For accurate three-dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one-third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half-value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.

An algorithm, which calculates the motions of the collimator jaws required to generate a given arbitrary intensity profile, is presented. The intensity profile is assumed to be piecewise linear, i.e., to consist of segments of straight lines. The jaws move unidirectionally and continuously with variable speed during radiation delivery. During each segment, at least one of the jaws is set to move at the maximum permissible speed. The algorithm is equally applicable for multileaf collimators (MLC), where the transmission through the collimator leaves is taken into account. Examples are presented for different intensity profiles with varying degrees of complexity. Typically, the calculation takes less than 10 ms on a VAX 8550 computer.

Differential pencil beam (DPB) is defined as the dose distribution relative to the position of the first collision, per unit collision density, for a monoenergetic pencil beam of photons in an infinite homogeneous medium of unit density. We have generated DPB dose distribution tables for a number of photon energies in water using the Monte Carlo method. The three-dimensional (3D) nature of the transport of photons and electrons is automatically incorporated in DPB dose distributions. Dose is computed by evaluating 3D integrals of DPB dose. The DPB dose computation model has been applied to calculate dose distributions for 60Co and accelerator beams. Calculations for the latter are performed using energy spectra generated with the Monte Carlo program. To predict dose distributions near the beam boundaries defined by the collimation system as well as blocks, we utilize the angular distribution of incident photons. Inhomogeneities are taken into account by attenuating the primary photon fluence exponentially utilizing the average total linear attenuation coefficient of intervening tissue, by multiplying photon fluence by the linear attenuation coefficient to yield the number of collisions in the scattering volume, and by scaling the path between the scattering volume element and the computation point by an effective density.

No abstract

The optimization of intensity distributions and the delivery of intensity-modulated treatments with dynamic multi-leaf collimators (MLC) offer important improvements to three-dimensional conformal radiotherapy. In this study, a nine-beam intensity-modulated prostate plan was generated using the inverse radiotherapy technique. The resulting fluence profiles were converted into dynamic MLC leaf motions as functions of monitor units. The leaf motion pattern data were then transferred to the MLC control computer and were used to guide the motions of the leaves during irradiation. To verify that the dose distribution predicted by the optimization and planning systems was actually delivered, a homogeneous polystyrene phantom was irradiated with each of the nine intensity-modulated beams incident normally on the phantom. For each exposure, a radiographic film was placed normal to the beam in the phantom to record the deposited dose. The films were calibrated and scanned to generate 2-D isodose distributions. The dose was also calculated by convolving the incident fluence pattern with pencil beams. The measured and calculated dose distributions were compared and found to have discrepancies in excess of 5% of the central axis dose. The source of discrepancies was suspected to be the rounded edges of the leaves and the scattered radiation from the various components of the collimation system. After approximate corrections were made for these effects, the agreement between the two dose distributions was within 2%. We also studied the impact of the "tongue-and-groove" effect on dynamic MLC treatments and showed that it is possible to render this effect inconsequential by appropriately synchronizing leaf motions. This study also demonstrated that accurate and rapid delivery of realistic intensity-modulated plans is feasible using a dynamic multi-leaf collimator.