High-energy beams of charged nuclear particles (protons and heavier ions) offer significant advantages for the treatment of deep-seated local tumors in comparison to conventional megavolt photon therapy. Their physical depth-dose distribution in tissue is characterized by a small entrance dose and a distinct maximum (Bragg peak) near the end of range with a sharp fall-off at the distal edge. Taking full advantage of the well-defined range and the small lateral beam spread, modern scanning beam systems allow delivery of the dose with millimeter precision. In addition, projectiles heavier than protons such as carbon ions exhibit an enhanced biological effectiveness in the Bragg peak region caused by the dense ionization of individual particle tracks resulting in reduced cellular repair. This makes them particularly attractive for the treatment of radio-resistant tumors localized near organs at risk. While tumor therapy with protons is a well-established treatment modality with more than 60 000 patients treated worldwide, the application of heavy ions is so far restricted to a few facilities only. Nevertheless, results of clinical phase I-II trials provide evidence that carbon-ion radiotherapy might be beneficial in several tumor entities. This article reviews the progress in heavy-ion therapy, including physical and technical developments, radiobiological studiesmore » and models, as well as radiooncological studies. As a result of the promising clinical results obtained with carbon-ion beams in the past ten years at the Heavy Ion Medical Accelerator facility (Japan) and in a pilot project at GSI Darmstadt (Germany), the plans for new clinical centers for heavy-ion or combined proton and heavy-ion therapy have recently received a substantial boost.« less

Heavy-ion tumor therapy: Physical and radiobiological benefits

Charged particle therapy has been largely driven and influenced by nuclear physics. The increase in energy deposition density along the ion path in the body allows reducing the dose to normal tissues during radiotherapy compared to photons. Clinical results of particle therapy support the physical rationale for this treatment, but the method remains controversial because of the high cost and of the lack of comparative clinical trials proving the benefit compared to x-rays. Research in applied nuclear physics, including nuclear interactions, dosimetry, image guidance, range verification, novel accelerators and beam delivery technologies, can significantly improve the clinical outcome in particle therapy. Measurements of fragmentation cross-sections, including those for the production of positron-emitting fragments, and attenuation curves are needed for tuning Monte Carlo codes, whose use in clinical environments is rapidly increasing thanks to fast calculation methods. Existing cross sections and codes are indeed not very accurate in the energy and target regions of interest for particle therapy. These measurements are especially urgent for new ions to be used in therapy, such as helium. Furthermore, nuclear physics hardware developments are frequently finding applications in ion therapy due to similar requirements concerning sensors and real-time data processing. In this review we will briefly describe the physics bases, and concentrate on the open issues.

https://iopscience.iop.org/article/10.1088/0034-4885/79/9/096702/pdf

Nuclear physics in particle therapy: a review.

Recently, ion beam radiotherapy (including protons as well as heavier ions) gained considerable interest. Although ion beam radiotherapy requires dose prescription in terms of iso-effective dose (referring to an iso-effective photon dose), absorbed dose is still required as an operative quantity to control beam delivery, to characterize the beam dosimetrically and to verify dose delivery. This paper reviews current methods and standards to determine absorbed dose to water in ion beam radiotherapy, including (i) the detectors used to measure absorbed dose, (ii) dosimetry under reference conditions and (iii) dosimetry under non-reference conditions. Due to the LET dependence of the response of films and solid-state detectors, dosimetric measurements are mostly based on ion chambers. While a primary standard for ion beam radiotherapy still remains to be established, ion chamber dosimetry under reference conditions is based on similar protocols as for photons and electrons although the involved uncertainty is larger than for photon beams. For non-reference conditions, dose measurements in tissue-equivalent materials may also be necessary. Regarding the atomic numbers of the composites of tissue-equivalent phantoms, special requirements have to be fulfilled for ion beams. Methods for calibrating the beam monitor depend on whether passive or active beam delivery techniques are used. QA measurements are comparable to conventional radiotherapy; however, dose verification is usually single field rather than treatment plan based. Dose verification for active beam delivery techniques requires the use of multi-channel dosimetry systems to check the compliance of measured and calculated dose for a representative sample of measurement points. Although methods for ion beam dosimetry have been established, there is still room for developments. This includes improvement of the dosimetric accuracy as well as development of more efficient measurement techniques.

https://iopscience.iop.org/article/10.1088/0031-9155/55/21/R01/pdf

Dosimetry for ion beam radiotherapy.

