Factors for converting dose measured in polystyrene phantoms to dose reported in water phantoms for incident proton beams.

PURPOSE: Previous dosimetry protocols allowed calibrations of proton beamline dose monitors to be performed in plastic phantoms. Nevertheless, dose determinations were referenced to absorbed dose-to-muscle or absorbed dose-to-water. The IAEA Code of Practice TRS 398 recommended that dose calibrations be performed with ionization chambers only in water phantoms because plastic-to-water dose conversion factors were not available with sufficient accuracy at the time of its writing. These factors are necessary, however, to evaluate the difference in doses delivered to patients if switching from calibration in plastic to a protocol that only allows calibration in water. METHODS: This work measured polystyrene-to-water dose conversion factors for this purpose. Uncertainties in the results due to temperature, geometry, and chamber effects were minimized by using special experimental set-up procedures. The measurements were validated by Monte Carlo simulations. RESULTS: At the peak of non-range-modulated beams, measured polystyrene-to-water factors ranged from 1.015 to 1.024 for beams with ranges from 36 to 315 mm. For beams with the same ranges and medium sized modulations, the factors ranged from 1.005 to 1.019. The measured results were used to generate tables of polystyrene-to-water dose conversion factors. CONCLUSIONS: The dose conversion factors can be used at clinical proton facilities to support beamline and patient specific dose per monitor unit calibrations performed in polystyrene phantoms.

Dose response of CaF2:Tm to charged particles of different LET.

Thermoluminescent dosimeters are well established for performing calibrations in radiotherapy and for monitoring dose to personnel exposed to low linear energy transfer (LET) ionizing radiation. Patients undergoing light ion therapy and astronauts engaged in space flight are, however, exposed to radiation fields consisting of a mix of low- and high-LET charged particles. In this study, glow curves from CaF2:Tm chips were examined after exposure to various electron and ion beams. The annealing and readout procedures for these chips were optimized for these beams. After a 10 min prereadout annealing at 100 degrees C, the optimized glow curve samples the light output between 95 and 335 degrees C with a heating rate of 2 degrees C/s. The ratio of the integral of the glow curve under peaks 4-6 to the integral under peak 3 was approximately 0.9 for electrons, 1.0 for entrance protons, 1.6 for peak protons, and 2.2 for entrance carbon, silicon, and iron ions. The integral light output per unit dose in water for the iron exposures was about half as much as for the electron exposures. The peak-area-ratio can be used to determine a dose response factor for different LET radiations.

Spill-to-spill and daily proton energy consistency with a new accelerator control system.

The Loma Linda University proton accelerator has had several upgrades installed including synchrotron dipole power supplies and a system for monitoring the beam energy. The consistency of the energy from spill-to-spill has been tested by measuring the depth ionization at the distal edge as a function of time. These measurements were made with a minimally equipped beamline to reduce interference from confounding factors. The consistency of the energy over several months was measured in a treatment room beamline using an ionization chamber based daily quality assurance device. The results showed that the energy of protons delivered from the accelerator (in terms of water equivalent range) was consistent from spill-to-spill to better than +/-0.03 mm at 70, 155, and 250 MeV and that the energy check performed each day in the treatment room over a several month period was within +/-0.11 mm (+/-0.06 MeV) at 149 MeV. These results are within the tolerances required for the energy stacking technique.

Leakage and scatter radiation from a double scattering based proton beamline.

Proton beams offer several advantages over conventional radiation techniques for treating cancer and other diseases. These advantages might be negated if the leakage and scatter radiation from the beamline and patient are too large. Although the leakage and scatter radiation for the double scattering proton beamlines at the Loma Linda University Proton Treatment Facility were measured during the acceptance testing that occurred in the early 1990s, recent discussions in the radiotherapy community have prompted a reinvestigation of this contribution to the dose equivalent a patient receives. The dose and dose equivalent delivered to a large phantom patient outside a primary proton field were determined using five methods: simulations using Monte Carlo calculations, measurements with silver halide film, measurements with ionization chambers, measurements with rem meters, and measurements with CR-39 plastic nuclear track detectors. The Monte Carlo dose distribution was calculated in a coronal plane through the simulated patient that coincided with the central axis of the beam. Measurements with the ionization chambers, rem meters, and plastic nuclear track detectors were made at multiple locations within the same coronal plane. Measurements with the film were done in a plane perpendicular to the central axis of the beam and coincident with the surface of the phantom patient. In general, agreement between the five methods was good, but there were some differences. Measurements and simulations also tended to be in agreement with the original acceptance testing measurements and results from similar facilities published in the literature. Simulations illustrated that most of the neutrons entering the patient are produced in the final patient-specific aperture and precollimator just upstream of the aperture, not in the scattering system. These new results confirm that the dose equivalents received by patients outside the primary proton field from primary particles that leak through the nozzle are below the accepted standards for x-ray and electron beams. The total dose equivalent outside of the field is similar to that received by patients undergoing treatments with intensity modulated x-ray therapy. At the center of a patient for a whole course of treatment, the dose equivalent is comparable to that delivered by a single whole-body XCT scan.

