Measurements of primary spectra from a kilocurie 60Co unit and a 6 MeV linear accelerator

The influence of field size on stopping-power ratios in- and out-of-field: quantitative data for the BrainLAB m3 micro-multileaf collimator

The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac‐based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically‐matched model of a Varian 600C linear accelerator with mounted BrainLAB micro‐multileaf collimator. Stopping‐power ratios were calculated for field sizes ranging from 6×6 mm2 up to the maximum (98×98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro‐multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer‐Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping‐power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro‐multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in‐field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out‐of‐field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in‐field dosimetry.

PACS number: 87.10.RT; 87.53.Ly; 87.56.jf; 87.56.jk

Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator

The purpose of this study is to provide detailed characteristics of incident photon beams for different field sizes and beam energies. This information is critical to the future development of accurate treatment planning systems. It also enhances our knowledge of radiotherapy photon beams. The EGS4 Monte Carlo code, BEAM, has been used to simulate 6 and 18 MV photon beams from a Varian Clinac-2100EX accelerator. A simulated realistic beam is stored in a phase space data file, which contains details of each particle's complete history including where it has been and where it has interacted. The phase space files are analysed to obtain energy spectra, angular distribution, fluence profile and mean energy profiles at the phantom surface for particles separated according to their charge and history. The accuracy of a simulated beam is validated by the excellent agreement between the Monte Carlo calculated and measured dose distributions. Measured depth-dose curves are obtained from depth-ionization curves by accounting for newly introduced chamber fluence corrections and the stopping-power ratios for realistic beams. The study presents calculated depth-dose components from different particles as well as calculated surface dose and contribution from different particles to surface dose across the field. It is shown that the increase of surface dose with the increase of the field size is mainly due to the increase of incident contaminant charged particles. At 6 MV, the incident charged particles contribute 7% to 21% of maximum dose at the surface when the field size increases from 10 x 10 to 40 x 40 cm2. At 18 MV, their contributions are up to 11% and 29% of maximum dose at the surface for 10 x 10 cm2 and 40 x 40 cm2 fields respectively. However, the fluence of these incident charged particles is less than 1% of incident photon fluence in all cases.

A multiplicative correction factor for tissue heterogeneities

A new multiplicative correction factor for tissue heterogeneities based on an exact expression for homogeneous non-unit-density media is proposed. O'Connor's density scaling theorem is invoked to evaluate the medium-specific tissue-maximum ratios used in the exact formulation. Monte Carlo calculations were performed at energies ranging from 60Co to 15 MV to test the technique for single- and multiple-slab geometries as well as for the clinical benchmark problem described recently by Orton et al consisting of a six-field lung plan. Within the level of statistical uncertainty (less than 2%) in the Monte Carlo calculations, the new correction factor agrees with the calculated data for single slabs of non-unit-density material. For the multiply layered geometries, the correction factor accurately represents the data beyond the build-up region at any interface, but is less reliable close to a boundary. For high energies, however, it is more accurate in the build-up region than other commonly used correction techniques such as the ratio-of-TMR or Batho methods. For all energies considered, the new correction factor agrees with the data measured by Orton et al to within 1.5%. It is more accurate than other correction factors considered by Orton et al at high energies, and is competitive with the Batho and equivalent TAR methods at low energies.

Stopping power data for high-energy photon beams

Spencer-Attix stopping power ratios for the dosimetry of high-energy photon beams used in radiation therapy have been calculated using the Monte Carlo method. The stopping power ratios are calculated in a more consistent way than previously and are given as a function of the attenuation properties of the beam. The dependence of the stopping power ratio on the electron contamination of the beam as well as on depth and field size has also been investigated. Results are compared with stopping power ratios recommended in different dosimetry protocols and to experimental results. The agreement with most dosimetry protocols is within about one per cent and with recent experimental data is better than half a per cent.

Absorbed dose from secondary electrons in high energy photon beams

The absorbed dose in high energy photon beams due to scattered electrons from the irradiated air volume and from beam-shaping platforms has been calculated using the Fermi-Eyges theory of multiple scattering. The results are presented as lateral surface absorbed dose distributions across the field for three different radiation qualities, namely 60Co, 6 MV and 21 MV X-rays. For 60Co the relative absorbed dose due to electrons expelled in air reaches a value as high as 30% of the absorbed dose at dose maximum at a field size 40 X 40 cm2 and an SSD of 100 cm. The absorbed dose from electrons emanating from beam-shaping platforms contribute significantly to the absorbed dose at the surface when the platform is placed closer than 20--40 cm from the surface for field sizes greater than 10 X 10 cm2 to 40 X 40 cm2 respectively.

Measurements of single event spectra with a wall-less proportional counter in low LET radiation fields

Measurements have been made on 60Co gamma rays from a point source and on 6 MV roentgen rays from a linear accelerator with a wall-less proportional counter. Single event spectra have been measured in sites of 0.5-2 mum in diameters stimulated with a tissue equivalent gas. Calculations of the relative variance in LET are performed and compared with the relative variance of the measured distributions for estimates about the straggling contribution.

Secondary radiation from a spherical tissue-equivalent phantom irradiated with 60Co gamma radiation and 6 MV X-rays

The angular dependence and the dependence on the phantom mass of the secondary exposure rate from spherical tissue-equivalent phantoms irradiated with 60Co gamma radiation and 6 MV X-rays have been investigated both by measurement and by calculation using the Monte Carlo method and approximate methods. A nonlinear dependence on the phantom mass was found. Calculation of the differential photon fluence was also made. These measurements and calculations can be used when calculating the required barrier thickness for walls which are not hit by primary radiation.

Spectral measurements and Monte Carlo calculations of scattered radiation from therapeutic radiation sources

Spectral measurements have been made on scattered radiation in a perspex phantom in various depths from a kilocurie 60Co unit and a 6 MeV linear accelerator using a NaI crystal spectrometer. The measurements are compared to Monte Carlo calculations and a good agreement is obtained. The measured total photon distributions make the basis for calculations of absorbed dose and track length distributions in LET and their average values for the different depths and fields.

Irradiation of small structures through the intact skull

The physical conditions for producing minute disc-shaped lesions for functional radiation surgery by means of narrow beam irradiation are discussed. The intention was a treatment procedure for routine clinical use and with high physical and mechanical reproducibility. The possibilities of using high energy electrons, supervoltage roentgen radiation, gamma emitting nuclides or fast neutrons in a technique for routine clinical use were investigated. The radiation of choice taking physical properties, radiation biologic factors and practical circumstances into account was considered to be 60-Co gamma radiation. A treatment apparatus containing 179 60-Co sources within a spherical sector of 70 degrees latitude and 160 degrees longitude was constructed.