Intermediate dosimetric quantities.

The transfer of energy from ionizing radiation to matter involves a series of steps. In wide ranges of their energy spectra photons and neutrons transfer energy to an irradiated medium almost exclusively by the production of charged particles which ionize and thereby produce electrons that can ionize in turn. The examination of these processes leads to a series of intermediate quantities. One of these is kerma, which has long been employed as a measure of the energy imparted in the first of the interactions. It depends only on the fluence of uncharged particles and is therefore--unlike absorbed dose and electron fluence--insensitive to local differences of receptor geometry and composition. An analogous quantity for charged-particle fields, cema (converted energy per unit mass), is defined, which quantifies the energy imparted in terms of the interactions of charged particles, disregarding energy dissipation by secondary electrons. Cema can be expressed as an integral over the fluence of ions times their stopping power. However, complications arise when the charged particles are electrons, and when their fluence cannot be separated from that of the secondaries. The resulting difficulty can be circumvented by the definition of reduced cema. This quantity corresponds largely to the concept employed in the cavity theory of Spencer and Attix. In reduced cema not all secondary electrons but all electrons below a chosen cutoff energy, delta, are considered to be absorbed locally. When the cutoff energy is reduced, cema approaches absorbed dose and thereby becomes sensitive to highly local differences in geometry or composition. With larger values of delta, reduced cema is a useful parameter to specify the dose-generating potential of a charged-particle field 'free in air' or in vacuo. It is nearly equal to the mean absorbed dose in a sphere with radius equal to the range of electrons of energy delta. Reduced cema is a function of the fluence at the specified location at and above the chosen cutoff energy. Its definition requires a modification of restricted linear collision stopping power, L delta, and it is recommended that the definition of L delta be so changed.     

On the revised concept of linear energy transfer

The quantities linear energy transfer or restricted linear energy transfer are utilized in calculations that link absorbed dose to the fluence distribution of a radiation field. The computations provide approximations to absorbed dose in terms of the intermediate quantity cema or reduced cema. With the definition of the restricted linear energy transfer,L , given in ICRU Report 33, the approximation remains imperfect. This study deals with the resulting need for a modified definition ofL , as proposed in a draft report of ICRU. Essential differences between the old and the new definitions are demonstrated. The changed definition permits a rigorous formulation of the dependence between fluence and absorbed dose.     

光动力治疗中光的有效吸收光剂量及其确定

光动力治疗中真正有效的光剂量是达到病变组织并且被组织中的光敏剂所吸收的那部分剂量,即有效吸收光剂量。明确组织中的有效吸收光剂量可以指导临床治疗,从而避免治疗剂量不足(治疗不彻底)或剂量过量(造成正常组织的热损伤)。而确定PDT中光的有效吸收剂量时,需测定组织中考察点的光辐射能流率。在目前计算和模拟组织中光辐射能流率的方法中,都需要首先确定所研究组织的光学特性参数。本文概述了常用的测定组织中光辐射能流率及组织光学特性参数的测量方法。     

Aspects of fast-ion dosimetry

A first step in the dosimetry of fast-ion beams is the determination of accurate Bragg (ionization) functions. Bragg functions for several substances have been measured and calculated for 3480 MeV carbon ions. In the measurements, the ions first traverse an absorber in which the energy is reduced to either 1900 or 1200 MeV, then a "range gauge" followed by a thin ionization chamber. Functions are calculated with an analytical method using convolutions of straggling functions. This approach gives results without the stochastic variations implicit in Monte Carlo methods. The comparison of measured and calculated functions shows how reliable the calculations are. An important part of the calculations is the determination of the total range of the ions. The range can be determined from the Bragg function. The measured range is given by the sum of the thickness of the absorber and the residual range measured with the range gauge. For water, the range is about 150 mm, and the precision of the measurements is +/-0.05 mm. Because the ion energy at the surface of the absorber fluctuates with time, measurements with water are used to define this energy. Thus the ranges (or average stopping powers) in absorbers are obtained relative to those in water. Measured ranges R(m) are compared with ranges R(0) calculated with a current version of the Bethe theory. For light absorbers (atomic number Z < 20), differences between R(m) and R(0) are less than +/-0.3 mm; for Z > 20 differences are between 0 and +/-0.6 mm. This agreement between calculated and measured ranges confirms the value I = 80 eV for water measured earlier for protons. The ionization by nuclear fragments is obtained from the difference between measured and calculated ionization functions, and has little influence on the ranges of the primary ions.     

