In vivo diode dosimetry for total marrow irradiation.

PURPOSE: To describe a method and evaluate the efficacy of using a p-type silicon diode as an alternative to thermoluminescent dosimeters for verifying the accuracy of total marrow irradiation setups and calculations. METHODS AND MATERIALS: A calibration factor has been measured for a 6 MV photon beam incident horizontally onto a polystyrene phantom inside an in-house built total marrow irradiation stand. Signal responses due to positioning and orientation of the diode with respect to the source were compared to a 0.6 cc cylindrical ionization chamber inside a polystyrene phantom. Procedures for predicting the diode reading and taking entrance measurements have been developed and action levels established to determine causes for discrepancies when ratios between predicted and actual values fell outside a +/- 5% tolerance range. Measurements were taken at the skin surface over the umbilicus calculation point for alternating 1.5 Gy anterior and posterior fractions given bidaily over a 3-day period. RESULTS: A total of 137 measurements taken from January to September 1994 for 23 patients were analyzed using a frequency histogram. The histogram indicated a mean reading of 1.002 +/- 2.6, and that three of the measurements fell outside the 5% tolerance. Investigation into the cause of the discrepancies showed that the diode had been improperly placed one time and that further patient immobilization needs to be considered. CONCLUSION: It is possible to use a diode as an in vivo dosimeter for a total marrow irradiation technique. The ease of implementation and immediate readouts make a diode system preferable to a thermoluminescent system for identifying systematic errors and verifying treatment configurations and monitor unit calculations.

Results of neutron radiosurgery for inoperable arteriovenous malformations of the brain.

Twenty patients with inoperable arteriovenous malformations (AVMs) of the brain were treated with neutron radiosurgery. A 50 MeV cyclotron was used to deliver 9 Gy in a single fraction through 7 to 14 isocentric portals. The size and shape of the portals were customized to each treatment site. Nineteen patients have been followed for a median duration of 24 months following neutron radiosurgery. A radiographically demonstrable partial response was seen in 8 patients (42%). No complete responses were observed. There has been one post-treatment hemorrhage, no fatalities, and no evidence of radiation toxicity. The low response rate and absence of toxicity suggest that the appropriate dose for neutron radiosurgical treatment of AVMs is higher than the dose of 9 Gy at isocenter used in this initial study.

Stereotactic neutron radiosurgery for arteriovenous malformations of the brain.

A technique employing single fraction neutron radiosurgery for treatment of intracranial vascular malformations has been developed at the University of Washington and is described in this report. The natural history of arteriovenous malformations of the brain is briefly reviewed, along with currently available therapeutic methods for treatment of these lesions. The characteristics of the neutron beam used for radiosurgery are described, along with methods for patient immobilization, radiation treatment planning, dosimetry, and delivery of treatment using this technique.

The Clatterbridge high-energy neutron facility: dosimetry intercomparisons.

Dosimetry intercomparisons have been performed between the Clatterbridge high-energy neutron facility and the following institutions, all employing beams with similar neutron energies: Université Catholique de Louvain, Belgium; University of Washington, Seattle, USA; MD Anderson Hospital, Houston, USA; and Fermi National Accelerator Laboratory, Batavia, USA. The purpose of the intercomparison was to provide a basis for the exchange of dose-response data and to facilitate the involvement of Clatterbridge in collaborative clinical trials. Tissue-equivalent ionization chambers were used by the participants in each intercomparison to compare measurements of total (neutron plus gamma) absorbed dose in the host institution's neutron beam, following calibration of the chambers in a reference photon beam. The effects of differences in exposure standards, chamber responses in the neutron beams and protocol-dependent dosimetry factors were all investigated. It was concluded that the overall difference in the measurement of absorbed dose relative to that determined by the Clatterbridge group was less than 2%.

Fast neutron radiotherapy for advanced malignant salivary gland tumors.

Thirty-two patients with inoperable, recurrent, or gross residual malignant salivary gland tumors received fast neutron radiotherapy at the University of Washington. Eleven patients were treated with low energy neutrons alone, four received a combined photon-low energy neutron treatment regimen ("mixed beam"), and 17 were treated with high energy neutrons alone. Patients treated for microscopic residual tumor after a surgical resection were excluded from this study. With a minimum follow-up period of one year, (maximum 12 years), the overall locoregional tumor control rate for the entire series was 81%. The 5-year locoregional tumor control rate was 69%. The overall 5-year survival rate was 33% (50% for T3 tumors and 0% for T4 tumors). Compared to results obtained with conventional photon and/or electron treatment for advanced salivary gland tumors, fast neutron radiotherapy appears to offer a significant advantage.

