Narrow Photon Beams Research Articles

Characterization of a novel micro-irradiator using Monte Carlo radiation transport simulations

Small animals are highly valuable resources for radiobiology research. While rodents have been widely used for decades, zebrafish embryos have recently become a very popular research model. However, unlike rodents, zebrafish embryos lack appropriate irradiation tools and methodologies. Therefore, the main purpose of this work is to use Monte Carlo radiation transport simulations to characterize dosimetric parameters, determine dosimetric sensitivity and help with the design of a new micro-irradiator capable of delivering irradiation fields as small as 1.0 mm in diameter. The system is based on a miniature x-ray source enclosed in a brass collimator with 3 cm diameter and 3 cm length. A pinhole of 1.0 mm diameter along the central axis of the collimator is used to produce a narrow photon beam. The MCNP5, Monte Carlo code, is used to study the beam energy spectrum, percentage depth dose curves, penumbra and effective field size, dose rate and radiation levels at 50 cm from the source. The results obtained from Monte Carlo simulations show that a beam produced by the miniature x-ray and the collimator system is adequate to totally or partially irradiate zebrafish embryos, cell cultures and other small specimens used in radiobiology research.

Strahlenphysikalische Einflussgrößen bei der Dosimetrie mit verschiedenen Detektortypen

The present study investigated the radiophysical influences on the measurement of dosimetry basic data, attributable to field size, photon energy and detector type. A natural diamond detector, two ionisation chambers, different Si-diodes and a EBT-Gafchromic film were studied for this purpose. The characteristics of the detectors were investigated with regard to the measurement of output factors, lateral beam profiles and relative depth-dose curves for narrow and wide photon beams of 15 MV. Significant differences in output factors were obtained with different detectors. For narrow fields, the natural diamond detector and the diodes PTW-60012 and SCX_WH-PFD measured output factors close to those of the EBT-Gafchromic film. The output factor for large fields was overestimated by the unshielded diode PTW 60012 and the PinPoint-chamber PTW-31006 because of their over-response to scattered photons. The relative depth dose distributions for wide beams at large depths agree well for the diamond, the ionisation chambers and the shielded Diode SCX_WH-PFD and PTW-60008, while the measured dose was overestimated by an unshielded diode PTW-60012. Considering the influence due to the sensitive materials and the construction of the detectors the manufacturers of dosimeters have specified the application ranges for the various types of detectors.

SU-FF-T-96: Characteristics of Narrow Field Photon Beam Measurements for a Micro-MLC Based Radiosurgery System

Purpose: To determine the response of various detectors in the commissioning of narrow field megavoltage photon beams for a micro-MLC based radiosurgery system. Method and Materials: We commissioned both a standard and a high dose rate 6 MV photon beams from a Trilogy unit (Varian Medical systems, Palo Alto CA) for the pencil beam algorithm using a micro-MLC based Radiosurgery system (BrainLAB AG Germany). The loss of lateral equilibrium, high dose gradients, and spectral changes within high energy narrow photon beams imposes restrictions on detector composition and size used for relative measurements. We investigated the response of two p-type Si-diodes (Scanditronix photon and stereotactic diodes), a diamond (PTW) detector, a cylindrical (Scanditronix RK chamber) and a micro-ion-chamber (Exradin #a16) for depth dose and relative output measurements. Results: Percent depth dose measurement with photon diode (guarded) yielded consistently higher measurements by 1–2% compared to ionization chamber results for larger depths with increasing field sizes. Stereotactic diode measurements of depth doses for field sizes ⩾18×18 mm2 yielded 1–2% lower values compared to ionization chamber results. For field sizes ⩽12×12mm2 photon diode depth dose measurements were higher by 1–5% at depths > 5cm compared to those measured by stereotactic diode. The relative output factors measured by photon diode, diamond detector and both ion chambers were within 1% agreement with each other for all field sizes ⩾ 30×30mm2. The stereotactic diode measurements were lower by ∼2.5% for all but the smallest field size of 6×6 mm2 compared to other detectors. Relative output factors for the high dose rate beam were higher by 2–4% for fields < 18×18mm2 compared to the standard 6 MV photon beam. Conclusion: Measurement of relative output factors and depth doses for megavoltage narrow photon fields require multiple detectors to correctly estimate the true response over the range of measurements.

