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

Clinical trials of carbon radiotherapy started at HIMAC in 1994 using three treatment rooms and four beam ports, two horizontal and two vertical. The broad beam method was adopted to make a three-dimensionally uniform field at an isocenter. A spot beam extracted from an accelerator was laterally spread out by using a pair of wobbler magnets and a scatterer. A bar ridge filter modulated the beam energy to obtain the spread out Bragg peak (SOBP). The SOBP was designed to be flat in terms of the biological dose based on the consideration that the field consisted of various beams with different LET. Finally, the field of 20 cm in diameter with +/- 2.5% uniformity was formed at the isocenter. The width of the maximum SOBP was 15 cm. When treating the lung or liver, organs that move due to breathing, the beam was irradiated only during the expiration period in a respiration-gated irradiation method. This reduced the treatment margin of the moving target. In order to prevent normal tissues adjacent to the target volume from irradiation by an unwanted dose, a layer-stacking method was developed. In this method, thin SOBP layers which have different ranges were piled up step by step from the distal end to the entrance of the target volume. At the same time, a multi-leaf collimator was used to change the aperture shape to match the shape of each layer to the cross-sectional shape of the target. This method has been applied to rather large volume cancers including bone and soft-tissue cancers. Only a few serious problems in the irradiation systems have been encountered since the beginning of the clinical trials. Overall the systems have been working stably and reliably.

Irradiation System for HIMAC.

Washout of 10C and 11C implanted by radioactive beams in brain and thigh muscle of rabbits was studied. The biological washout effect in a living body is important in the range verification system or three-dimensional volume imaging in heavy ion therapy. Positron emitter beams were implanted in the rabbit and the annihilation gamma-rays were measured by an in situ positron camera which consisted of a pair of scintillation cameras set on either side of the target. The ROI (region of interest) was set as a two-dimensional position distribution and the time–activity curve of the ROI was measured. Experiments were done under two conditions: live and dead. By comparing the two sets of measurement data, it was deduced that there are at least three components in the washout process. Time–activity curves of both brain and thigh muscle were clearly explained by the three-component model analysis. The three components ratios (and washout half-lives) were 35% (2.0 s), 30% (140 s) and 35% (10 191 s) for brain and 30% (10 s), 19% (195 s) and 52% (3175 s) for thigh muscle. The washout effect must be taken into account for the verification of treatment plans by means of positron camera measurements.

https://iopscience.iop.org/article/10.1088/0031-9155/48/15/302/pdf

Washout measurement of radioisotope implanted by radioactive beams in the rabbit

Heavy ion therapy has two definite advantages: good dose localization and higher biological effect. Range calculation of the heavy ions is an important factor in treatment planning. X-ray CT numbers are used to estimate the heavy ion range by looking up values in a conversion table which relates empirically photon attenuation in tissues to particle stopping power; this is one source of uncertainty in the treatment planning. Use of positron emitting radioactive beams along with a positron emission tomograph or a positron camera gives range information and may be used as a means of checking in heavy ion treatment planning. However, the metabolism of the implanted positron emitters in a living object is unpredictable because the chemical forms of these emitters are unknown and the metabolism is dependent on the organ species and may be influenced by many factors such as blood flow rate and fluid components present. In this paper, the washout rate of 11C activity implanted by injecting energetic 11C beams into thigh muscle of a rear leg of a rabbit is presented. The washout was found to consist of two components, the shorter one was about 4.2 ± 1.1 min and the longer one ranged from 91 to 124 min. About one third of the implanted β+ activity can be used for imaging and the rest was washed out of the target area.

https://iopscience.iop.org/article/10.1088/0031-9155/48/7/305/pdf

Washout studies of 11C in rabbit thigh muscle implanted by secondary beams of HIMAC

A treatment planning of heavy ion cancer therapy is performed with an aid of X-ray CT, which has uncertainties in an estimation of ion range. In order to obtain a dose distribution with high accuracy, irradiation and range measurement systems are developed at HIMAC using positron emitter beams. Exact stopping points of the ions can be measured by detecting annihilation γ-rays from positron emitters themselves. A new irradiation system has been installed, for creating an irradiation field with 3-dimensionally desired shape. A positron camera, which is similar to an Anger-type γ camera, is used to determine a range distribution of the heavy ions in a patient’s body. An existing PET system can be also used to measure an image of the 3dimensional irradiation-field.

/pdf/medical-application-of-the-positron-emitter-beam-at-himac-2bmhe1t51u.pdf

Medical application of the positron emitter beam at himac

A new beam course, a projectile fragment separator to provide radioactive beams, is under construction at HIMAC. A primary purpose of the new beam course is to study medical applications of radioactive beams. The specifications and status of the beam course and a preliminary result using carbon beams are summarized.

New secondary beam course for medical use in HIMAC

In heavy-ion radiotherapy, an accelerated beam is modified to realize a desired dose distribution in patients. The setup of the beam-modifying devices in the irradiation system is changed according to the patient, and it is important to check the depth dose distributions in the patient. In order to measure dose distributions realized by an irradiation system for heavy-ion radiotherapy, a multi-layer ionization chamber(MLIC) was developed. The MLIC consists of 64 ionization chambers, which are stacked mutually. The interval between each ionization chamber is about 4.1 mm water. There are signal and high voltage plates in the MILC, which are used as electrodes of the ionization chambers and phantom. Depth dose distribution from 5.09 mm to 261.92 mm water can be measured in about 30 seconds using this MLIC. Thus, it is possible to check beam quality in a short amount of time.

[Development of a multi-layer ion chamber for measurement of depth dose distributions of heavy-ion therapeutic beam for individual patients].

Since 1994 clinical trials have been performed successfully with carbon beam. To improve the clinical result further, new irradiation systems are under development such as a 3D-irradiation system and a verification system of range with positron emitter. There are also improvements on the accelerator performances. One is the wide range of ion species; the others are concerned with the machine devices and new beam monitors to get good machine operation. In this report we present current status of HIMAC.

/pdf/present-status-of-himac-at-nirs-2xyj8w1gxc.pdf

N. Matsufuji

Papers

New secondary beam course for medical use in HIMAC

[Development of a multi-layer ion chamber for measurement of depth dose distributions of heavy-ion therapeutic beam for individual patients].

Present status of HIMAC at NIRS