Uniformity in dosimetry protocols for therapeutic applications of fast neutron beams.

In this supplement to both the American and the European protocols for clinical neutron dosimetry, new recommendations are given with respect to the basic physical parameters and experimental techniques employed. For neutron dosimetry, the air kerma or exposure calibration in a photon beam is the most suitable method for the calibration of tissue-equivalent ionization chambers until calibration in a standard neutron field becomes available. More recent data are recommended for the physical parameters required for the photon calibration as well as for the measurements in the neutron beam. Water is recommended as the reference phantom material due to its similarity in absorption and scattering properties to muscle. The resulting overall change in absorbed dose calculated according to this supplement, compared with the original protocols, will be smaller than about +/- 2% due to differences in the basic physical parameters. An additional change of several percent occurs at depth in a phantom as a result of the difference between water and the muscle-equivalent liquid formerly recommended as the reference phantom material.

Neutron beam characteristics from 50 MeV protons on beryllium using a continuously variable multi-leaf collimator.

The dose distributional properties of a p(50) Be neutron beam using a continuously variable multi-leaf collimator are presented and compared with a 6 MV photon beam. The differences in physical dose delivery between these two radiation modalities are generally insignificant for radiation therapy, and stringent comparisons of neutron and photon treatments should therefore be possible. The flexibility in field shaping with the multi-leaf collimator opens new possibilities in the treatment of complex irregular target volumes. The collimator consists of 40 wedge-shaped leaves that are independently moved under computer control with their collimating surfaces always aligned with the effective radiation source to minimize the penumbra. The leaf collimator eliminates the need for handling of heavy insert collimators and beam blocks at the same time that it allows dynamic conformation therapy with neutrons.

Calorimetric and ionimetric dosimetry intercomparisons I: U.S. neutron radiotherapy centers.

In the U.S. neutron radiotherapy trial centers, absorbed dose is routinely measured using commercially available A-150 tissue equivalent (TE) plastic ionization chambers. The collecting volumes of these chambers are filled with either methane-based tissue equivalent gas or air. Absorbed dose in A-150 plastic, determined with these ionization chambers, was compared to that measured by an A-150 plastic calorimeter in an A-150 plastic phantom. These comparisons have yielded the following information: (1) Agreement of the total absorbed dose measured using the ionization chambers was within 2.5% of the calorimeter at all the centers visited to date. (2) For all the neutron fields measured, the product of the stopping power ratio (sw,g)N' between the A-150 plastic chamber wall and TE gas, and the average energy expended in the gas per ion pair formed, WN/e, was computed assuming Bragg-Gray theory and found to be 31.0 +/- 0.7 J/C. (3) The displacement correction factor employed to normalize measurements at a depth in a phantom using the type IC-17 ionization chamber was verified to be approximately 0.97 +/- 0.01.

Dosimetric properties of neutrons from 21-MeV deuteron bombardment of a deuterium gas target.

Spectra, yields, average energies, and kerma rates in tissue of neutrons from 21-MeV deuteron bombardment of deuterium gas targets have been calculated for target thicknesses of 1, 3.5, and 5 MeV. A high pressure gas cell was constructed and was filled with 33 atm of D2 gas (equivalent to an energy loss of 3.5 MeV for 21-MeV deuterons); dose rate, dose buildup, and depth-dose properties of neutrons produced by the D(d,n) reaction were measured. Dosimetric properties of these neutrons are superior to those of neutrons from a thick Be target bombarded by a deuteron beam of the same energy.

Measurement of photon dose fraction in a neutron radiotherapy beam.

Photon dose fractions (PDFs) have been measured in and around a neutron radiotherapy beam with a tissue-equivalent proportional counter (TEPC) and with paired ion chambers. The PDFs were found to increase linearly with increasing field size and width depth in phantom. PDFs were shown to decrease with decreasing phantom size and to be larger in the shielded region of the phantom than in the direct beam. Uncertainties in the PDF values were estimated to be 10%-15% for the TEPC measurements but about 50% for the measurement made with ion chambers.