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Radiation Dosimetry
Published in Kwan Hoong Ng, Ngie Min Ung, Robin Hill, Problems and Solutions in Medical Physics, 2023
Kwan Hoong Ng, Ngie Min Ung, Robin Hill
An ionisation chamber operates based on the principle of measuring the number of ion pairs produced in a volume of air due to radiation. In the simplest arrangement, the ionisation chamber exists as two-electrode plates spaced apart in air. A large potential (100–400 V) is applied to the plates. Radiation dose is delivered by charged particles in excitation and ionisation events. Charged particles are either the radiation of interest themselves (e.g., electron and proton radiotherapy) or indirectly produced by non-charged radiation (e.g., photons and neutrons). When charged particles traverse between the plates, they ionise the air producing free negative electrons and positive ions. The positive- and negative-charged particles are then swept by the electric field between the plates towards the appropriate electrodes, producing a steady current flow in the external circuit, which can be measured by an electrometer. The important premise of an ionisation chamber is that each interaction of the charged particle produces exactly one ion pair and therefore allows for accurate quantification of dose.
Ionisation Chambers
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
Multiple designs of ionisation chambers have been developed for use in different situations. These are discussed in Section 16.3, where it is shown that the ionisation chamber is a highly versatile radiation detector, capable of operating over a wide range of radiation intensity (or fluence rate). For the determination of absorbed dose to water in radiotherapy beams, the most common types are cylindrical (thimble) and plane-parallel (parallel-plate or coin) chambers, with the gas almost always being air (see Section 16.3.2). The international standard 60731 from the International Electrotechnical Commission on dosimeters with ionisation chambers as used in radiotherapy and its amendment 1 (IEC 2011) provide recommended limits for the performance characteristics of these devices. This IEC standard defines an ionisation chamber as ‘a detector consisting of a chamber filled with air, in which an electric field insufficient to produce gas multiplication is provided for the collection at the electrodes of charges associated with the ions and the electrons produced in the measuring volume of the detector by ionising radiation'. The term dosimeter describes the entire charge measurement apparatus, including chamber, electrometer and cables – although some manufacturers use this term solely to refer to the electrometer. IEC 60731 standard includes in the term radiotherapy dosimeter not only the chamber and measuring assemblies but also stability check sources and phantoms.
Radiotherapy Physics
Published in Debbie Peet, Emma Chung, Practical Medical Physics, 2021
Andrea Wynn-Jones, Caroline Reddy, John Gittins, Philip Baker, Anna Mason, Greg Jolliffe
The ionisation chamber is an integral part of the dose monitoring and control system. It is designed to provide signals in four quadrants, which are used in feedback circuits connected to the beam steering system. If the beam transport system is not set up correctly or has a fault, baseline properties of the beam such as its symmetry and flatness can go out of tolerance making it clinically unacceptable. Such fault conditions can be monitored using this ionisation chamber and feedback circuits. The ionisation chamber is also designed to define and control the amount of radiation delivered by the accelerator. The dual design is a safety feature to provide resilience, so a fault occurring in one ionisation chamber won’t cause excess dose delivery to the patient.
Properties of IBA Razor Nano Chamber in small-field radiation therapy using 6 MV FF, 6 MV FFF, and 10 MV FFF photon beams
Published in Acta Oncologica, 2021
Mari Partanen, Jarkko Niemelä, Jarkko Ojala, Jani Keyriläinen, Mika Kapanen
In this work, properties of the Nano chamber ionization chamber were assessed in small RT fields. The Nano chamber was compared with two commercially available and small-field recommended detectors, namely the IBA Razor Diode semiconductor detector and the PTW microDiamond (PTW-Freiburg GmbH, Freiburg, Germany) synthetic diamond detector. Moreover, the traditional PTW Semiflex ionization chamber was used for comparison in larger fields. The IBA Stealth ionization chamber (attached to the linac using the interface mount) was used as a reference signal detector relative measurements in small fields of 2 and the IBA CC13 ionization chamber (positioned to be outside the radiation beam) in larger fields. The following bias voltages were used in the measurements: 300 V for the Nano chamber, 0 V for the Razor Diode, 0 V for the microDiamond, 400 V for the Semiflex, −420 V for the Steath and 300 V for the CC13. In this study, the measurement results were not corrected for the polarity effect.
Towards a novel small animal proton irradiation platform: the SIRMIO project
Published in Acta Oncologica, 2019
Katia Parodi, Walter Assmann, Claus Belka, Jonathan Bortfeldt, Dirk-André Clevert, George Dedes, Ronaldo Kalunga, Sonja Kundel, Neeraj Kurichiyanil, Paulina Lämmer, Julie Lascaud, Kirsten Lauber, Giulio Lovatti, Sebastian Meyer, Munetaka Nitta, Marco Pinto, Mohammad J. Safari, Katrin Schnürle, Jörg Schreiber, Peter G. Thirolf, Hans-Peter Wieser, Matthias Würl
The current beamline design features a triplet of permanent magnet quadrupoles (PMQ) optimized for focusing 20–60 MeV proton beams at the treatment isocenter, approximately 70 cm downstream of a variable energy degrader of graphite followed by two dynamic brass collimators to adjust the beam emittance in front of the magnets. Resulting laterally-integrated dose distributions simulated in water show considerably improved entrance-to-peak and plateau-to-peak ratios with respect to a collimator-only passive beam delivery (Figure 1 Supplementary Material). The proposed design is estimated to provide spot sizes smaller than 1 mm FWHM at the focal position at isocenter for an energy spread within 4% and with transmission up to 1%, along with a neutron fluence below 10% relative to the considered passive-only configuration [15]. Additional simulations and treatment planning studies are currently ongoing to further optimize the beamline performance, especially concerning the choice of degrader material, PMQ sensitivity to stray radiation, transmission efficiency and potentially more stringent low-energy transport requirements, prior to finalizing the magnetic lattice design. For the beam monitor, the segmented ionization chamber prototype performed as required. The first experimental data analysis suggests achievable spatial resolution of few tens of micrometers along with accurate (within ∼1%) fluence monitoring in a wide dynamic range (5·105 to 1·1010 protons/s).
Raman spectroscopy monitoring of MCF10A cells irradiated by protons at clinical doses
Published in International Journal of Radiation Biology, 2019
Maria Lasalvia, Giuseppe Perna, Lorenzo Manti, Javier Rasero, Sebastiano Stramaglia, Vito Capozzi
Cell irradiation was carried out at the ocular melanoma treatment facility of the Laboratori Nazionali del Sud of the Istituto Nazionale di Fisica Nucleare (Catania, Italy), where a 62 MeV proton beam is generated by a super-conducting cyclotron. In particular, glass coverslips containing adherent MCF10A cells were located into identical flasks, which were placed at the distal end position of a SOBP. The SOBP range was 30 mm in water and cells were located at the depth of 29.04 mm water equivalent (LET ∼16.4 keV/µm), using Poly-methyl-methacrilate (PMMA) beam degraders. The relative dose profile was measured with a MarkusTM ionization chamber. Detailed description of beam line and dosimetry is described elsewhere (Cirrone et al. 2004). Single fractions of 0, 0.5 and 4 Gy were delivered to the flasks. The uncertainty in dose measurements was within 3%. The MCF10A cells were fixed immediately after the end of the exposure process, by means of 3.7% PFA in PBS solution.