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A History of Surface Guidance Methods in Radiation Therapy
Published in Jeremy D. P. Hoisak, Adam B. Paxton, Benjamin Waghorn, Todd Pawlicki, Surface Guided Radiation Therapy, 2020
Jeremy D. P. Hoisak, Todd Pawlicki
The drive to minimize normal tissue dose and enable a maximization of tumor dose led to the addition of shielding by blocks, cones and later, the multileaf collimator (MLC) so that the fields could conform to the shape of the target in three dimensions. With 3D conformal radiation therapy, it became both more important and more technically challenging to ensure the target was within the planned high-dose area. Modern linear accelerators equipped with MLCs together with intensity modulated radiation therapy have now made it possible to deliver radiation dose distributions that are highly conformal to the target while sparing adjacent healthy tissues. These finely modulated dose distributions require that the target be localized as accurately as possible to ensure the dose is delivered as intended. Precise and accurate localization of the target can reduce geometric uncertainties and therefore reduce the impact of treatment-limiting side effects. This is a technical problem of considerable difficulty. Many technologies have been developed to help ensure precise and accurate localization of the therapy target, including rigid immobilization devices3 (e.g., thermoplastic masks and vacuum bags), X-ray image guidance with kV4,5 or MV6,7 sources, and nonradiographic localization devices.
MRI-based IGRT for lung cancer
Published in Jing Cai, Joe Y. Chang, Fang-Fang Yin, Principles and Practice of Image-Guided Radiation Therapy of Lung Cancer, 2017
The MR-linac system from the University Medical Center Utrecht, the Netherlands, was developed in collaboration with Philips Medical Systems (Best, The Netherlands), and Elekta AB (Stockholm, Sweden) [10,11]. This system consists of 1.5 Tesla Philips MRI scanner, modified to allow the incorporation of a 6 mega-voltage (MV) Elekta linear accelerator on a ring in the transverse midplane [11,12]. The accelerator head is equipped with a multileaf collimator (MLC) and can rotate in either direction [10] enabling delivery of intensity-modulated radiotherapy (IMRT). This system does not allow for couch shifts to correct for patient setup; however, it uses a virtual couch-shift technique where the treatment plan is adjusted daily based on the anatomy of the day accounting for any translations, rotations, or deformations [11,12]. The treatment planning system for the MR-linac has a Monte Carlo dose calculation algorithm capable of simulating the magnetic field and accounting for the electron return effect in the dose calculation. This is an important part of any MR-IGRT system, as it can have a significant impact on the dose distribution at interfaces, depending on the strength of the magnetic field, its orientation relative to the beam, and other patient- and field-specific parameters [13–15]. In this design the MR and the RT systems share an isocenter, thus allowing for image acquisition during treatment delivery. Proof-of-concept studies were performed showing the system's capability to track the target in phantom geometry; however, additional details regarding the latency and spatial resolution of these images, as well as practical considerations for applications to human subjects, are needed prior to clinical deployment [16,17].
Real-time tumor tracking
Published in Ross I. Berbeco, Beam’s Eye View Imaging in Radiation Oncology, 2017
A multitude of tumor-localization methods utilizing the EPID have been developed over the past two decades. All methods face the aforementioned challenges of a low-image contrast and a restricted field of view (FOV) imposed by the treatment beam aperture. The FOV limitations are particularly pronounced in tumor sites exhibiting large motion ranges, such as tumors located in the thorax or upper abdomen, which can lead to periodic temporary occlusions of the tracking target. For intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), the multileaf collimator (MLC) leafs moving through the treatment field cause further temporary FOV occlusions. In addition, the dose rate is often varied for intensity-modulated delivery types. This typically leads to stripe artifacts on the portal images (cf. Chapter 2). Although the boundary between cancerous and normal tissues may provide sufficient contrast in some situations such as early stage lung tumors, many other treatment sites do not provide sufficient soft tissue contrast for target localization. In these situations alternative surrogate structures may be tracked, for example, the diaphragm for tumors in proximity. When no natural surrogates are available, implanting radiopaque fiducial markers (typically made from gold) inside or adjacent to the tumor volume may offer an alternative (cf. Figure 9.3). However, the percutaneous implantation procedure may cause clinical concern over side effects tied to the invasiveness of the procedure (e.g., spread of microscopic disease along the insertion path, infection, and pneumothorax). For lung tumors, bronchoscopic implantation may also be an option—however, not all locations in the lung are reachable through this technique.
