beneficial only for a system close to 100% recapture efficiency. We systematically studied the effect of each machine operating parameter on the reduction in total BDT and its correlation with treatment plan characteristics. Our findings will aid new and existing synchrotron-based proton beam therapy centers to make balanced decisions on BDT benefits vs. costs when considering machine upgrade or new system selection.

Creation, evolution, and future challenges of ion beam therapy from a medical physicist's viewpoint (part 1). Introduction and Chapter 1. accelerator and beam delivery system.

Radiation therapy for cancer using the Bragg peak of an ion beam has been making steady progress after being proposed by Robert Wilson in 1946. At the end of 2020, 12 dedicated treatment devices existed in operation worldwide, and approximately 40,000 patients have been treated with ion beams (mostly carbon ions). To date, ion beam therapy is superior to other treatments for rare cancers in the head and neck as well as bone and soft tissues; however, most recently, evidence submitted in Japan for the 2022 revision of public health insurance shows that ion beam therapy outperforms photon therapy for intractable common cancers such as pancreatic cancer and liver cancer. This may greatly expand its indications. Lawrence Berkeley Laboratory in the United States started research of ion beam therapy, National Institute of Radiological Sciences in Japan built the first dedicated device Heavy Ion Accelerator in Chiba and started systematic clinical research, and GSI in Germany developed the scanning irradiation method and rotating gantry for the first time. This paper presents the history and future challenges of ion beam therapy in three fields: accelerator and beam delivery system, physical/biological model and treatment planning system, and clinical research. This study is divided into three parts describing the achievements and roles of the three laboratories. In Part 1, accelerator and beam delivery system are described.

Optical fibre array detector to monitor irradiations for medical radioisotope production

Low-energy cyclotrons are in use worldwide to produce medical radioisotopes for nuclear medicine. Beam monitoring during the irradiation of targets is difficult due to the high-power density of low-energy protons, space limitations, and interference with beam delivery. Doped silica fibres are sensitive to prompt ionizing radiation from the bombarded target, and produce radiation induced luminescence (RIL) when exposed. The fibres can be attached to the outside of the target in a low-profile fibre array, ensuring efficient and safe operation. Here, we present the results from our prototype of such a fibre array. It consists of four Ce-doped fibres with a diameter of 200 μm and has been installed at the TR13 medical cyclotron at TRIUMF where it has been tested at different irradiation conditions.

Evaluation of the feasibility of cardiac gating for SBRT of ventricular tachycardia based on real-time ECG signal acquisition.

To investigate the feasibility of cardiac synchronized gating in stereotactic body radiation therapy (SBRT) of ventricular tachycardia (VT) using a real-time electrocardiogram (ECG) signal acquisition. Stability of beam characteristics during simulated ECG gating was examined by developing a microcontroller interface to a Varian Clinac iX linear accelerator allowing gating at frequencies and duty cycles relevant to cardiac rhythm. Delivery accuracy was evaluated by measuring dose linearity with an ionization chamber, and flatness and symmetry with a two-dimensional detector array, for different gating windows within typical human cardiac cycle periods. To establish a practical method of gating based on actual ECG signals, an AD8232 Heart Monitor board was used to acquire the ECG signal and synchronize the beam delivery. Real-time cardiac gated delivery measurements were performed for a single 10×10cm2 field and for a VT-SBRT plan using intensity-modulated radiation therapy (IMRT). Dose per monitor unit (MU) values were found to be linear within most gating windows investigated with maximum differences relative to non-gated delivery of <2% for gating windows ≥200ms and for >10MUs. Beam profiles for both gated and non-gated modes were also found to agree with maximum differences of 0.5% relative to central axis dose for all sets of beam-on/beam-off combinations. Comparison of dose distributions for intensity-modulated SBRT plans between non-gating and cardiac gating modes provided a gamma passing rate of 97.2% for a 2% 2mm tolerance. Beam output is stable with respect to linearity, flatness, and symmetry for gating windows within cardiac cycle periods. Agreement between dose distributions for VT-SBRT using IMRT in non-gated and cardiac cycle gated delivery modes shows that the proposed methodology is feasible. Technically, gating for delivery of SBRT for VT is possible with regard to beam stability.

