In heavy ion therapy (HIT) it is of paramount importance to ensure that the radiobiological dose (RBED) calculated in the treatment plan is identical to that given to the patient. However, due to a lack of instrumentation for simple and routine RBE Quality Assurance use, both the radiobiological effectiveness (RBE) and RBED are quantities that remain relatively unstudied. Whilst preliminary theoretical calculations and cell studies have been undertaken , the effect that organ motion has on RBE and RBED is an issue that needs further investigation, especially with pencil scanning beam delivery due to interplay effect.
In order to fully understand these effects, high spatial resolution microdosimetry detectors are required as the size of the gold standard tissue equivalent proportional counter sensitive volume can be as large as the amplitude of movement, rendering it useless in detecting differences in RBED due to motion. In this study, a high spatial resolution silicon microdosimeter with 3D sensitive volumes (SVs) has been used to investigate a typical 290 MeV/u 12C beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan and out-of-field of proton pencil beam scanning at Mayo Clinics, USA. The effect of motion on RBED in 12C passive beam treatment was studied with polyethylene bolus placed upstream along the central axis of the beam.
The new microdosimeter (also called Bridge Microdosimeter) is an n-type SOI detector based on an array of 30 x 30 μm2 sensitive volumes with a thickness of just 10μm, achieved through the use of ion plasma etching. Silicon “bridges” are left unetched to support the aluminium tracks between each sensitive volume. The Bridge Microdosimeter is connected to an in house-built low noise preamplifier and shaping amplifier optimized for the Bridge microdosimeter, enabling detection of lineal energies as low as ~0.3keV/um.
Derived RBE10 values based on the Microdosimetric Kinetic Model (MKM) model  and SOI bridge microdosimetric spectra were obtained in 12C ion therapy. The RBE values match very well with those obtained from the TEPC measurements. Due to high spatial resolution of the microdosimeter, more detailed RBE10 measurements were obtained at the end of the SOBP compared to the TEPC. Significant difference has been observed between the stationary microdosimetric spectra at distal part of the SOBP (148mm in water) and the case where the detector mimicked lung motion between 141mm and 149mm. Extra RBED has been delivered to the non-target volume as measured by microdosimeter and ionization chamber mimiking lung motion.Microdosimetric spectra and dose mean lineal energy obtained out-of-field in proton beam scanning allow the determination of neutron dose equivalent and the comparison with passive treatment delivery.
The Centre for Medical Radiation Physics, University of Wollongong has developed and implemented a new microdosimeter probe, enabling measurement of microdosimetric spectra with lineal energies as low as ~ 0.3 keV/µm. We have demonstrated that effect of motion can lead to changes in the microdosimetric spectrum and consequently the RBE. The microdosimeter also has ability of measuring neutrons dose outside of the treatment field.
 A Gemmel, E Rietzel, G Kraft, M Durante and C Bert, “Calculation and experimental verification of the RBE-weighted dose for scanned ion beams in the presence of target motion”, Phys. Med. Biol. 56 (2011) 7337–7351
 Kase Y et al Microdosimetric measurements and estimation of human cell survival for heavy-ion beams. Radiat. Res., 2006. 166(4): p. 9
Lecturer: Prof. Marco Petasecca (University of Wallongong, Australia)
Title: Advanced Quality Assurance instrumentation for radiotherapy: the experience at Centre for Medical Radiation Physics.
Cancer is rapidly becoming the largest cause of mortality in this century. Better technologies for diagnosis and treatment of cancer are required to address this problem and save more lives. During the last decade, due to the continuing development of innovative radiation therapy techniques, very conformal delivery of radiation has been achieved, improving the treatment of the cancer.
A review of major radiation therapy modalities related instrumentation for Adaptive Motion radiotherapy (ART) and Proton and Heavy Ions radiotherapy will be presented: Innovative External beam radiotherapies such as Adaptive Motion radiotherapy (ART) and Proton and heavy ion radiation therapies (C-12 ions) are growing due to progress in accelerator and beam delivery technologies, offering unique conformality in dose delivery.
Such Highly conformal modern radiation therapy modalities allow dose enhancement in a target while sparing normal tissue, and they are always associated with steep dose gradients. Safe application of these radiotherapy modalities requires sophisticated tools for quality assurance during the treatment –real time dosimetry.
Semiconductor electronic dosimetry is an option for QA due to the small size of the radiation detectors, the ability to produce pixelated detectors for 2D dosimetry with high spatial resolution and large area, and reproducible in manufacturing on microelectronics foundries. A family of new semiconductor dosimeters was developed at Centre for Medical Radiation Physics (CMRP):
• The technology of silicon strips and pixelated detectors allow dose mapping in phantom with spatial resolution down to 0.2 mm. The so named Dose Magnifying Glass (DMG), was developed at CMRP and used for QA of steep dose fall off at the target and organ-at-risk interfaces in both EBRT and Hadron therapy modalities.
• MagicPlate512, evolution of DMG from 1D to 2D array, will be also presented as in-vivo dosimetry system for in-phantom dosimetry verification of SBRT/SRS modalities and pristine Bragg peak position and profile verification in Hadron therapies, respectively.
All this instrumentation has been develop with the intent to provide effective tools for medical physicists and designed with the “customer” needs as priority specifications.
Contact：Ken Takayama (takayama at post.kek.jp)