Investigation on the performances of multiple microstrip CdTe detectors

A study of the spectroscopic response of several large-size CdTe detectors equipped with a conventional cathode and a multi-microstrip anode (15 strips, each 150 μm wide, 500 μm pitch) was carried out. Measurements were performed scanning the detector across the strips, by means of a narrow rectangular photon beam ( 57Co and 109Cd sources), with its longer side oriented parallel to the strip direction, at normal incidence, in order to evaluate the detector behavior when the photon beam impinges at the strip position and at different positions inside the gaps. The detectors were also irradiated changing the value of the angle θ between the photon beam direction and the normal to the cathode surface in the range 0°< θ<70°. The behaviors of the 14, 22, 88 and 122 keV energy peaks have been analyzed. From the scanning along and across the strips some degree of spatial non-uniformity has been observed. As expected, the spectroscopic performance of microstrip detectors is only slightly dependent on the angle, i.e. our strip design is working well. Further measurements are in progress to study the observed charge sharing between adjacent strips, in order to understand if these detectors are suitable to be used as position sensitive devices with spectroscopic capabilities to build up focal planes of X-ray telescopes for astrophysics applications.

Dosimetric comparison of linear accelerator-based stereotactic radiosurgery systems

Stereotactic radiosurgery (SRS) is a special radiotherapy technique used to irradiate intracranial lesions by 3-D arrangements of narrow photon beams eliminating the needs of invasive surgery. Three different tertiary collimators, namely BrainLab and Radionics circular cones and BrainLab micro multileaf collimator (mMLC), are used for linear accelerator-based SRS systems (X-Knife). Output factor (St), tissue maximum ratio (TMR) and off axis ratio (OAR) of these three SRS systems were measured using CC01 (Scanditronix/ Welhofer) and Pinpoint (PTW) cylindrical and Markus plane parallel ionization chambers as well as TLD and radiochromic film. Measurement results of CC01 and Pinpoint chambers were very close to each other which indicate that further reduction in volume and physical dimensions of cylindrical ionization chamber is not necessary for SRS/SRT dosimetry. Output factors of BrainLab and Radionics SRS cones were very close to each other while output factors of equivalent diameter mMLC field were different from SRS circular cones. TMR of the three SRS systems compared were very close to one another. OAR of Radionics cone and BrainLab mMLC were very close to each other, within 2%. However, OARs of BrainLab cone were found comparable to OARs of Radionics cone and BrainLab mMLC within maximum variation of 4%. In addition, user-measured similar data of other three mMLC X-Knives were compared with the mMLC X-Knife data measured in this work and found comparable. The concept of switching over to mMLC-based SRS/SRT is thus validated from dosimetric characteristics as well.

Use of polymer gel dosimetry for the determination of the detector size effect on profile measurements of a 5 mm diameter photon beam

Narrow photon beams of less than 10 mm in diameter play a significant role in modern radiotherapy since they are increasingly used in stereotactic radiosurgery and in IMRT. The conventional treatment planning systems rely on narrow beam profile measurements either for relative dose calculations or for beam modeling. However, these measurements are problematic since narrow beams are characterized by large dose gradients and electronic dis-equilibrium. The aim of the current study was to investigate the detectors' size effect on the profile measurement of a 5 mm diameter 6 MV x-ray narrow beam using polymer gel MRI dosimetry.

Experimental determination of the effect of detector size on profile measurements in narrow photon beams

The aim of this work is to investigate experimentally the detector size effect on narrow beam profile measurements. Polymer gel and magnetic resonance imaging dosimetry was used for this purpose. Profile measurements (Pm(s)) of a 5 mm diameter 6 MV stereotactic beam were performed using polymer gels. Eight measurements of the profile of this narrow beam were performed using correspondingly eight different detector sizes. This was achieved using high spatial resolution (0.25 mm) two-dimensional measurements and eight different signal integration volumes A X A X slice thickness, simulating detectors of different size. "A" ranged from 0.25 to 7.5 mm, representing the detector size. The gel-derived profiles exhibited increased penumbra width with increasing detector size, for sizes >0.5 mm. By extrapolating the gel-derived profiles to zero detector size, the true profile (Pt) of the studied beam was derived. The same polymer gel data were also used to simulate a small-volume ion chamber profile measurement of the same beam, in terms of volume averaging. The comparison between these results and actual corresponding small-volume chamber profile measurements performed in this study, reveal that the penumbra broadening caused by both volume averaging and electron transport alterations (present in actual ion chamber profile measurements) is a lot more intense than that resulted by volume averaging effects alone (present in gel-derived profiles simulating ion chamber profile measurements). Therefore, not only the detector size, but also its composition and tissue equivalency is proved to be an important factor for correct narrow beam profile measurements. Additionally, the convolution kernels related to each detector size and to the air ion chamber were calculated using the corresponding profile measurements (Pm(s)), the gel-derived true profile (Pt), and convolution theory. The response kernels of any desired detector can be derived, allowing the elimination of the errors associated with narrow beam profile measurements.