EDR-2 film response to charged particles.

A useful tool for verifying segmental or dynamic treatments with multiple multi-leaf collimator positions, spinning range modulator propellors or magnetically scanned beams would be a film with a linear dose response up to several hundred centiGray, as typical for delivered treatments. Kodak has released an extended range film (EDR-2) that may satisfy this desire. In this study, dose response curves were obtained for several electron, proton, carbon ion and iron ion beams of different energies to determine the utility of this film.

Calibration of a proton beam energy monitor.

Delivery of therapeutic proton beams requires an absolute energy accuracy of +/-0.64 to 0.27 MeV for patch fields and a relative energy accuracy of +/-0.10 to 0.25 MeV for tailoring the depth dose distribution using the energy stacking technique. Achromatic switchyard tunes, which lead to better stability of the beam incident onto the patient, unfortunately limit the ability of switchyard magnet tesla meters to verify the correct beam energy within the tolerances listed above. A new monitor to measure the proton energy before each pulse is transported through the switchyard has been installed into a proton synchrotron. The purpose of this monitor is to correct and/or inhibit beam delivery when the measured beam energy is outside of the tolerances for treatment. The monitor calculates the beam energy using data from two frequency and eight beam position monitors that measure the revolution frequency of the proton bunches and the effective offset of the orbit from the nominal radius of the synchrotron. The new energy monitor has been calibrated by measuring the range of the beam through water and comparing with published range-energy tables for various energies. A relationship between depth dose curves and range-energy tables was first determined using Monte Carlo simulations of particle transport and energy deposition. To reduce the uncertainties associated with typical scanning water phantoms, a new technique was devised in which the beam energy was scanned while fixed thickness water tanks were sandwiched between two fixed parallel plate ionization chambers. Using a multitude of tank sizes, several energies were tested to determine the nominal accelerator orbit radius. After calibration, the energy reported by the control system matched the energy derived by range measurements to better than 0.72 MeV for all nine energies tested between 40 and 255 MeV with an average difference of -0.33 MeV. A study of different combinations of revolution frequency and radial offsets to test the envelope of algorithm accuracy demonstrated a relative accuracy of +/-0.11 MeV for small energy changes between 126 and 250 MeV. These new measurements may serve as a data set for benchmarking range-energy relationships.

Generation and characterization of a proton microbeam for experimental radiosurgery.

A proton microbeam has been developed to support various research endeavors. Test subjects may be irradiated from any angle with respect to the vertical because the beamline is contained within a rotating gantry used for human patients. Converting from the treatment to experimental arrangement is quick and straightforward as is the reverse. Using a series of collimators, the final beam diameter at the surface of the subject is 1 mm. The depth from the surface to the Bragg peak in water is 15 mm. Fluence distributions perpendicular to the beam axis were determined by scanning radiographic film exposed at various depths with a scanner having a pixel size of 84.7 microm. The depth dose integrated over the beam area was measured using a parallel plate ionization chamber. Central axis depth doses were calculated by multiplying the ionization chamber signal by the ratio of film doses for the central axis pixels to the integrated beam doses at each depth. A Faraday cup was used to confirm the dose at the surface while TLDs, diodes, and film were used to verify the dose at depth. The usefulness of this beamline for experimental situations has been demonstrated in a feline neurological study. The dosimetry techniques are useful for narrow beams such as used for functional radiosurgery treatments of humans.

Studies of physiology and the morphology of the cat LGN following proton irradiation.

PURPOSE: We have examined the effects of proton irradiation on the histologic and receptive field properties of thalamic relay cells in the cat visual system. The cat lateral geniculate nucleus (LGN) is a large structure with well-defined anatomical boundaries, and well-described afferent, efferent, and receptive field properties. METHODS AND MATERIALS: A 1.0-mm proton microbeam was used on the cat LGN to determine short-term (3 months) and long-term (9 months) receptive field effects of irradiation on LGN relay cells. The doses used were 16-, 40-, and 60-gray (Gy). RESULTS: Following irradiation, abnormalities in receptive field organization were found in 40- and 60-Gy short-term animals, and in all of the long-term animals. The abnormalities included "silent" areas of the LGN where a visual response could not be evoked and other regions that had unusually large or small compound receptive fields. Histologic analysis failed to identify cellular necrosis or vascular damage in the irradiated LGN, but revealed a disruption in retinal afferents to areas of the LGN. CONCLUSIONS: These results indicate that microbeam proton irradiation can disrupt cellular function in the absence of obvious cellular necrosis. Moreover, the area and extent of this disruption increased with time, having larger affect with longer post-irradiation periods.

Methodologies and tools for proton beam design for lung tumors.