FLUKA simulation of target fragmentation in proton therapy

In proton therapy, secondary fragments are created in nuclear interactions of the beam with the target nuclei. The secondary fragments have low kinetic energies and high atomic numbers as compared to primary protons. Fragments have a high LET and deposit all their energy close to the generation point. For their characteristics, secondary fragments can alter the dose distribution and lead to an increase of RBE for the same delivered physical dose. Moreover, the radiobiological impact of target fragmentation is significant mostly in the region before the Bragg peak, where generally healthy tissues are present, and immediately after Bragg peak. Considering the high biological impact of those particles, especially in the case of healthy tissues or organs at risk, the inclusion of target fragmentation processes in the dose calculation of a treatment planning system can be relevant to improve the treatment accuracy and for this reason it is one of the major tasks of the MoVe IT project.In this study, Monte Carlo simulations were employed to fully characterize the mixed radiation field generated by target fragmentation in proton therapy. The dose averaged LET has been evaluated in case of a Spread Out Bragg Peak (SOBP). Starting from LET distribution, RBE has been evaluated with two different phenomenological models. In order to characterize the mixed radiation field, the production cross section has been evaluated by means of the FLUKA code. The future development of present work is to generate a MC database of fragments fluence to be included in TPS.     

Effect of anode angle on photon beam spectra and depth dose characteristics for X-RAD320 orthovoltage unit

Background

In radiation therapy with orthovoltage units, the tube design has a crucial effect on its dosimetric features.

Aim

In this study, the effect of anode angle on photon beam spectra, depth dose and photon fluence per initial electron was studied for a commercial orthovoltage unit of X-RAD320 biological irradiator.

Materials and methods

The MCNPX MC code was used for modeling in the current study. We used the Monte Carlo method to model the X-RAD320 X-ray unit based on the manufacturer provided information. The MC model was validated by comparing the MC calculated photon beam spectra with the results of SpekCalc software. The photon beam spectra were calculated for anode angles from 15 to 35 degrees. We also calculated the percentage depth doses for some angles to verify the impact of anode angle on depth dose. Additionally, the heel effect and its relation with anode angle were studied for X-RAD320 irradiator.

Results

Our results showed that the photon beam spectra and their mean energy are changed significantly with anode angle and the optimum anode angle of 30 degrees was selected based on less heel effect and appropriate depth dose and photon fluence per initial electron.

Conclusion

It can be concluded that the anode angle of 30 degrees for X-RAD320 unit used by manufacturer has been selected properly considering the heel effect and dosimetric properties.     

On pathlength and energy straggling of megavoltage electrons slowing down

Purposeto elucidate the effects of multiple scattering and energy-loss straggling on electron beams slowing down in materials.MethodsEGSnrc Monte Carlo simulations are done using a purpose-written user-code.ResultsPlots are presented of the primary electron’s energy as a function of pathlength for 20 MeV electrons incident on water and tantalum as are plots of the overall distribution of pathlengths as the 20 MeV electrons slow down under various Monte Carlo scenarios in water and tantalum. The distributions range from 1 % to 135 % of the CSDA range in water and from 1 % to 186 % in tantalum. The effects of energy-loss straggling on energy spectra at depth and electron fluence at depth are also presented.ConclusionsThe role of energy-loss straggling and multiple scattering are shown to play a significant role in the range straggling which determines the dose fall-off region in electron beam dose vs depth curves and a significant role in the energy distributions as a function of depth.     