Evaluating Risk of FLASH Experiments on a Clinical LINAC with Failure Mode and Effects Analysis

<h3>Purpose/Objective(s)</h3> Use of clinical linear accelerators in ultra-high dose rate (UHDR) mode can provide a conduit for wider access to UHDR FLASH effects, sparing normal tissue, but care needs to be taken in the use of such systems to ensure errors are avoided. A Failure Modes and Effects Analysis (FMEA) was conducted with a team involved in converting a LINAC between clinical use and UHDR experimental mode for more than one year, following the proposed methods of AAPM (The American Association of Physicists in Medicine) Task Group 100 report. <h3>Materials/Methods</h3> A team of 9 professionals with extensive experience outlined the process map and workflow for analysis, and developed fault trees for potential errors, as well as failure modes that would results. The team scored the categories of severity magnitude (S), occurrence likelihood (O), and detectability potential (D) on a scale of 1 to 10, so that a risk priority number (RPN=S*O*D) could be assessed for each potential failure. <h3>Results</h3> A total of 46 potential failure modes were identified, including 5 with RPN>100, all during converting and experimental use in UHDR mode and none during conversion to clinical mode. These failure modes involved 1) patient set up, 2) gating mechanisms in delivery, and 3) detector in the beam stop mechanism. Identified methods to mitigate errors included 1) use of a checklist post conversion, 2) use of robust radiation detectors, 3) automation of QA and beam consistency checks, and 4) implementation of surface guidance during beam delivery. <h3>Conclusion</h3> The FMEA process was considered critically important in this setting of a new use of a LINAC, and the expert team developed a higher level of confidence in the ability to safely move UHDR LINAC use towards expanded research access.

Can Proton Linear Accelerator Beams and Minibeams Achieve Superior Plan Quality over Cyclotron-Generated Proton Beams and Photons in the Treatment of Multiple Brain Metastases?

<h3>Purpose/Objective(s)</h3> To compare plan quality and delivery timing among photon Volumetric Arc Therapy (VMAT), Intensity-Modulated Proton Therapy using cyclotron-generated proton beams (CPB), linear accelerator proton beams (LPB), and linear accelerator proton minibeams (LPMB). <h3>Materials/Methods</h3> Thirty-five brain lesions from twenty patients were planned using CPBs and compared against LPBs and LPMBs and VMAT. All proton plans were optimized with multi-field optimization technique using the Monte Carlo algorithm. Benchmarked on the photon plans, dose coverage to each lesion for each proton plan was set to 99% of the GTV receiving the prescription (Rx) dose for all plans. Conformity index (CI), gradient index (GI), V_12Gy, V_4.5Gy and mean dose of the brain were compared. To achieve similar PTV dose coverage to photons, ±2 mm setup uncertainty and ±2% range uncertainty were included in robust evaluation using V_100%Rx > 95% of the GTV for proton plans. Additionally, the plan delivery timing was assessed based on the proton modalities' energy switching and spot scanning times. <h3>Results</h3> All metrics of CI, GI, V_12Gy, V_4.5Gy, and mean brain dose for VMAT photon and three different proton plans are listed in Table 1. When compared to CPB generated plans, CI and GI were significantly better for the LPB and LPMB plans with a mean CI of 1.7 ± 0.5, 1.57 ± 0.4, and 1.4 ± 0.6 and a mean GI of 4.64 ± 1.48, 3.7 ± 1.1 and 2.8 ± 0.7, respectively. In comparison, mean CI and GI from photon VMAT plans were 2 ± 1.2, 4.7 ± 1.4. All proton plans achieved better V_4.5Gy than photon VMAT plans. LPBs reduced the mean brain dose by 17% compared to the CPB plans, and LPMB plans reduced the mean brain dose by 44%. While all plans had V_100%Rx of the GTV > 99% in the nominal scenario, in the voxel-wise worst scenario, 97.1%, 92.8% and 84.2% of the lesions had V_100%Rx > 95% for CPB, LPB, and LPMB, respectively, and in the worst-case scenario, 97.1%, 94.3% and 78.6% of the plans showed V_100%Rx > 95% among all lesions. The LPB energy switching time (5 ms) enables a rapid beam delivery, 58% faster on average than for CPB plans. LPMB beam delivery time can be further reduced by spot reduction planning (SRP) with equal plan quality. <h3>Conclusion</h3> We successfully quantified plan quality and evaluated robustness for CPBs, LPBs and LPMBs for brain metastases. The LPB and LPMB stand as an excellent alternative to CPB therapy and can significantly increase the preservation of normal tissue while ensuring fast delivery time.