TH‐C‐VaIA‐02: In's and Out's of Non‐Equilibrium Dosimetry

Typical procedures for clinical dosimetry are well established for measurements in charged particle equilibrium situations. These situations only form a subset of the measurement challenges of a clinical physicist. Dose measurements in regions of charged particle disequilibrium require special considerations before the measurement can be assumed accurate. These disequilibrium conditions occur in association with measurements performed for standard clinical treatment techniques as well as for special treatment techniques. In the symposium we identified four major areas that complicate accurate clinical dosimetry: (1) the photon build‐up region, (2) narrow photon beams, (3) measurements in heterogeneous phantoms and (4) modulated fields and dynamic measurements.In the present lecture we describe the principles of accurate dose measurements in general in equilibrium and non‐equilibrium situations. Principles of detector response, energy dependence and characteristics of practical detectors are described with emphasis on the understanding of detector behaviour in non‐equilibrium conditions. By way of introduction, practical examples will be provided of non‐equilibrium measurements in the build‐up region, narrow beams, heterogeneous phantoms and IMRT beams. The lecture will provide general guidelines about detector suitability for commercially available devices for the discussed areas of application. Finally, uncertainties of the procedures in a clinical context will be discussed.Educational Objectives:1. To understand the fundamental complications of non‐equilibrium measurements.2. To understand the impact of detector properties on measurement accuracy in non‐equilibrium situations.3. To place measurement uncertainties in non‐equilibrium conditions in a clinical perspective.

SU‐FF‐T‐414: The Dose Area Product in Radiation Therapy‐ a New Concept for the Parameterisation of Small Fields

Purpose: To establish a new parameterisation of small fields.Material and Methods: The traditional parameterisation of a narrow photon field via the central axis dose and the relative transversal dose profile has met considerable methodical complications. These difficulties are to find a sufficiently small detector, to adjust the detector accurately on the axis of the narrow beam and to find a detector material not responding to lateral changes of the electron spectrum within the small field. These obstacles can be avoided by reconsidering the parameterisation of the narrow‐field dose distribution. The new parameter recommended for characterising the absolute dose values in a plane perpendicular to the beam axis is the dose‐area product DAP (the area integral of the dose in this plane). It can be measured with a flat ionisation chamber of large cross section of the sensitive volume. Results and Conclusions: The radial adjustment of the large area chamber is by no means critical. The dose‐area product provides a simple normalisation of the relative transversal dose distribution which can be measured with radiochromic film. We have investigated the abilities of a large‐area flat ionisation chamber of PTW Freiburg (PTW TM 34070‐2,5) of 8,1 cm diameter and 2 mm thickness of the sensitive volume to measure the DAP of narrow photon beams with side lengths up to 5 cm. A modified output factor has been defined as the quotient of the DAP, measured at 5 cm phantom depth for SSD 100 cm distance of the phantom, and the monitor reading. Besides the useful feature of the DAP is its direct measurability during patient treatment by means of the DAVID chamber, an on‐line monitor arranged in the accessory holder, so that non‐negligible deviations of the actual from the chosen field size of narrow photon fields can be immediately detected.

