This study validated the feasibility of XACT in accurately obtaining relative dose maps of tissue-mimicking phantoms. XACT offers a practical method for verifying the beam position against the target and quantifying the relative dose delivered to the target during external beam radiotherapy.

Objective. The goal is to increase the precision of radiation delivery during radiotherapy by tracking the movements of the tumor and other surrounding normal tissues due to respiratory and other body motions. Introduction. This work presents the recent advancement of X-ray-induced radiation acoustic imaging (xRAI) technology and the evaluation of its feasibility for real-time monitoring of geometric and morphological misalignments of the X-ray field with respect to the target tissue by combining xRAI with established ultrasound (US) imaging, thereby improving radiotherapy tumor eradication and limiting treatment side effects. Methods. An integrated xRAI and B-mode US dual-modality system was established based on a clinic-ready research US platform. The performance of this dual-modality imaging system was evaluated via experiments on phantoms and ex vivo and in vivo rabbit liver models. Results. This system can alternatively switch between the xRAI and the US modes, with spatial resolutions of 1.1 mm and 0.37 mm, respectively. 300 times signal averaging was required for xRAI to reach a satisfactory signal-to-noise ratio, and a frame rate of 1.1 Hz was achieved with a clinical linear accelerator. The US imaging frame rate was 22 Hz, which is sufficient for real-time monitoring of the displacement of the target due to internal body motion. Conclusion. Our developed xRAI, in combination with US imaging, allows for mapping of the dose deposition in biological samples in vivo, in real-time, during radiotherapy. Impact Statement. The US-based image-guided radiotherapy system presented in this work holds great potential for personalized cancer treatment and better outcomes.

Purpose: FLASH radiotherapy (FLASH-RT) is a novel irradiation modality with ultra-high dose rates (>40 Gy/s) that have shown tremendous promise for its ability to enhance normal tissue sparing while maintaining comparable tumor cell eradication toconventional radiotherapy (CONV-RT). Due to its extremely high dose rates, clinical translation of FLASH-RT is hampered by risky delivery and current limitations in dosimetric devices, which cannot accurately measure, in real time, dose at deeper tissue. This work aims to investigate ionizing radiation acoustic imaging (iRAI) as a promising image-guidance modality for real-time deep tissue dose measurements during FLASH-RT. The underlying hypothesis is that iRAI can enable mapping of dose deposition with respect to surrounding tissue with a single linear accelerator (linac) pulse precision in real time. In this work, the relationship between iRAI signal response and deposited dose was investigated as well as the feasibility of using a proof-of-concept dual-modality imaging system of ultrasound and iRAI for treatment beam co-localization with respect to underlying anatomy. Methods: Two experimental setups were used to study the feasibility of iRAI for FLASH-RT using 6 MeV electrons from a modified Varian Clinac. First, experiments were conducted using a single element focused transducer to take a series of point measurements in a gelatin phantom, which was compared with independent dose measurements using GAFchromic film. Secondly, an ultrasound and iRAI dual-modality imaging system utilizing a phased array transducer was used to take coregistered two-dimensional (2D) iRAI signal amplitude images as well as ultrasound B-mode images, to map the dose deposition with respect to surrounding anatomy in an ex vivo rabbit liver model with a single linac pulse precision. Results: Using a single element transducer, iRAI measurements showed a highly linear relationship between the iRAI signal amplitude and the linac dose per pulse (r 2 = 0.9998) with a repeatability precision of 1% and a dose resolution error <2.5% in a homogenous phantom when compared to GAFchromic film dose measurements. These phantom results were used to develop a calibration curve between the iRAI signal response and the delivered dose per pulse. Subsequently, a normalized depth dose curve was generated that agreed with film measurements with an RMSE of 0.0243, using correction factors to account for deviations in measurement conditions with respect to calibration. Experiments on the ex-vivo rabbit liver model demonstrated that a 2D iRAI image could be generated successfully from a single linac pulse, which was fused with the B-mode ultrasound image to provide information about the beam position with respect to surrounding anatomy in real time.

Ionizing radiation acoustic imaging (iRAI) allows online monitoring of radiation’s interactions with tissues during radiation therapy, providing real-time, adaptive feedback for cancer treatments. We describe an iRAI volumetric imaging system that enables mapping of the three-dimensional (3D) radiation dose distribution in a complex clinical radiotherapy treatment. The method relies on a two-dimensional matrix array transducer and a matching multi-channel preamplifier board. The feasibility of imaging temporal 3D dose accumulation was first validated in a tissue-mimicking phantom. Next, semiquantitative iRAI relative dose measurements were verified in vivo in a rabbit model. Finally, real-time visualization of the 3D radiation dose delivered to a patient with liver metastases was accomplished with a clinical linear accelerator. These studies demonstrate the potential of iRAI to monitor and quantify the 3D radiation dose deposition during treatment, potentially improving radiotherapy treatment efficacy using real-time adaptive treatment.