Intrafraction Prostate Motion Management during Dose-Escalated Linac-Based SBRT

<h3>Purpose/Objective(s)</h3> This study reports the first clinical experience worldwide using a novel electromagnetic (EM) tracking device for intrafraction prostate motion management during dose-escalated Linac-based stereotactic body radiation therapy (SBRT). <h3>Materials/Methods</h3> Thirteen patients with organ-confined prostate cancer underwent dose-escalated SBRT using flattening filter-free (FFF) volumetric modulated arc therapy (VMAT). The prescribed dose was 40 Gy in 5 fractions or 38 Gy in 4 fractions. The EM tracking device consisted of an integrated Foley catheter with a transmitter. Patients were simulated and treated with a filled bladder and an empty rectum. Setup accuracy was achieved by ConeBeam-CT (CBCT) matching and the prostate motion was tracked during all the procedure. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, target position was re-defined by repeating CBCT. Moreover, the displacements that would have occurred without any intrafraction organ motion management (i.e., no interruptions and repositionings) were simulated and analyzed. <h3>Results</h3> In 31 out of 56 monitored fractions (55%), no intervention was required to correct the target position as a result of an excessive displacement. In 25 (45%) a correction was mandated, but only in 10 (18%), the beam delivery was interrupted. Total treatment time lasted on average 10.2 minutes, 6.7 minutes for setup, and 3.5 minutes for beam delivery. The prostate was found inside the 2 mm threshold from its initial position in 96% of the treatment time, i.e., in 94% of the time during the setup, and in 98% during the delivery (beam on + interruptions). Without any intrafraction motion management, the overall mean treatment time and the mean delivery time would have been 6.9 minutes and 3.2 minutes, respectively. The prostate would have been found outside the tolerance in 8% of the total session time, in 4% of the time during the setup, and in 14% during the beam-on phase. The probability of motion > 2 mm in the lateral, longitudinal, and vertical direction after 5 minutes was 3.6% (2/56), 8.9% (5/56), and 14.3% (8/56), respectively. The predominant motion pattern was posterior with a mean motion ≤ 2 mm occurring within 10 minutes. Moreover, the analysis of the rotation angles from all patients showed a systematic rotation in the pitch axis which is absent in the yaw axis. Concurrently, the range and standard deviation of rotation angles were larger in the pitch axis. <h3>Conclusion</h3> EM real-time tracking was successfully implemented for intrafraction motion management during dose-escalated prostate SBRT. Results showed that most of the observed displacements were < 2 mm in any direction; however, there were a non-insignificant number of fractions with motion exceeding the predefined threshold, which would have otherwise gone undetected without intrafraction motion management.

Machine-Specific Delivery Sequence Model of Compact Superconducting Synchrocyclotron Proton Therapy Systems – A Multi-Institutional Investigation

<h3>Purpose/Objective(s)</h3> Proton pencil beam scanning delivery sequence varies among institutions due to the vendor, machine type, and configuration. However, having a reliable model of delivery time and sequence is critical to maximizing throughput when proton beam time is limited. In this study, we used an experimental approach to build a precise beam delivery time (BDT) and sequence prediction model for a new compact superconducting synchrocyclotron proton system (PTC#1) and compare it with a different institution using an older version of the same system (PTC#2). We investigated treatment efficiency and the interplay effect for the treatment of mobile tumors. <h3>Materials/Methods</h3> The machine delivery log files from PTC#1 were retrospectively analyzed to model beam delivery parameters. We compared the PTC#1 delivery sequence with PTC#2's published machine-specific model using the same type of proton system. A total of twenty clinical beams from head neck, lung, breast, and esophagus treatment sites were used for model comparison. Also, we used a single clinical treatment day to compare treatment efficiency. The interplay effect is evaluated and compared based on these two different delivery sequence models. A digital thoracic 4DCT phantom image set with a rigid target in the right lung was used for the study. Different delivery techniques such as standard delivery (N_vol=1) and volumetric repainting delivery (N_vol=2-5) were simulated. <h3>Results</h3> The results showed PTC#1 has a much faster spot scanning speed than PTC#2 thanks to a new generation of scanning magnets. The spot switching time (SSWT) is 0s within 11.3mm in x-direction and 8 mm in y-direction. Compared to PTC#2, the average SSWT reduces by 94.37%±3.82% (50.76s±35.10s) in PTC#1. PTC#1 also demonstrated improved burst switching transmission efficiency, which reduced burst switching time (BST) by 39.84%±3.64% (6.00s±2.21s) on average. The energy layer switching time (ELST) dominates the PTC#1 delivery time instead of SSWT for PTC#2 (Table). The total BDT was reduced by 54.61%±12.12% (53.45s±34.47s) on average. Based on the single day patient mix, it was estimated that 67 min irradiation time could be saved by upgrading the scanning magnets in PTC#2 (10% daily patient treatment throughput improvement). The results also indicated the interplay effect would be reduced as volumetric repainting number increased. The system in PTC#2 had better target coverage and homogeneity than PTC#1 for=1-3. Their target coverage converges at=5. <h3>Conclusion</h3> A precise machine-specific delivery sequence model is highly recommended in the interplay effect evaluation and clinical decision. It provided the first quantitative analysis of the impact of the new scanning magnets regarding the improvement in the treatment delivery efficiency.