The dose-area product, a new parameter for the dosimetry of narrow photon beams

In the dosimetry of narrow photon fields with side lengths of the order of 1 cm, the traditional parametrisation via the absolute dose on the beam axis and the relative lateral dose distribution has to deal with the difficulty to find sufficiently small detectors and to adjust them accurately on the narrow-beam axis. This can be avoided by reconsidering the parametrisation, using as normalization factor the surface integral of the dose in the plane perpendicular to the beam axis, abbreviated as the “dose-area product” (DAP). We investigated and confirmed the ability of a large-area parallel-plate ionisation chamber, with a sensitive volume shaped as a flat cylinder of 81.6 mm diameter and 2 mm thickness, to perform the integration over the full lateral dose profile of narrow photon beams with side lengths up to 5 cm. The lateral adjustment of this large-area detector relative to a narrow photon beam is not critical. The large-area ionisation chamber was calibrated in terms of the DAP by reference to a 0.3 cm3 ionisation chamber. A field-size dependent “modified output factor” was defined as the ratio of the DAP, measured at 5 cm phantom depth for 100 cm SSD, and the monitor reading. A prominent phenomenon of narrow photon fields is the field-size and source-distance independence of the relative axial profile of the DAP as function of the thickness of a pre-absorber or of the depth in a phantom. For narrow-beam treatment planning in IMRT, the DAP is combined with the energy- and field size-dependent relative lateral dose distribution which is represented, for example, by a Gaussian convolution kernel. Another useful feature of the DAP is the possibility of its direct control during patient irradiation by means of an on-line monitor with spatial resolution, arranged in the accessory holder.

Radiation transport calculations for 50 MV photon therapy beam using the Monte Carlo code GEANT4

A new thin transmission target technique for fast dose delivery using narrow scanned photon beams has been developed. High-energy, 50-100 MeV, electron beams of low emittance incident on thin low-Z targets produce narrow and intense high-energy bremsstrahlung beams. However, electrons transmitted through the target are bent from the therapeutic beam by a purging magnet and have to be effectively absorbed in a dedicated electron collector. The electron-photon transport through a treatment head has been studied using the Monte Carlo simulation toolkit Geant4. The Geant4 electromagnetic physics processes have been compared with experimental data of radial dose profiles. The differences between calculated and measured radial dose distributions are approximately 2-10%. Preliminary investigations of the collector design have been carried out in order to minimise secondary electron and photon contamination of the therapeutic beam. The toolkit presented here is promising for further development of narrow photon beam therapy.

217 Development of a scanning system for intensity modulated radiation therapy with photon pencil beams

The increasing cancer incidence requires more accurate and cost-effective diagnostic as well as therapeutic methods. Intensity modulated radiation therapy is now rapidly being introduced at many radiation therapy departments with a clear trend towards radiobiological treatment planning using 4D-diagnostic imaging and -treatment delivery. Therefore a magnetic system for scanning of narrow photon pencil beams has been developed for fast 4D-IMRT. The magnetic scanning system has been modelled in full 3D taken into account all aspects such as fringing field, magnetic leakage and cooling. The magnetic system consists of two perpendicular scanning magnets, a bending magnet and a purging magnet placed after a thin bremstrahlung target and in the same plane as the last scan magnet. The scanning magnets operate in orthogonal planes and the first scanning magnet (bending plane) is placed so that the image of its scanning centre lies near the effective centre of the second scan magnet (cross plane). In this way a very compact deflection system is obtained, with the position of the effective radiation source coinciding for the two orthogonal planes. The focus of the intrinsic electron beam is at the top of the target placed near the end of the last scanning magnet. The composition and thickness of this target determines the energy of the transmitted electrons and the full width half maximum (FWHM) of the photon beam. Here the target is made of 3 mm beryllium creating a therapeutic beam of 10-15 mm (FWHM) for an incident electron beam of 70-50 MeV. The transmitted electron beam has only lost around 0.5-1.0 MeV in the target and must be removed from the therapeutic beam. Therefore downstream of the target a strong and fast purging magnet integrated with an electron collector has been developed. The purging magnet operates in the same plane as the last scanning magnet and deflects the transmitted high energy electrons to the same area on the electron collector independent of the direction of the scanned photon beam. The overall design of the treatment head is compact and the narrow photon beam will allow a very accurate dose delivery particularly when combined with a thin efficient leaf collimator designed for pencil beam IMRT.

