P. Shapiro

Displacement correction factor for fast‐neutron dosimetry in a tissue‐equivalent phantom

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

A comparison of two nitroimidazoles and a dihydroquinoline as radiosensitizers and cytotoxic agents

Abstract The search for electron affinic compounds that exhibit the properties needed for a clinically useful hypoxic cell radiosensitizer has led to the setting up of clinical trials with misonidazole, which is a 2-nitroimidazole. Recently, MTDQ a new drug of novel design, a dihydroquinoline (6,6-methylene-bis-2,2,4-trimethyl-1,2-dihydroquinoline)has been synthesized and suggested as a radiosensitizer. The radiosensitizing and cytotoxic properties of misonidazole are compared with MTDQ and a second 2-nitroimadazole Ro-07-0741, which has warranted attention as a possible alternative to misonidazole for clinical use.

Stripping-theory analysis of thick-target neutron production for D+Be (and tissue dose calculation)

The Serber theory for deuteron stripping is employed to predict the shape of the neutron energy spectrum produced by 35 MeV deuterons (D+) on a thick beryllium target. In particular, the observation that the maximum of the neutron energy spectrum (at 0degrees relative to the deuteron beam direction) occurs at approximately 0-4Ed, where Ed is the incident deuteron energy, is explained reasonably well by the calculations. The explanation stems mainly from the fact that the stripping theory for thin targets predicts a narrow maximum at 0-5Ed, and thick target effects shift the maximum downward in energy to approximately 0-4Ed. A number of recent spectral measurements are in agreement with these predictions for a wide range of target materials and incident deuteron energies. The application of this theory also accounts for the previously observed Dd2-99 dependence of the absorbed dose in tissue,per unit charge of D+ ions on target, in the direction of the incident beam. This approximate Ed3 dependence is shown to be a characteristic property of deuteron stripping in a thick target and follows directly from the calculations that predict the neutron energy spectrum.

In vivo activation analysis following neutron therapy in cancer patients

A new mode of local i n v i v oactivation analysis following neutron therapy in cancer patients has been explored at the Naval Research Laboratory Cyclotron Facility. The level of the therapeuticneutron dose administered to a patient presents a unique opportunity to use a Ge(Li) detector to obtain rapidly a high‐resolution γ‐ray spectrum of the activity induced in and around the tumor mass. Methods of analysis and local geometrical effects are discussed.

The light-ion flux mixed with collimated fast-neutron beams

Experimental results are presented which show that neutron beams of the type currently employed in fast-neutron cancer therapy are contaminated by a light ion flux. Tantalum foils were used to demonstrate the existence of the light ions.

Computer generation of dose distributions for a fast-neutron therapy beam

A system has been developed for the computer generation of dose distributions for the MANTA-NRL fast-neutron radiotherapy beam. This program is based on scatter-air ratio (SAR) techniques. A method has been developed to unfold the effect of the neutron-beam profile in the derivation of SARs so that the SARs obtained are those which would be derived if the beam profile were flat. Tables of zero-area tissue-air ratios and SARs are presented. Comparisons of calculated and measured dose distributions are shown. An empirical correction to the usual SAR methods was required to obtain agreement between calculated and measured dose distributions at source-to-skin distances (SSD) which are different from the SSD at which the SAR are derived.

Scattered radiation from a neutron collimator.

Fast-neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three-component model, from measurements made with three ionization chambers (tissue-equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35-MeV D+ on Be). This method may prove useful for measurements in phantoms as well.