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Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting ‘‘dose spread arrays’’ were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three‐dimensional dose distributions. The method gives good agreement for the 15‐MV x‐ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low‐density heterogeneities.

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.

The EGS Monte Carlo code was used to generate photon energy deposition kernels which describe the energy deposited by charged particles set in motion by primary, first scattered, second scattered, multiple scattered and bremsstrahlung plus annihilation photons. These were calculated for a water medium irradiated with monoenergetic photons with energies in the range 0.1-50 MeV. In addition to the primary energy deposition kernels, primary charged particle transport was further characterised by computing the effective centre of the voxels, and the effective penetration depth, effective radius and effective lateral distance travelled by these particles. The dose per unit collision kerma for parallel monoenergetic primary photons beta ' was calculated. Additional applications of the energy deposition kernels are discussed.

The knowledge of accumulated dose in a specified tissue on a functional subunit's basis is of crucial importance for the application of biological models to estimate control and complication probabilities. It is known that geometrical uncertainties of the patient's anatomy lead to differences between the planned and delivered dose distributions. In this work, the absorbed dose in a moving and deforming object is calculated by means of a direct Monte Carlo calculation. The Monte Carlo code EGS4/BEAM was modified to incorporate temporal dynamics of the simulation geometry. Lateral one-dimensional dose distributions were studied in a moving and deforming water slab adjacent to an air interface. A linear motion and a simple deformation of the water volume were investigated. The position of the boundary of the water volume was changing as a function of particle history. The Monte Carlo code is able to directly calculate the dose in the local coordinates of the moving object. The results show that a convolution algorithm to determine the resulting dose distribution is not sufficient for highly inhomogeneous situations and if internal deformations are present.

We present a new multipurpose phantom for routine dosimetric and image quality assurance (QA) testing of a tomotherapy unit. The phantom is designed for verification of delivered dose from the intensity modulated radiation therapy (IMRT) fan beams used to provide conformal dose distributions. The phantom is used to assess imaging characteristics and verify CT number calibration for the integrated megavoltage computed tomography (MVCT) detector mounted on the gantry across from the therapy linac. Functional requirements are outlined for the unique dosimetric and imaging needs of the QA tests of the tomotherapy system. The 36 cm long phantom is constructed mostly of Solid Water/sup TM/ and consists of 6 adjacent discs. The phantom consists of head and torso modules. The phantom is divided into imaging and dosimetric regions. For fan beam IMRT dosimetry, both a standard Farmer-style ionization chamber and a CT pencil chamber are used to measure the dose and dose rate under a variety of delivery conditions and phantom heterogeneities. The feasibility of using an existing kV CT chamber at therapy energies was investigated. The response to irradiation by small individual fields (7.6 mm) along its length is within one standard deviation of the average for the central 80% of its active volume (10 cm). For imaging QA, this phantom contains cavities to accommodate standard inserts to test spatial resolution, low contrast detectability, noise and edge contrast. Addition inserts are required for helical pitch verification and detector centering.

A consequence of the advent of intensity modulated radiation therapy (IMRT) is the presence of large dose gradients surrounding the tumor, and the regions at risk (RAR's) near the tumor. Unlike standard radiotherapy, these gradients are not the chance result of a few beam choices, but rather they are strategically placed by the optimization software. A first issue is the magnitude of these gradients. Optimal dose plans were simulated for four clinical cases using experimental helical tomotherapy optimization software. It was found that high gradients could be divided into two categories: avoidance gradients and incidental gradients. Avoidance gradients are associated with tumor/RAR boundaries and tend to be larger, on the order of 30% per cm or higher. Incidental gradients are a secondary consequence of the optimizer doing its job: placing a large amount of dose confined to the target volume. Incidental gradients tend to be on the order of 15%-20% per cm.

Helical tomotherapy is an innovative means of delivering intensity-modulated radiation therapy (IMRT) using a device that merges features of a linear accelerator and a helical computed tomography (CT) scanner. The tomotherapy unit can generate CT images from the megavoltage radiation it uses for treatment as often as needed during a course of radiation therapy. These megavoltage CT (MVCT) images offer verification of patient position prior to and potentially during radiation therapy, and provide considerably more anatomical detail than the conventional radiation therapy port films used for patient set-up verification. Also, MVCT imaging may enable reconstruction of the radiation dose delivered, thereby providing unprecedented verification of the actual treatment. These key features of helical tomotherapy distinguish it from other IMRT approaches. We report results from a pilot feasibility trial of 10 patients with non-small-cell lung cancer (NSCLC) on whom we obtained MVCT images using a prototype helical tomotherapy system. All patients underwent conventional CT imaging for radiation therapy treatment planning. Specific aims were to subjectively compare MVCT and conventional CT images and then to objectively compare the 2 modalities by contouring tumors and performing a volumetric comparison. Seven patients had disease located primarily in the lung parenchyma, 2 primarily in the mediastinum, and 1 in both. When evaluated by location, all 7 patients with lesions primarily in the lung parenchyma had subjectively high-quality MVCT images. Objectively, the volumetric agreement between conventional and MVCT for parenchymal lesions was excellent in 5 of the 7 patients. Megavoltage CT imaging via the helical tomotherapy prototype provided adequate information for use in verification of patient position and dose reconstruction for lesions within the pulmonary parenchyma, but presently appears suboptimal for primarily mediastinal disease. Further studies are ongoing to optimize MVCT imaging and better define its utility in patients with NSCLC.