Potential pitfalls of the PTV concept in dose-to-medium planning optimization

In typical treatment planning of 3D IMRT, the incident energy fluence is optimized to achieve a homogeneous dose distribution to the PTV. The PTV includes the tumour but also healthy tissues that may have a different dose response for the same incident energy fluence, like bony structures included in the PTV (mandibles in head and neck tumours or femoral bones in sarcomas). Dose to medium optimization compensates for this heterogeneous response, leading to a non-homogeneous energy fluence in the PTV and a non-homogeneous dose in the CTV in the presence of geometric errors. We illustrate qualitatively this statement in a cylindrical geometry where the PTV includes a CTV (7 cm diameter) made of water surrounded by ICRU compact bone (1.2 cm thickness); such configuration was chosen to exaggerate the aforementioned effect. Optimization was performed assuming dose equals photon energy fluence times mass energy absorption coefficient. Bone has a 4% lower dose response in a 6 MV flattening filter free spectrum. After optimization either in medium or assuming everything as water composition, the geometry was shifted by 1.2 cm and dose recomputed. As expected, compensating for the under-response of the bone material during optimization in medium leads to an overdosage of the CTV when patient geometric errors are taken into account. Optimization in dose assuming everything as water composition leads to a uniform coverage. Robust optimization or forcing a uniform atomic composition in the PTV margin may resolve this incompatibility between the PTV concept and dose to medium optimization.     

Analytical model of ion transport and conversion of light energy in chloroplasts

An analytical model, which describes the stationary transformation of light energy to the energy of pigment electronic excitation, has been constructed. A proton pump of the thylakoid membrane has been considered as a two-level conformon. The difference between the energies of the excited and ground states of both the pigment and the protein complex is assumed to be the energy of an absorbed photon. It has been found how the concentration of ions in a lumen and the potential across the thylakoid membrane depend on the concentration of ions in the stroma and the brightness temperature of absorbed radiation. Conditions for the maximum efficiency of the photosynthesis process have been analyzed. This model has been used to determine the electric potential (φ≈6.7 mV) at the chloroplast thylakoid membrane. The calculated value of the electric potential is in good agreement with the experimental data. A limitation on the stoichiometric coefficient of the proton transport through ATP-synthase, m>3, has been found theoretically.     

Examination of fragment dose contribution in heavy ion radiotherapy

Heavy-ion radiotherapy is an efficient method for the treatment of deep-seated tumors, because the stopping of ions in a tissue delivers the maximal absorbed dose to the tumor-affected areas with minimal damage to the healthy tissues. However, heavy ions can undergo nuclear reactions, giving products with lower Z-values and hence a longer range in the tissue. This causes a dose increase beyond the mean range of the primary beam. The contribution of such reaction products was examined in an experiment where a stack of tissue-like targets interleaved with CR-39 etched track detectors (ETD) was irradiated with heavy ions. The analysis was performed using a recently developed technique of trajectory tracing, which enables the spectroscopy of fragments with different Z-values.     

Nuclear fragmentation of clinical silicon beam

Heavy ions used for biomedical studies fragment when they pass through matter. It is known that different fragments cause different types of biological damage. In this paper, results pertaining to different kinds of secondary fragments produced by a 28Si beam at 463 MeV/nucleon in its interaction with nuclear emulsion are presented. It is observed that the production frequencies of secondary fragments are independent of the target. Partial production cross sections and fluences of the secondary fragments are also discussed. The partial production cross sections for different fragments at three lower energy ranges are practically the same within their statistical errors except for the cross section for Li fragments, which is larger by almost a factor of 1.5 at the lowest energy investigated. The dose contributions of the primary Si beam and its components at different points of the Bragg curve are presented for two different energy regions.     