SU-FF-T-396: Analytical Derivation of 6 MV Radiosurgery Cone Factors

Purpose: To derive cone factors analytically for 6 MV radiosurgery beams Method and Materials: Lack of lateral electronic equilibrium and finite detector size make experimental measurements of stereotactic radiosurgery cone factors a difficult task. In this work, cone factors for 6 MV radiosurgery beams of various cross sections are determined using an analytical model for absorbed dose in narrow photon beams and validated by means of experimental measurements in water. The model allows one to calculate absorbed dose under electronic disequilibrium conditions using measured broad‐beam data. For each cone ranging from 4 mm to 20 mm in diameter, cone factor is derived analytically and compared to that measured in water experimentally using a high resolution diode and a computerized data acquisition system. Results: The agreement between the analytical and measured cone factors is within 2.5 % of local value. Conclusion: Based on the results of this study for 6 MV x‐rays, the analytical method described here can be employed to determine radiosurgery cone factors. It provides a viable alternative to experimental methods where measurements in water are arduous. It can be used as an independent method or to validate experimental results.

TU‐FF‐A2‐01: A Heterogeneity Inclusive Algorithm for Calculation of Central‐Axis Absorbed Dose in Finite‐Size Pencil Beams

Purpose: To develop algorithms for calculating central‐axis absorbed dose from high‐energy photon beams in heterogeneous media of arbitrarily varying density. Method and Materials: We employ and extend a phenomenological model for the central‐axis absorbed dose in therapeutic photon beams (Nizin, Med. Phys. 26, 1893–1900 (1999)) in order to describe absorbed dose in heterogeneous media. The parameters defined by the model are extracted from beam data for a homogeneous water phantom. For heterogeneous media, the model's parameters are determined by rescaling these for water using the local values of the relative density of the medium. Results: The phenomenological model was generalized for arbitrarily varying density media by introducing a differential equation that modulates the field size dependent normalization factor. This factor is multiplied by the photon fluence, which decays exponentially with the radiological depth. The generalized model was tested using slab geometries where layers of water and lung‐like material alternate. We found very close agreement between absorbed dose calculated using the generalized model and Monte Carlo simulations (Kawrakow et al, NRCC PIRS‐701 (2003)) for finite‐size pencil beams and for a wide range of field sizes for broad beams. Conclusion: Accurate algorithms for calculation of central‐axis absorbed dose in heterogeneous media are proposed. For both narrow and broad photon beams, the model generates results that are in close agreement with Monte Carlo data for slab geometries. Conflicts of Interest: This research was supported by NOMOS Radiation Oncology Division of North American Scientific.

Microionization chamber for reference dosimetry in IMRT verification: clinical implications on OAR dosimetric errors

Intensity modulated radiotherapy (IMRT) has become a treatment of choice in many oncological institutions. Small fields or beamlets with sizes of 1 to 5 cm2 are now routinely used in IMRT delivery. Therefore small ionization chambers (IC) with sensitive volumes ⩽0.1 cm3are generally used for dose verification of an IMRT treatment. The measurement conditions during verification may be quite different from reference conditions normally encountered in clinical beam calibration, so dosimetry of these narrow photon beams pertains to the so-called non-reference conditions for beam calibration. This work aims at estimating the error made when measuring the organ at risk's (OAR) absolute dose by a micro ion chamber (μIC) in a typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions. We have selected two clinical cases, treated by IMRT, for our dose error evaluations. Detailed geometrical simulation of the μIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the dose measured by the chamber as a dose averaged over the air cavity within the ion-chamber active volume (Dair). The absorbed dose to water (Dwater) is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water in the absence of the ion chamber. Therefore, the Dwater/Dair dose ratio is the MC estimator of the total correction factor needed to convert the absorbed dose in air into the absorbed dose in water. The dose ratio was calculated for the μIC located at the isocentre within the OARs for both clinical cases. The clinical impact of the calculated dose error was found to be negligible for the studied IMRT treatments.

An experimental method to evaluate the dead Layer thickness of X- and gamma-ray semiconductor detectors

In the classic irradiation configuration of solid-state X- and Gamma-ray detectors, where the photons impinge normally to the cathode, the electrode and detector dead layer thickness affect the detection efficiency and the spectroscopic performance of the device, mainly at the lowest incident photon energies. The concentration of defects in the near-electrode regions greatly depends on the mechanical and chemical treatments used for the surface preparation (polishing and etching), before electrode deposition, as well as on metallic diffusion in the crystal, which result in differently thick dead layers. In this paper, we evaluate, in an easy, experimental way, the dead layer thickness, irradiating a detector by a narrow photon beam, at various incident angles. The areas relevant to the 14-keV (/sup 57/Co) and to 22-keV (/sup 109/Cd) photopeaks, at different angles of incidence, are independently used to solve a linear equation depending on the photons absorption in the electrodes (Pt) and the dead layer (CdTe) material. As an example, data obtained with a CdTe detector 2-mm-thick and 3/spl times/10 mm/sup 2/ electrode area are presented and discussed.

