Central-axis Data Research Articles

SU‐F‐T‐473: Evaluation of Off‐Axis And Peripheral Dose Using Different Detectors

Purpose:In radiation therapy, measurement of off‐axis and peripheral dose is a tedious task. The dose distribution along the beam central axis give only part of the information required for an accurate dose description inside the patient. Dose distributions in 2‐D and 3‐D are determined with central axis data in conjunction with off‐axis dose profiles. Combining a central axis dose distribution with off‐axis data results in volume dose matrix that provides 2‐D and 3‐D information on dose distribution. By considering the importance of these two parameters this study investigates and compares the off axis and peripheral dose measurement using Ionization chamber, MOSFET, Radiochromic film and EDR2 film.Methods:In the measurement of off‐axis and peripheral doses 0.6cc farmer type chamber, EDR‐2 film, MD‐55 radiochromic film and MOSFET detectors were used for both 6MV and 15MV beams. The off‐axis and peripheral dose was measured at every 1 cm interval by changing the position of the couch without disturbing the other experimental setup in particular, the phantom with detector position.Results:Readings were obtained for both 6MV and 15MV photon beams at SSD technique for various field size using MOSFET, Ionization chamber and Radiochromic film. The results shows the percentage difference between various detectors for various field sizes. For Peripheral dose measurement were taken from the edge of the field size and for off axis it is measured form central axis dose.Conclusion:In conclusion, our study shows that no detector is ideal and only a comparison between different detectors highlights the weaknesses of each detector. MOSFET provides adequate dose assessment in off‐axis and peripheral regions in 6MV and 15MV photon beams. Film dosimetry in general a convenient method to generate one‐ and two‐dimensional dose distributions. The result of this indicates that MOSFET is flexible tool for relative dosimetry as films.

Calculation of the off-axis in-phantom scatter functions

Semi-empirical algorithms used for dose calculation in high energy photon beams are usually based in central axis data. The applicability of them to points off axis has been assessed only for old calculation techniques, such those using tissue air ratio functions. Recently, a new formulation based on phantom scatter quantities i.e. the [D/Ψ]rel formalism, has been introduced for use in central axis and homogeneous media. The extension of this calculation methodology for points off axis has to deal mainly with spectral changes that must to be accounted for. In this work, a new multiplicative function P(d,r) is introduced to account for variations in the phantom scatter quantities, which depends on the phantom depth (d) and on the off-axis position (r). This function was calculated from measurements of dose profiles (off-axis profiles in water) and in-air profiles corresponding to photon beams of 6 and 15 MV. Additionally, the function [D/Ψ]rel was determined experimentally and compared with the theoretical prediction, with an agreement better than 2% in any case. This difference could be attributed to experimental uncertainties and is within the clinical tolerances for dose calculation.

SU-FF-T-323: On the Adequacy of Shielding for 15-22 MeV Electron Beams

Purpose: It is customary to define block transmission as a fraction of open field dose at the depth of maximum dose (dmax) for an electron beam. The purpose of this study is to investigate the depth at which transmission should be measured and the adequacy of shielding for 15–22MeV electron beams to reduce the transmission to the regulatory level. Method and Materials: Central axis data were collected in open and blocked beams in water phantom. Measurements were taken for a set of cones from 6×10 to 25×25cm2, 100 cm SSD and 5 cm gap between the cone and water surface. Transmission factors as a function of depth and cone size were calculated. For the 22MeV electron beam, the effect of adding thicknesses of different attenuators (cerrobend, lead, bolus) was investigated. The data were verified independently in three institutions with similar accelerator model and electron energies. Monte Carlo simulations with PENELOPE code were performed to compare with measurements. Results: Photon contamination of 15–22MeV electron beams in the open beam was in agreement with the manufacturer's specification. Maximum transmission for the blocked beam for all energies and cones occurred between 4–7 mm depth. The 15MeV electron beam was well attenuated for the standard cerrobend thickness. For 18 and 22MeV beams, maximum transmission varied from 5.9% to 7.5% and from 9.5% to 11% for the 6×10 to 25×25cm2 cones, respectively. For the 15×15cm2 cone, 30 mm thick cerrobend attenuated 22MeV beam to 5%. For 21MeV beam, Monte Carlo simulations showed maximum transmission from 10.6% to 11% for 10×10cm2 and 25×25cm2 cones, respectively. Conclusion: The dmax dose under a blocked beam occurs closer to the surface than for the open beam. In order to decrease transmission to the regulatory level of 5%, 18–22MeV electron beams require additional shielding to supplement the standard cerrobend block.

Analysis of the photon beam treatment planning data for a scanning beam machine

The characteristics of photon beams from the Scanditronix MM50 radiation therapy machine that are necessary for treatment planning are described. The MM50 uses a scanning beam instead of a conventional flattening filter to achieve flat dose distributions. At each beam energy, a scan pattern is chosen, depending on the field size; the small scan pattern (S) is used for field sizes up to 10 x 10 cm, the medium scan pattern (M) is used for field sizes up to 20 x 20 cm, and the large scan pattern (L) is used for the larger field sizes. The dose distributions of the beams associated with the 10 MV S, M, and L scan patterns, the 25 MV S, M, and L patterns, and the 50 MV S and M patterns are described. The data reported includes central axis data, beam profiles, and output factors. In addition to the measured data, our dose calculation model requires a pencil beam kernel for each beam. The kernel is constructed using the average photon energy spectrum, which is generated using a Monte Carlo simulation of the MM50. The simulation, based on EGS4, is also used to generate the radial variation of fluence and energy fluence, which is required by a new dose calculation model that does not require the measurement of beam profiles. The Monte Carlo generated data; the photon energy spectrum, the fluence, and the energy fluence are presented.