Neutron scattering and energy deposition spectra

Summary The influence of neutron scattering in the wall of a spherical proportional counter on the energy deposition spectra and the absorbed dose is investigated. Event probabilities, and frequency and dose-averaged deposited energies are calculated with and without scattering contribution and compared. The change of absorbed dose due to attenuation of the primary neutron flux is also evaluated.Contribution No. 1157 of the Biology Programme, Directorate General XII of the Commission of the European Communities.     

Propagation of focused and multibeam laser energy in biological tissue

The results of a Monte Carlo simulation of laser beam propagation in turbid media are presented. The study was performed to determine whether using a focused beam or multiple beams instead of a single collimated beam could improve subsurface laser energy delivery in biological tissue. A parametric study was carried out to determine both the laser fluence at a target depth and the ratio of fluence at the target over surface fluence as a function of tissue properties and the mode of energy delivery. It was found that the reduced scattering coefficient was the primary determinant as to whether multibeam or focused beam delivery could be effective. A focused beam was found to be extremely effective in increasing fluence at the target if the dimensionless reduced scattering coefficient was less than 2. The delivered fluence, however, was found to be extremely sensitive to tissue properties. A five-beam laser system was found to be less effective at increasing fluence at the target than a focused beam; but the fluence delivered by a five-beam system was far less sensitive to tissue properties, thereby making accurate dosimetry more feasible.     

Effect of light fluence rate on mammalian cells photosensitization by chloroaluminium phthalocyanine tetrasulphonate

Phthalocyanines are photosensitizers evaluated for use in photodynamic therapy of cancer. As such, the dependence of the bioresponse on the light fluence rate may be of clinical importance. The effect of the fluence rate of white light from 0.165 to 3.3 kJ m-2 min-1 was studied in Chinese hamster cells and human lymphocytes, using as endpoints colony-forming ability and inhibition of [3H]thymidine incorporation following mitogenic stimulation and dye-photoactivation, respectively. Using Chinese hamster cells exposed to photoexcited chloroaluminium phthalocyanine tetrasulphonate in PBS solution, cytotoxicity was diminished as the fluence rate was reduced. In human lymphocytes changing the fluence rate between 0.33 and 3.3 kJ m-2 min-1 affected the response in a way similar to that of Chinese hamster cells. Human lymphocytes, when exposed to incremental increasing light fluences, 4 h after a conditioning dose, were able to recover from phthalocyanine-induced photodamage, as evidenced by the reappearance of a shoulder on the dose-effect curve. This recovery process during a protracted light exposure, could explain the reduced sensitivity to phthalocyanine photosensitization, compared to exposure at high fluence rates.     

Measurements of neutron energy using a recoil-proton telescope and a high-pressure ionization chamber

Two very different techniques for measuring the energy of neutrons in the energy range 0.1-10 MeV are presented and compared. A recoil-proton spectrometer is used to determine the energy spectra of neutrons produced by the d(4)-Be and p(4)-Be reactions down to the low-energy threshold of 0.7 MeV. The same radiation fields are also measured with a recently developed method using a high-pressure ionization chamber that can be used to determine the mean energy of the neutrons in a mixed neutron-gamma radiation field provided the gamma-ray absorbed dose fraction is determined independently. An intercomparison of the two methods shows that the high-pressure ionization chamber compares well and supplements the established recoil-proton spectrometer technique. The almost isotropic response of the chamber has enabled measurements to be made of the variation of mean neutron energy with depth in water for the two radiation fields.     

The effect of light fluence rate in photodynamic therapy of normal rat brain.

This paper reports the effect of incident light fluence rate on the depth to which necrotic lesions are produced by photodynamic therapy (PDT) in the brains of normal Fisher rats. The rats were injected intraperitoneally with Photofrin (12.5 mg kg-1) 48 h prior to PDT with a fixed incident fluence of 35 J cm-2. The treatment was performed at 10, 50, 100, and 200 mW cm-2 and also in a periodic manner (30 s "on" at 100 mW cm-2, 30 s "off"). The depth to which necrosis occurred was determined 24 h after treatment by microscopic examination of tissue sections. No differences were found in the depth to which necrosis was produced by any of the five irradiation schedules. This finding is discussed in the context of other published dose-rate experiments.     