The enormous advances in the understanding of human anatomy, physiology and pathology in recent decades have led to ever-improving methods of disease prevention, diagnosis and treatment. Many of these achievements have been enabled, at least in part, by advances in ionizing radiation detectors. Radiology has been transformed by the implementation of multi-slice CT and digital x-ray imaging systems, with silver halide films now largely obsolete for many applications. Nuclear medicine has benefited from more sensitive, faster and higher-resolution detectors delivering ever-higher SPECT and PET image quality. PET/MR systems have been enabled by the development of gamma ray detectors that can operate in high magnetic fields. These huge advances in imaging have enabled equally impressive steps forward in radiotherapy delivery accuracy, with 4DCT, PET and MRI routinely used in treatment planning and online image guidance provided by cone-beam CT.The challenge of ensuring safe, accurate and precise delivery of highly complex radiation fields has also both driven and benefited from advances in radiation detectors. Detector systems have been developed for the measurement of electron, intensity-modulated and modulated arc x-ray, proton and ion beams, and around brachytherapy sources based on a very wide range of technologies. The types of measurement performed are equally wide, encompassing commissioning and quality assurance, reference dosimetry, in vivo dosimetry and personal and environmental monitoring.In this article, we briefly introduce the general physical characteristics and properties that are commonly used to describe the behaviour and performance of both discrete and imaging detectors. The physical principles of operation of calorimeters; ionization and charge detectors; semiconductor, luminescent, scintillating and chemical detectors; and radiochromic and radiographic films are then reviewed and their principle applications discussed. Finally, a general discussion of the application of detectors for x-ray nuclear medicine and ion beam imaging and dosimetry is presented.

https://iopscience.iop.org/article/10.1088/0031-9155/59/20/R303/pdf

Review on the characteristics of radiation detectors for dosimetry and imaging

In this study, the authors investigated the feasibility of using a 3D liquid scintillator (LS) detector system for the verification and characterization of proton beams in real time for intensity and energy-modulated proton therapy. A plastic tank filled with liquid scintillator was irradiated with pristine proton Bragg peaks. Scintillation light produced during the irradiation was measured with a CCD camera. Acquisition rates of 20 and 10 frames per second (fps) were used to image consecutive frame sequences. These measurements were then compared to ion chamber measurements and Monte Carlo simulations. The light distribution measured from the images acquired at rates of 20 and 10 fps have standard deviations of 1.1% and 0.7%, respectively, in the plateau region of the Bragg curve. Differences were seen between the raw LS signal and the ion chamber due to the quenching effects of the LS and due to the optical properties of the imaging system. The authors showed that this effect can be accounted for and corrected by Monte Carlo simulations. The liquid scintillator detector system has a good potential for performing fast proton beam verification and characterization.

Exploration of the potential of liquid scintillators for real-time 3D dosimetry of intensity modulated proton beams

Two detectors for fast two-dimensional (2D) and quasi-three-dimensional (quasi-3D) verification of the dose delivered by radiotherapy beams have been developed at University and Istituto Nazionale di Fisica Nucleare (INFN) of Torino. The Magic Cube is a stack of strip-segmented ionization chambers interleaved with water-equivalent slabs. The parallel plate ionization chambers have a sensitive area of 24 x 24 cm2, and consist of 0.375 cm wide and 24 cm long strips. There are a total of 64 strips per chamber. The Magic Cube has been tested with the clinical proton beam at Loma Linda University Medical Centre (LLUMC), and was shown to be capable of fast and precise quasi-3D dose verification. The Pixel Ionization Chamber (PXC) is a detector with pixel anode segmentation. It is a 32 x 32 matrix of 1024 cylindrical ionization cells arranged in a square 24 x 24 cm2 area. Each cell has 0.4 cm diameter and 0.55 cm height, at a pitch of 0.75 cm separates the centre of adjacent cells. The sensitive volume of each single ionization cell is 0.07 cm3. The detectors are read out using custom designed front-end microelectronics and a personal computer-based data acquisition system. The PXC has been used to verify dynamic intensity-modulated radiotherapy for head-and-neck and breast cancers.

https://iopscience.iop.org/article/10.1088/0031-9155/49/16/017/pdf

D Santuari

Papers

Two-dimensional and quasi-three-dimensional dosimetry of hadron and photon beams with the Magic Cube and the Pixel Ionization Chamber.