Proton beam penumbra: effects of separation between patient and beam modifying devices.

The sharp lateral penumbra of a proton beam is often used to spare sensitive normal structures in treating clinical sites in which the target volume abuts, or even wraps around, these structures. Using Monte Carlo calculations and measurements, the factors which influence the penumbra of the proton beam at the Harvard Cyclotron Laboratory were investigated, with particular emphasis on the effects of separation between the patient and any beam modifying devices. Penumbra broadening, characterized by the distance over which the dose rises from 20% to 80% of the central dose, increases with greater amounts of scatterer introduced into the beam line. The broadening due to separation of the beam modifying devices and the patient is essentially linear with increasing air gap; the rate of increase depends on the details of these devices and on the depth of interest in the patient. For a particular portal, most of the parameters which affect the penumbra width are fixed by the patient's anatomy and the target volume. Only the thickness of the compensating bolus around the aperture edge and any air gap between the patient and the beam modifying devices can vary. Families of curves relating combinations of bolus thickness and air gap that maintain a constant penumbra width have been developed for guidelines during patient setup.

Sources of electron contamination for the Clinac-35 25-MV photon beam.

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80-100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.

Measurements and calculations of the influence of thin inhomogeneities on charged particle beams.

The predictions of an analytic technique for calculating fluence and dose distributions beneath thin inhomogeneities are presented for a number of structures, including a rectangular cavity or bar, a cylinder, a disk, and an angled or diffuse edge. Experiments with both electrons and protons for several geometries are presented and compared with predictions based on this technique. We offer some clinical guidelines for avoiding large perturbations due to scattering effects.

Partial body calcium determination in bone by proton activation analysis.

The feasibility of a new method for in vivo regional bone calcium measurement has been studied in phantoms using the 160 MeV Harvard cyclotron, Advantages include the capability of measuring bone calcium directly directly in a well defined anatomical region (such as one or several vertebrae) and restriction of the dose to the immediate region of interest. Proton activation of 40Ca (97% natural abundance) produces radionuclide 38K. Its 2-17 MeV gamma ray (T 1/2 = 7-71 min) is detected by a NaI counter. Separation of this activity from room background and interfering nuclides, 14O and 34Clm, proceeds by decay curve analysis. Phantom studies showed the dependence of 38K activity to be highly linear with calcium content (r= 0-998). Non-linearities with dose did not appear below the 20 rad level. The precision of measurements on a phantom with two calf vertebrae exposed to 2-4 rad was 3% and was mostly limited by counting statistics. System reproducibility on phantoms given a high dose proved better than 0-5%.

Flattening of proton dose distributions for large-field radiotherapy.

Methods for obtaining flattened charged-particle dose distributions over large areas are described. The system being used at Harvard for proton radiotherapy is discussed in detail. It is an extension of usual multiple-scattering techniques to include blocking out some of the central peak, followed by rescattering to fill in the profile, resulting in flat distributions up to 30 cm in diameter. The unusually long source-to-skin distance (4.6-m SSD) plus the small lateral spread (2 mm rms) from multiple scattering in the patient results in little divergence of the beam throughout the treatment volume.