Victor A. Otte

Composition of A‐150 tissue‐equivalent plastic

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.

Dosimetry intercomparisons between fast-neutron radiotherapy facilities.

Neutron dosimetry intercomparison visits have been made by physicists from the M. D. Anderson Hospital-Texas A&M University Project to the Naval Research Laboratory, the University of Washington, and the MRC Cyclotron at Hammersmith Hospital. The Naval Research Laboratory and University of Washington physicists have made dosimetry intercomparisons at the Texas A&M Variable-Energy Cyclotron (TAMVEC). The parameters that are usually measured during these visits are tissue kerma in air, tissue dose at depth of dose maximum, relative central-axis depth dose, neutron/gamma ratios in air and in phantom, and photon calibrations of ionization chambers. In addition, beam profiles and dose buildup curves are sometimes measured. Other parameters that are compared are values of W, stopping power ratios, kerma corrections, and calculations that lead to the statement of tumor doses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.

Dosimetric properties of the fast neutron therapy beams at TAMVEC

Neutrons produced by bombarding beryllium with 16-, 30-, and 50-MeV deuterons are compared in terms of dose rate, skin sparing, depth dose, and field flatness. In general, the dosimetric properties improved as the energy increased, but at 50 MeV flattening filters were necessary. Isodose curves for treatment planning were generated, using the decrement-line method, and were compared to curves measured by a computer-controlled isodose plotter. This system was also employed to measure the isodose curves for wedge fields. Dosimetry checks on various patients were made, using silicon diodes as in vivo fast neutron dosimeters.

Shielding for neutron radiotherapy sources created by the Be(d, n) reaction.

Shielding data for neutrons produced by the Be(d, n) reaction at deuteron energies of 16, 30 and 50 MeV are presented. This information includes narrow beam and broad beam total kerma attenuation coefficients, a? examination of the benefits of laminated shields, and a discussion of the design basis for the therapy shield and collimators used at the Texas A&M Variable Energy Cyclotron. The effectiveness of this clinical shield is discussed.

Fast‐neutron dose rate vs energy for the d+Be reaction—a reanalysis

The differences in the published information concerning tissue kerma in air vs deuteron energy for the d+Be reaction are analyzed in light of some recent measurements. The reason for the discrepancy is determined to be a lack of electron suppression on the Be target in some earlier measurements, and the relation ln(tissue kerma)=ln(1.356 X 10(-4)+2.97lnE is found to fit the measured data over the deuteron energy range 11-50 MeV.

Computer dosimetry for flattened and wedged fast-neutron beams

Beam flattening by the use of polyethylene filters has been developed for the 50-MeV d in equilibrium Be fast-neutron therapy beam at the Texas A&M Variable-Energy Cyclotron (TAMVEC) as a result of the need for a more uniform dose distribution at depth within the patient. A computer algorithm has been developed that allows the use of a modified decrement line method to calculate dose distributions; standards decrement line methods do not apply because of off-axis peaking. The dose distributions for measured flattened beams are transformed into distributions that are physically equivalent to an unflattened distribution. In the transformed space, standard decrement line theory yields a distribution for any field size which, by applying the inverse transformation, generates the flattened dose distribution, including the off-axis peaking. A semiempirical model has been constructed that allows the calculation of dose distributions for wedged beams from open-beam data.

Design of flattening filters for the fast‐neutron beam at TAMVEC by use of decrement lines

Isodose distributions in a tissue-equivalent phantom produced by fast neutrons from 50-MeV deuterons incident on a thick beryllium target exhibit strong forward peaking, particularly for large fields. The design by use of decrement lines and the construction of polyethylene filters used to flatten those distributions are discussed and the results are illustrated. Also, the compromises of central-axis attenuation versus effective filter width and of off-axis peaking versus depth of flattening are discussed.

Measurement of dose distributions using film in therapeutic electron beams

The feasibility of using film dosimetry data as the input data for patient treatment planning was evaluated. The central-axis depth dose and the off-axis ratios obtained from film measurements in a solid phantom were compared with those of ion-chamber measurements in water. Two techniques were used to generate isodose distributions. The first technique used only the film data, i.e., the central-axis depth dose and the off-axis ratios used for the reconstruction were determined from the film optical density (corrected for film nonlinearity). In the second technique, the central-axis depth dose measured by an ion chamber in a water phantom was combined with the off-axis ratios measured using film in the solid water phantom. The resulting isodose distributions from both techniques were compared with the ion-chamber measurements in water for 7-, 12-, and 18-MeV electrons, and the second technique showed better agreement with the ion-chamber measurements than did the first technique. The differences were within a clinically acceptable range.

Control and monitor systems for a fast neutron radiotherapy facility

The fast neutrons are produced by bombarding an infinitely thick 4Be target with deuterons from a variable energy cyclotron The control system, relay logic and safety interlocks for turning the deuteron beam on and off were described. Also, the monitoring systems for measuring the integrated dose, for checking the focus and intensity of the deuteron beam, for observation of the patients, etc. were illustrated. Clinical use has shown the operation of the overall system to be convenient and reliable.