A Monte Carlo multiple source model applied to radiosurgery narrow photon beams.

Monte Carlo (MC) methods are nowadays often used in the field of radiotherapy. Through successive steps, radiation fields are simulated, producing source Phase Space Data (PSD) that enable a dose calculation with good accuracy. Narrow photon beams used in radiosurgery can also be simulated by MC codes. However, the poor efficiency in simulating these narrow photon beams produces PSD whose quality prevents calculating dose with the required accuracy. To overcome this difficulty, a multiple source model was developed that enhances the quality of the reconstructed PSD, reducing also the time and storage capacities. This multiple source model was based on the full MC simulation, performed with the MC code MCNP4C, of the Siemens Mevatron KD2 (6 MV mode) linear accelerator head and additional collimators. The full simulation allowed the characterization of the particles coming from the accelerator head and from the additional collimators that shape the narrow photon beams used in radiosurgery treatments. Eight relevant photon virtual sources were identified from the full characterization analysis. Spatial and energy distributions were stored in histograms for the virtual sources representing the accelerator head components and the additional collimators. The photon directions were calculated for virtual sources representing the accelerator head components whereas, for the virtual sources representing the additional collimators, they were recorded into histograms. All these histograms were included in the MC code, DPM code and using a sampling procedure that reconstructed the PSDs, dose distributions were calculated in a water phantom divided in 20000 voxels of 1 x 1 x 5 mm3. The model accurately calculates dose distributions in the water phantom for all the additional collimators; for depth dose curves, associated errors at 2sigma were lower than 2.5% until a depth of 202.5 mm for all the additional collimators and for profiles at various depths, deviations between measured and calculated values were less than 2.5% or 1 mm.

Spectroscopic response of a CdTe microstrip detector when irradiated at various impinging angles

A study of the spectroscopic response of a CdTe detector, 10mm×3mm×3mm in size, equipped with a 10mm×3mm full cathode and a 10mm×0.3mm central strip anode placed between two parallel focusing strips spaced of about 0.050mm from the collecting anode, was carried out. This configuration increases the intensity of the weighing field surrounding the anode strip, thus improving the collection of the electrons. The detector was irradiated through the cathode by a narrow photon beam (57Co source) at various impinging angles θ in the range 0–87° , θ being the angle between the photon beam axis and the normal to the cathode surface, aiming at obtaining better spectroscopic performance. The measurements were carried out with the long side of the beam spot (0.90mm×0.18mm) oriented both transversal and parallel to the central strip anode. In this last case, significant improvements for some spectroscopic parameters were obtained; e.g. the peak-to-valley ratio of the 122-keV peak changes from 13.5 at 0° to 25.8 at 82°, i.e. an almost two times better value.

An overview on performance and possible applications of X- and gamma-ray semiconductor detectors irradiated at various impinging angles

The spectroscopic response of CdTe detectors has been investigated with an unusual irradiation geometry, i.e., with the beam impinging at various angles through the cathode surface. In fact, the classical planar parallel field (PPF) irradiation geometry, i.e., with photons impinging perpendicularly to the cathode, limits the spectroscopic performance at high energies (E>60 keV), if thick detectors are used, owing to the large spread in the paths of the charge carriers. These paths for the holes can be greatly reduced by using larger impinging angles, thus improving the energy resolution without losses in detection efficiency. Several sets of measurements have been carried out with a narrow photon beam, coming from collimated /sup 57/Co, /sup 109/Cd and /sup 241/Am sources; measurement angles ranged from 0/spl deg/ (orthogonal to the cathode) to 87/spl deg/ (beam grazing the cathode). The spectroscopic responses of the detectors for 14, 22, 60 and 122 keV photon energies have been analyzed. Detectors equipped with both full anodes and microstrip anodes were used. In this last case the rectangular collimated spot of the beam was used with the long side both parallel and orthogonal to the strip direction. The experimental results show that an actual improvement in the spectroscopic performance is obtained, mostly for the high-energy X-rays, by this tilting procedure. Further, this technique allows the evaluation of the detector dead layer thickness. The obtained performance and possible medical and space applications are discussed.