Investigation of Buildup Region and Surface Dose: Comparison of Parallel Plane Ion Chamber, Treatment Planning System, and MC Simulation
Published in Nuclear Technology, 2022
The buildup region and surface dose measurements were performed using the Siemens Artiste linear accelerator (Siemens Global System, Siemens AG) with 160 multileaf collimator using 6-MV photon beams. The measurements were taken using a solid water phantom with 30 × 30 cm2 (PTW-RW3) and parallel plane ion chamber (PTW-Markus 23343) starting from the surface with 1-mm-depth intervals in a field of 10 × 10 cm2 at gantry 0 deg [source skin distance (SSD) = 100 cm]. The percent depth dose (PDD) was described as the ratio of absorbed dose to the maximum absorbed dose along the beam axis. The dose measurement results were read using a PTW-UNIDOS E electrometer. Since the polarity effect shown by parallel plane ion chambers can cause significant effects in electronic disequilibrium areas such as the buildup region, perturbation corrections should be applied to results obtained with parallel plane ion chambers under electron disequilibrium conditions.4,18,26,27 The polarity effect was corrected by mean positive and negative bias voltage measurements (the voltage of ±300 V):
Spectroscopy of High-Intensity Bremsstrahlung Using Compton Recoiled Electrons
Published in Nuclear Science and Engineering, 2020
C. V. Midhun, M. M. Musthafa, Shaima Akbar, Swapna Lilly Cyriac, S. Sajeev, Antony Joseph, K. C. Jagadeesan, S. V. Suryanarayana, S. Ganesan
Performing research experiments with photons of interest in nuclear physics requires a precise knowledge of the produced bremsstrahlung spectrum. But, the critical components in the beam path, such as the primary collimator, flattening filter, jaws, multileaf collimator, etc., will modify the bremsstrahlung spectrum from the inherent behavior. In most of the cases, the spectrum is generated using Monte Carlo codes to account for the perturbing effects of these components.1–3 However, instantaneous changes of the beam current and energy spread will induce severe reverberation leading to a deviation between the final bremsstrahlung spectrum and the estimated “virgin” theoretical spectrum. It may be noted further that particularly when the bremsstrahlung spectrum is generated with an external bremsstrahlung target, the electron beam energy gets modified while it passes through the air. To the best of our knowledge, none of the reported spectra have taken into account such effects.
Design and Optimization Based on Monte Carlo Method of a Range Shifter for Proton Therapy
Published in Nuclear Technology, 2021
Yecheng Yu, Ping Tan, Yinjie Lin, Yuying Hu, Huidong Guo, Hao Lei, Zhongqi Zhang, Jiadong Li, Delin Hu
In some of the mainstream proton and heavy-ion therapy centers, range shifters are applied in both active-scanning and passive-scattering nozzles. For example, the MD Anderson Cancer Center5 uses synchrotron-based proton radiotherapy equipment with an energy filter to achieve continuous adjustment of the energy and an energy absorber to obtain lower proton energy. The Nagoya Proton Therapy Center6 in Japan uses a multileaf collimator in conjunction with a range compensator to reduce energy while decreasing beam scattering. The Department of Radiation Oncology in the Mayo Clinic3 uses a range shifter made of wax or polyethylene to cooperate with a bolus made of Lexan or Lucite for conformal treatment.