Light quality growth promotion of Spiraea nipponica: the influence of a low photon fluence rate and transfer time to a higher fluence rate

Combinations of different light quality and fluence exposure times were investigated for their effects on in vitro growth of the woody plant Spiraea nipponica. An interaction was demonstrated between different levels of benzyladenine (BA) used for in vitro propagation and the specific light regimes investigated. This relationship was affected by the length of exposure to either white or red/FR light and the time of transfer from one fluence rate to another. In all instances exposure to red/FR light resulted in more extensive growth than under white light. Thus explants cultured under 0.25 and 0.4 mg l^-1 of BA exhibited high shoot proliferation rates when transferred, after 4 weeks of low photon fluence red/FR light, to higher fluence white light for a further week. The proliferation rates obtained were higher than any white light treatment including that with the highest BA level of 0.5 mg l^-1. In addition, the combination of red/FR light exposure with a white light stage of higher fluence improved proliferation at lower exogenous BA levels.     

Wave-guide spectroscopy on planar lipid bilayers doped with hydrophobic ions

Wave-guide spectroscopy exploits the light pipe properties of planar lipid bilayers by propagating a light wave along the plane of the bilayer. Applying this technique to the optical absorption of chromophore in the membrane, results in an enhanced sensitivity when compared to normal incidence spectroscopy. This gain factor is of the order of 100 per mm optical path along the bilayer, thus transforming the weak absorbances in lipid membranes into easily measurable quantities. Wave-guide spectroscopy has been used to measure the adsorption isotherm of hydrophobic dipicrylamine ions in a phosphatidylcholine membrane. The adsorption isotherm is linear for low aqueous concentrations, in the micromolar range however, it changes into a sublinear dependence. The addition of an inert alkali salt to the electrolyte favours the adsorption of hydrophobic ions. Current saturation is observed with the transition to the sublinear isotherm. When using the time constant for current relaxation as an indicator of changes in the magnitude of the surface potential, it does not seem to vary with the additional dipicrylamine which adsorbs in the presence of high concentrations of alkali salt in the electrolyte. A compensation of hydrophobic charge by the alkali ions from the inert electrolyte is proposed.     

A novel method for assessment of fragmentation and beam-material interactions in helium ion radiotherapy with a miniaturized setup

Radiotherapy with protons and carbon ions enables to deliver dose distributions of high conformation to the target. Treatment with helium ions has been suggested due to their physical and biological advantages. A reliable benchmarking of the employed physics models with experimental data is required for treatment planning. However, experimental data for helium interactions is limited, in part due to the complexity and large size of conventional experimental setups.We present a novel method for the investigation of helium interactions with matter using miniaturized instrumentation based on highly integrated pixel detectors. The versatile setup consisted of a monitoring detector in front of the PMMA phantom of varying thickness and a detector stack for investigation of outgoing particles. The ion type downstream from the phantom was determined by high-resolution pattern recognition analysis of the single particle signals in the pixelated detectors. The fractions of helium and hydrogen ions behind the used targets were determined. As expected for the stable helium nucleus, only a minor decrease of the primary ion fluence along the target depth was found. E.g. the detected fraction of hydrogen ions on axis of a 220 MeV/u ⁴He beam was below 6% behind 24.5 cm of PMMA. Monte-Carlo simulations using Geant4 reproduce the experimental data on helium attenuation and yield of helium fragments qualitatively, but significant deviations were found for some combinations of target thickness and beam energy.The presented method is promising to contribute to the reduction of the uncertainty of treatment planning for helium ion radiotherapy.