FLASH radiotherapy (FLASH-RT) is an emerging ultra-high dose (>40 Gy/s) delivery that promises to improve the therapeutic potential by limiting toxicities compared to conventional RT while maintaining similar tumor eradication efficacy. Image guidance is an essential component of modern RT that should be harnessed to meet the special emerging needs of FLASH-RT and its associated high risks in planning and delivering of such ultra-high doses in short period of times. Hence, this contribution will elaborate on the imaging requirements and possible solutions in the entire chain of FLASH-RT treatment, from the planning, through the setup and delivery with online in vivo imaging and dosimetry, up to the assessment of biological mechanisms and treatment response. In patient setup and delivery, higher temporal sampling than in conventional RT should ensure that the short treatment is delivered precisely to the targeted region. Additionally, conventional imaging tools such as cone-beam computed tomography will continue to play an important role in improving patient setup prior to delivery, while techniques based on magnetic resonance imaging or positron emission tomography may be extremely valuable for either linear accelerator (Linac) or particle FLASH therapy, to monitor and track anatomical changes during delivery. In either planning or assessing outcomes, quantitative functional imaging could supplement conventional imaging for more accurate utilization of the biological window of the FLASH effect, selecting for or verifying things such as tissue oxygen and existing or transient hypoxia on the relevant timescales of FLASH-RT delivery. Perhaps most importantly at this time, these tools might help improve the understanding of the biological mechanisms of FLASH-RT response in tumor and normal tissues. The high dose deposition of FLASH provides an opportunity to utilize pulse-to-pulse imaging tools such as Cherenkov or radiation acoustic emission imaging. These could provide individual pulse mapping or assessing the 3D dose delivery superficially or at tissue depth, respectively. In summary, the most promising components of modern RT should be used for safer application of FLASH-RT, and new promising developments could be advanced to cope with its novel demands but also exploit new opportunities in connection with the unique nature of pulsed delivery at unprecedented dose rates, opening a new era of biological image guidance and ultrafast, pulsebased in vivo dosimetry.

Cerenkov Emission (CE) during external beam radiation therapy (EBRT) from a linear accelerator (Linac) has been demonstrated as a useful tool for radiotherapy quality assurance and potentially other applications for online tracking of tumors during treatment delivery. However, some of the current challenges that are impacting the potential of CE are related to the limited detection sensitivity and the lack of flexible tools to fit into an already complex treatment delivery environment. Silicon photomultiplier (SiPM) solid-state devices are new promising tools for low light detection due to their extreme sensitivity that mirrors photomultiplier tubes and yet have a form factor that is similar to silicon photodiodes, allowing for improved flexibility in device design that may help in the process of wider clinical applicability. In this paper, we assess the feasibility of using SiPMs to detect CE during EBRT from a Linac and contrast their performance with commercially available silicon photodiodes (PDs). We demonstrate the feasibility of the SiPM-based probes for standard dosimetry measurements. We also demonstrate that CE optical signals can be detected from tissue depths about five times greater than that for standard probes based on PDs, making our SiPM probe an enabling technology of CE measurements, particularly for deep tissue applications.

Purpose To develop and implement an efficient and accurate commissioning procedure for small‐field static beam animal irradiation studies on an MV research linear accelerator (Linatron‐M9) using radiochromic gel dosimetry. Materials The research linear accelerator (Linatron‐M9) is a 9 MV linac with a static fixed collimator opening of 5.08 cm diameter. Lead collimators were manually placed to create smaller fields of 2 × 2 cm2, 1 × 1 cm2, and 0.5 × 0.5 cm2. Relative dosimetry measurements were performed, including profiles, percent depth dose (PDD) curves, beam divergence, and relative output factors using various dosimetry tools, including a small volume ionization chamber (A14), GAFCHROMIC™ EBT3 film, and Clearview gel dosimeters. The gel dosimeter was used to provide a 3D volumetric reference of the irradiated fields. The Linatron profiles and relative output factors were extracted at a reference depth of 2 cm with the output factor measured relative to the 2 × 2 cm2 reference field. Absolute dosimetry was performed using A14 ionization chamber measurements, which were verified using a national standards laboratory remote dosimetry service. Results Absolute dosimetry measurements were confirmed within 1.4% (k = 2, 95% confidence = 5%). The relative output factor of the small fields measured with films and gels agreed with a maximum relative percent error difference between the two methods of 1.1 % for the 1 × 1 cm2 field and 4.3 % for the 0.5 × 0.5 cm2 field. These relative errors were primarily due to the variability in the collimator positioning. The measured beam profiles demonstrated excellent agreement for beam size (measured as FWHM), within approximately 0.8 mm (or less). Film measurements were more accurate in the penumbra region due to the film’s finer resolution compared with the gel dosimeter. Following the van Dyk criteria, the PDD values of the film and gel measurements agree within 11% in the buildup region starting from 0.5 cm depth and within 2.6 % beyond maximum dose and into the fall‐off region for depths up to 5 cm. The 2D beam profile isodose lines agree within 0.5 mm in all regions for the 0.5 × 0.5 cm2 and the 1 × 1 cm2 fields and within 1 mm for the larger field of 2 × 2 cm2. The 2D PDD curves agree within approximately 2% of the maximum in the typical therapy region (1–4 cm) for the 1 × 1 cm2 and 2 × 2 cm2 and within 5% for the 0.5 × 0.5 cm2 field. Conclusion This work provides a commissioning process to measure the beam characteristics of a fixed beam MV accelerator with detailed dosimetric evaluation for its implementation in megavoltage small animal irradiation studies. Radiochromic gel dosimeters are efficient small‐field relative dosimetry tools providing 3D dose measurements allowing for full representation of dose, dosimeter misalignment corrections and high reproducibility with low inter‐dosimeter variability. Overall, radiochromic gels are valuable for fast, full relative dosimetry commissioning in comparison to films for application in high‐energy small‐field animal irradiatio...

No abstract