Recent advances in light ion radiation therapy

Background The fast development of energy- and intensity-modulated radiation therapy during the last two decades using photon and electron beams has when implemented resulted in a considerable improvement of radiation therapy, particularly if combined with radiobiologically based treatment optimization techniques. This has made intensity-modulated electron and photon beams as powerful as today's uniform dose proton therapy. To be able to cure also the most advanced hypoxic and radiation-resistant tumors of complex local spread, intensity-modulated light ion beams are really the ultimate tool and in clinical practice 2 to 3 times less expensive per patient treated than proton therapy. This development and the recent development of advanced tumor diagnostics based on PET-CT imaging of the tumor cell density open the field for new powerful radiobiologically based treatment optimization methods. The ultimate step is to use the unique radiobiologic and dose distributional advantages of light ion beams for truly optimized bioeffect planning where the integral three-dimensional dose delivery and tumor cell survival can be monitored by PET-CT imaging and corrected by biologically based adaptive therapy optimization methods. Purpose The main purpose of the present paper is to discuss the principal areas of development of therapy optimization, by considering the therapy chain from tumor diagnostics and the use of three-dimensional predictive assay to biologically based treatment optimization with special focus on the rapid clinical development of advanced light ion therapy. Methods Besides the “classical” approaches using low ionization density hydrogen ions (protons, but also possibly deuterons and tritium nuclei) and high ionization density carbon ions, two new approaches will be discussed. In the first one, lithium or beryllium or boron ions, which induce the least detrimental biologic effect to normal tissues for a given biologic effect in a small volume of the tumor, will be key particles. In the second approach, referred patients will be given a high-dose, high-precision “boost” treatment with carbon or oxygen ions during 1 week preceding the final treatment with conventional radiation in the referring hospital. The rationale behind these approaches is to minimize the high ionization density dose to the normal-tissue stroma outside but sometimes also inside the tumor bed and to ensure a more uniform and optimal biologic effectiveness in the tumor, also on the microscopic scale. The present discussion indicates that BIologically Optimized predictive Assay based light ion Radiation Therapy (Bio-Art) is really the ultimate way to perform high-precision radiation therapy using checkpoints of the integral dose delivery and the tumor response and, based on this information, perform compensating corrections of the dose delivery. By using biologically optimized scanned high-energy photon or ion beams, it is possible to measure in vivo the three-dimensional dose delivery using the same PET-CT camera that was used for diagnosing the tumor spread. This method thus opens up the door for truly three-dimensional biologically optimized adaptive radiation therapy, where the measured dose delivery to the true target tissues can be used to fine-adjust the incoming beams, so that possible errors in the integral therapy process are eliminated toward the end of the treatment. Interestingly enough, practically all major error sources—such as organ motion, treatment planning errors, patient setup errors, and dose delivery problems due to gantry, multileaf, or scanning beam errors—can be corrected for in this way. Results and conclusions Radiobiologically optimized dose delivery using intensity and radiation quality modulation based on high-resolution PET-CT or Magnetic Resonance Spectroscopic Imaging (MRSI)-based tumor and normal-tissue imaging is probably the ultimate development of radiation therapy, taking the unique physical and biologic advantages of light ions fully into account in truly patient-individualized curative treatment schedules. Using recently available biologically based treatment optimization algorithms, it is possible to improve the treatment outcome for advanced tumors by as much as 10–40%. The adaptive radiotherapy process based both on three-dimensional tumor cell survival and dose delivery monitoring has the potential of percent accuracy in tumor response and dose delivery monitoring, using two-dimensional, narrow high-energy photon beam scanning and three-dimensional 11C Bragg peak scanning for radiation quality and intensity-modulated dose delivery. There is no doubt that the future of radiation therapy is very promising, and gradually more and more patients may not even need advanced surgery. Instead, they could be cured by biologically optimized electron, photon, or light ion therapy, where the densely ionizing Bragg peak is placed solely in the gross tumor, and a lower ionization density is used in microscopically invasive tumor volumes.