Use of Spreadsheets to Manipulate Field Scanning Data

Computerized field scanning systems are now standard tools in most radiotherapy departments. These systems can easily measure central axis depth dose curves and beam profiles. Data acquired from these systems forms the basis of what is used for machine calibrations and patient dosimetry. Unfortunately, the data is often not in the best format for these applications. Each data point is often acquired in a fraction of a second with a small volume ionization chamber resulting in “noisy” data. Many dosimetric applications require tissue maximum ratio data which is technically difficult to directly measure with these scanning systems. And finally, it is desirable to have the data clearly organized in tables according to the specific needs of the department. Commercial spreadsheets can be used to solve these problems. We have used Lotus 1-2-3@ to smooth the central axis data, calculate additional data points, calculate tissue maximum ratios, and clearly present this data in neatly organized tables.

Using liquid dielectrics to obtain spatial thermal distributions.

A liquid dielectric has been used to determine thermal distributions for several clinically useful waveguide applicators as used in microwave hyperthermia. The system uses a saline/ethanol filled compartment placed within a temperature controlled water bath set to 37.5 degrees C. Thermal distributions are taken with a thermistor probe placed perpendicular to the electric field. In this way longitudinal and transverse cross sections of the heat deposition due to microwave interaction with the liquid dielectric can be elicited. A similar arrangement was carried out using phantom material and an infrared camera as described by Guy. Comparison of the central axis data and cross-sectional thermal distribution of these two systems shows good agreement.

超小型電算機を使用した線量分布の計算

Several pannel diecussions concerned with the use of the electric computer in radiotherapy have been recently hold, and their reports have been published. The use of the time-sharing system with the big machine would be reasonable for the rapid calculation of the dose distribution. If the accuracy of the results is clinically acceptable, the mini-computer will be able to applied in routine daily work instead of the time-consuming hand-calculation. In this paper. the outline of the application of the mini-computer for calculating the dose distribution in external. irradiation was described. The system is so simple to use, and input errors can be easily found out and can be quickly corrected by the key board. Central axis data obtained directly from the dose measurements were stored as a function of the ratio between the field area and the perimeter of the field, which was presented by Sterling et al. The shift method was applied for the correction of the dose inhomogeneity factor in lung tissue. The decrement value and the correction factor for the wedge filters were also stored. The difference between the calculated value and the value obtained by the actual measurement at the 10×10cm field with 6 MV X-ray, were 1% at the depth of 10cm, and 3% at the depth of 25cm. Access time were about 2 hours per patient at the most, therefore, 4-6 cases can be carried out in a day.

A general purpose method for computer generation of isodose contours from external X-ray sources with applications to treatment planning

A method has been developed for the generation of isodose contours from external 60Co sources which requires as input only sources to skin distance (SSD), field size (FS), trimmer setting, apparent source width and a field size dependent attenuation coefficient M. Fields are calculated directly, at any grid size and normalization desired, allowing continuous variation of beam-defining parameters. The penumbra is successfully approximated, and central axis data are within 3% of data tabulated at different SSD and FS. The same accuracy applies in the calculation of central axis data for X-rays in the energy range 2-31 MeV; thus, quite general applicability is indicated for many types of photon beams. Running time is about 5 s per symmetric 10*10 field on moderate computers. In this paper, a brief description of the method was presented, emphasis being placed subsequently on its application to treatment planning situations involving (1) split fields, (2) wedged fields, (3) oblique entry and (4) multiple ports, by making the coefficient M a function of position for a given FS. Finally, comparisons were made with hand-calculated results for multiple port, wedged field therapy in certain head and neck cases.

The 4 MeV linear accelerator at Newcastle upon Tyne.

An account, based on four years operation, is given of the first of the 4 MeV linear accelerators (Mullard) installed for medical use. Its “isocentric” mounting in conjunction with the specially designed single-pillar couch makes for rapid and accurate setting up of patients and it may be used for fixed or moving field therapy. The X-ray output is up to 250 r/minute at 100 cm F.S.D. and 400 p.p.s.; the H.V.L. 15·9 mm Cu. A sharp penumbra is obtained through the small focal spot and the use of beam defining diaphragms pivoted at target level. Central axis dosage measurements give a build-up peak at 1 cm depth in water and the dose at 10 cm depth for a 10 × 10 cm field is 62·8 per cent. The surface dose for this same size field is 17 per cent, largely due to secondary electrons from the air. Isodose curves were obtained by direct measurement and also by calculation from the central axis data and some typical curves for oblique fields, fields applied to curved surfaces and wedge fields are also given. The lo...

A method of calculating isodose curves from central axis depth dose data.

The method described permits calculation of isodose curves from any self consistent depth dose data such as that presented in the recent Survey of Central Axis Depth Dose Data (Suppl. 5 to this Journal), and applies to fields of any shape, both at medium energies and in the megavoltage region. It is simple and speedy in use and the basic charts prepared from central axis data may be rapidly replaced if the data changes. A new method of extending scatter: radius curves to large radii is described, which avoids the use of a formula and consequent “fitting”.