J. R. Cunningham

Lung dose corrections for 6‐ and 15‐MV x rays

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Corrections to absorbed dose calculations for tissue inhomogeneities.

Traditional methods for correcting for the presence of tissue inhomogeneities may produce errors as great as 10% at points within or closese to the inhomogeneities. Measurements were made in phantoms containing aluminum or cork inhomogeneities. Agreement between measured and predicted results was usually within 2%-3%.

Evaluation of lung dose correction methods for photon irradiations of thorax phantoms

Radiation absorbed dose in lung is measured and calculated using several algorithms available on commercial treatment planning systems. Phantoms resembling the human thorax are used and irradiated with small and large photon beams of 60Co, 4, 6, and 10 MV X ray energies. The applicability and usefulness of the different calculation methods in clinical situations is discussed.

Quality assurance in dosimetry and treatment planning

The considerations of tissue response to radiation absorbed dose suggest a need for an accuracy of +/- 5% in its delivery. This is very demanding and its regular achievement requires careful quality control. There are three distinct phases to the delivery of the planned treatment: calibration of the radiation beam in a reference situation, calculation of the dose distribution for a patient relative to the reference dose and the delivery of the radiation to the patient as planned. Each has distinctly different quality assurance requirements and must be diligently observed if the desired accuracy is to be achieved.

Calculation of the average energy absorbed in photon interactions

High energy electrons set into motion by photon interactions with matter lose some of their energy by bremsstrahlung. This loss must be evaluated before energy absorption coefficients may be calculated. Recent extensive tables of data published by Plechaty et al. contain an appreciable error in this quantity. The error results from two simplifying assumptions and for the case of very high photon energies interacting with high atomic number materials can be as much as a factor of two. This has important implications for the evaluation of quantities used in radiation dosimetry.

On the selection of stopping‐power and mass energy‐absorption coefficient ratios for high‐energy x‐ray dosimetry

A method for the selection of average stopping-power (L/rho)medair and energy-absorption coefficient (mu en/rho)medair ratios has been developed. The quality of the x-ray beam is characterized by the ratio of ionization chamber readings at depths of 20 and 10 cm in water (TMR)2010. For convenience, a relationship is established between experimental (TMR)2010 and the nominal accelerating potential (MV) of the accelerator. Experimental (TMR)2010 are related to (L/rho)medair and (mu en/rho)medair in a three-step process. First, using experimental and theoretical spectra in the range 60Co to 45 MV, (TMR)2010 were calculated for primary and first-scatter photons, and a graph of experimental versus calculated (TMR)2010 for these same spectra was constructed. Second, (L/rho)medair and (mu en/rho)medair were calculated for a large number of primary spectra [for most of which experimental (TMR)2010 were not available] and a graph constructed that related these quantities and (TMR)2010 calculated as above for this group of spectra. Third, using the graphs from the preceding steps, graphs relating the calculated (L/rho)medair and (mu en/rho)medair with experimental (TMR)2010 were constructed. Data are presented for water, polystyrene, acrylic, graphite, A-150, C-552, Bakelite, and nylon for beams with nominal accelerating potentials in the range 2-45 MV.

KEYNOTE ADDRESS: DEVELOPMENT OF COMPUTER ALGORITHMS FOR RADIATION TREATMENT PLANNING

Abstract As a result of an analysis of data relating tissue response to radiation absorbed dose the ICRU has recommended a target for accuracy of ±5 for dose delivery in radiation therapy. This is a difficult overall objective to achieve because of the many steps that make up a course of radiotherapy. The calculation of absorbed dose is only one of the steps and so to achieve an overall accuracy of better than ±5% the accuracy in dose calculation must be better yet. The physics behind the problem is sufficiently complicated so that no exact method of calculation has been found and consequently approximate solutions must be used. The development of computer algorithms for this task involves the search for better and better approximate solutions. To achieve the desired target of accuracy a fairly sophisticated calculation procedure must be used. Only when this is done can we hope to further improve our knowledge of the way in which tissues respond to radiation treatments.

The optimization of pencil beam widths for use in an electron pencil beam algorithm

Pencil beam algorithms for the calculation of electron beam dose distributions have come into widespread use. These algorithms, however, have generally exhibited difficulties in reproducing dose distributions for small field dimensions or, more specifically, for those conditions in which lateral scatter equilibrium does not exist. The work described here has determined that this difficulty can arise from the manner in which the width of the pencil beam is calculated. A unique approach for determining the pencil beam widths required to accurately reproduce small field dose distributions in a homogeneous phantom is described and compared with measurements and the results of other calculations. This method has also been extended to calculate electron beam dose distributions in heterogeneous media and the results of this work are presented. Suggestions for further improvements are discussed.

A semi-automatic cutter for compensating filters

Abstract A machine for cutting filters to composite for the irregular shape of a patients surface is described. It has been in use for almost 10 years and has become an essential part of our clinic equipment. The cutting of the compensator proceeds automatically and each compensator is individually tailored to the shape of the patient, the field size used and the depth of the target volume True compensation can take place only at a point, however compensation well within the demands of clinical use can easily be achieved.

The Application Of Computed Tomographic (CT) Imaging To Radiotherapy Planning

X-ray transmission computed tomography (CT) has been used primarily in medical diagnostic radiology since 1972. More recently, the application of CT to radiation therapy planning has been developed. In comparison to conventional radiography, CT images provide vastly improved anatomical localization of the tumour and of surrounding healthy tissue. Qualitatively, this information assists in the selection of appropriate radiation fields. Quantitatively, CT data can be processed to yield the electron densities of the tissues for calculation of the corresponding radiation distribution in the patient. In such calculations, the variations in tissue density and the three dimensional nature of structures are considered by employing a series of contiguous transverse section CT images which cover the entire patient volume to be irradiated. In this paper, the utilization of computed tomography for radiotherapy planning and for monitoring tissue response following radiotherapy will be demonstrated with a clinical example (tumour of the chest wall). The results presented were obtained with a computerized treatment planning system (TP-11, Atomic Energy of Canada) modified to interact with a commercial CT scanner (TR-40 Synerview, Picker X-ray Corporation). This system has been in clinical use for the past six months at the Princess Margaret Hospital.