The Feasibility of Biologically Guided Radiation Therapy Implemented in a Small-Animal Radiation Therapy Platform Integrated with Quad-Modal On-Board Imaging

<h3>Purpose/Objective(s)</h3> Unparallel evolution between clinical radiation therapy (RT) paradigms and preclinical RT techniques, especially in biologically guided RT, has posed a challenge to completely validate the benefits and pitfalls of novel RT techniques prior to their translation to bedside practices. In this study, we firstly investigated the feasibility of biologically guided RT implemented within a small animal radiation therapy (SART) platform, with quad-modal on-board positron emission tomography (PET), single-photon emission computed tomography (SPECT), spectral CT, and cone-beam computed tomography (CBCT) imaging integrated, in a Monte Carlo model as a proof-of-concept. <h3>Materials/Methods</h3> The SART platform with quad-modal imaging guidance was modeled by using a planning tool. A preclinical biologically guided RT workflow with quad-modal imaging guidance was designed, in which image-guided RT and emission-guided radiation therapy (EGRT) can be implemented. To demonstrate the potential of contrast-enhanced spectral CT used in biologically guided RT for simultaneously guiding dose prescription based on tracer uptakes and dose calculation based on underlying tissue electron densities in the platform, iodine uptake fractions and virtual non-contrast electron densities in the Kidney1 inserts within a simulated phantom, mixed with an iodine contrast agent at electron fractions of 0.02 and 0.03, were decomposed quantitatively by using the Bayesian eigen tissue decomposition method and the maximum a posterior (MAP) estimation. To demonstrate EGRT implemented in the platform, the preliminary algorithm of EGRT was illustrated by introducing a 2-mm setup error to a target in the simulated phantom. The dose calculation was implemented after the set of binary micro-multileaf collimator (bmMLC) configurations was determined. The dose distributions delivered by EGRT were compared to that delivered by the conventional RT method with three treatment beams with and without setup error. <h3>Results</h3> For the Kidney1 insert with an iodine contrast agent fraction of 0.02, the average relative errors in the iodine contrast agent fraction and the underlying electron density estimated were 0.64% and 0.95%, respectively. In EGRT, the simulations resulted in 4339 and 5197 beamlet responses in an exemplary dose slice delivery with and without the 2-mm setup error, respectively. Compared to the conventional RT method, EGRT scheme introduced into the SART platform was validated to be more robust to setup errors while providing improved tumor targeting and a lesser dose burden on normal tissues during the treatment beam delivery in preclinical RT setting. <h3>Conclusion</h3> The results demonstrated the successful implementation of biologically guided RT within the SART platform with quad-model imaging guidance. With the comprehensive image- and emission-guided RT schemes integrated, this novel SART platform affords one-stop preclinical RT studies.

Feasibility of Treating Brain Lesions with a Horizontal Beam Line and a Couch with Adjustable Backrest Angle

<h3>Purpose/Objective(s)</h3> A gantry enabling beam delivery from any angle is technically complex and expensive. This is especially true for particle beams. An alternative is a horizontal beam line and upright positioning of the patient. This requires a vertical CT scanner and dedicated positioning tools for accurate delivery. We propose a couch with an adjustable backrest in order to mimic a large range of gantry and table rotation combinations. In this work, we investigated which dose distributions are achievable with such a unit and compared it with those achieved with a gantry and a conventional couch. <h3>Materials/Methods</h3> Ten stereotactic treatment plans of cerebral metastases (1.8 cm³ to 26.5 cm³) comprising 8-10 beams were investigated. The treatments were re-planned by mimicking backrest angles of 0°, 30° and 60° by gantry angles of 90°, 120° and 150° respectively. Ten beams with meaningful isocentric table angles were set. The beam shapes and weights were optimized. The calculations were performed with a treatment planning system, simulating a 6MV FFF photon beam. Dose-volume parameters of the clinically used plan were compared to the one mimicking the horizontal beam line. <h3>Results</h3> For the target and a 2 mm shell around the target, approximately identical dose distributions were achieved. The mean dose to the shell decreased by 0.5% on average. The coverage index enhanced from a value of 1.41 ± 0.2 to 1.38 ± 0.2 on average. Only the gradient index got worse with an average of 3.8 ± 0.7 compared to 3.0 ± 0.3. <h3>Conclusion</h3> Despite the restriction to only 3 backrest angles, for photon beams, comparable dose distributions were achieved with the proposed couch unit. For protons we expect at least similar results, as the range of possible angles of incidence is even larger. For the head, displacement of anatomical structures is not expected. Therefore, saving a gantry and a huge amount of radiation shielding, the proposed device is a cost-effective treatment option for brain lesions. Conventional CT and supportive image data could be used.