PURPOSE: Proton beams can potentially increase the dose delivered to lung tumors without increasing the dose to critical normal tissues because protons can be stopped before encountering the normal tissues. This potential can only be realized if tissue motion and planning uncertainties are correctly included during planning. This study evaluated several planning strategies to determine which method best provides adequate tumor coverage, minimal normal tissue irradiation, and simplicity of use. METHODS AND MATERIALS: Proton beam treatment plans were generated using one or more of three different planning strategies. These strategies included designing apertures and boluses to the PTV, apertures to the PTV and boluses to the CTV, and aperture and bolus to the CTV. RESULTS: The planning target volume as specified in ICRU Report 50 can be used only to design the lateral margins of beams, because the distal and proximal margins resulting from CT number uncertainty, beam range uncertainty, tissue motions, and setup uncertainties, are different than the lateral margins resulting from these same factors. The best strategy for target coverage with the planning tools available overirradiated some normal tissues unnecessarily. The available tools also made the planning of lung tumors difficult. CONCLUSIONS: This study demonstrated that inclusion of target motion and setup uncertainties into a plan should be performed in the beam design step instead of creating new targets. New computerized treatment planning system tools suggested by this study will ease planning, facilitate abandonment of the PTV concept, improve conformance of the dose distribution to the target, and improve conformal avoidance of critical normal tissues.

Phase I/II study of proton beam irradiation for the treatment of subfoveal choroidal neovascularization in age-related macular degeneration: treatment techniques and preliminary results.

PURPOSE: Age-related macular degeneration is the prevalent etiology of subfoveal choroidal neovascularization (CNV). The only effective treatment is laser photocoagulation, which is associated with decreased visual acuity following treatment in most patients. This study assessed both the response of subfoveal CNV to proton beam irradiation and treatment-related morbidity. We evaluated preliminary results in patients treated with an initial dose of 8 Cobalt Gray Equivalents (CGE) using a relative biological effectiveness (RBE) of 1.1. METHODS AND MATERIALS: Twenty-one patients with subfoveal CNV received proton irradiation to the central macula with a single fraction of 8 CGE; 19 were eligible for evaluation. Treatment-related morbidity was based on Radiation Therapy Oncology Group (RTOG) criteria; response was evaluated by Macular Photocoagulation Study (MPS) guidelines. Fluorescein angiography was performed; visual acuity, contrast sensitivity, and reading speed were measured at study entry and at 3-month intervals after treatment. Follow-up ranged from 6 to 15 months. RESULTS: No measurable treatment-related morbidity was seen during or after treatment. Of 19 patients evaluated at 6 months, fluorescein angiography demonstrated treatment response in 10 (53%); 14 (74%) patients had improved or stable visual acuity. With a mean follow-up of 11.6 months, 11 (58%) patients have demonstrated improved or stable visual acuity. CONCLUSION: A macular dose of 8 CGE yielded no measurable treatment morbidity in patients studied. Fluorescein angiography demonstrated that regressed or stabilized lesions were associated with improved visual acuity as compared with MPS results. In the next phase, a dose of 14 CGE in a single fraction will be used to further define the optimal dose fractionation schedule.

Proton therapy for pediatric cranial tumors: preliminary report on treatment and disease-related morbidities.

PURPOSE: Accelerated protons were used in an attempt to limit treatment-related morbidity in children with tumors in or near the developing brain, by reducing the integral dose to adjacent normal tissues. METHODS AND MATERIALS: Children treated with protons at Loma Linda University Medical Center between August 1991 and December 1994 were analyzed retrospectively. Twenty-eight children, aged 1 to 18 years, were identified as at risk for brain injury from treatment. Medical records, physical examinations, and correspondence with patients, their parents, and referring physicians were analyzed. The investigators tabulated post-treatment changes in pre-treatment signs and symptoms and made judgments as to whether improvement, no change, or worsening related to disease or treatment had supervened. Magnetic resonance images were correlated with clinical findings and radiographic impressions were tabulated. RESULTS: Follow-up ranged from 7 to 49 months (median 25 months). Four instances of treatment-related morbidity were identified. Forty-one instances of site-specific, disease-related morbidity were identified: 15 improved or resolved and 26 remained unchanged after treatment. Four patients had radiographic evidence of local failure. Three of these patients, including two with high-grade glioma, have died. CONCLUSION: Early treatment-related morbidity associated with proton therapy is low. Tumor progression remains a problem when treating certain histologies such as high-grade glioma. Escalating the dose delivered to target volumes may benefit children with tumors associated with poor rates of local control. Long-term follow-up, including neurocognitive testing, is in progress to assess integral-dose effects on cognitive, behavioral and developmental outcomes in children with cranial tumors.

Proton dosimetry intercomparison.

BACKGROUND AND PURPOSE: Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols. MATERIALS AND METHODS: Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV). RESULTS: The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%. CONCLUSION: The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Proton dosimetry intercomparison based on the ICRU report 59 protocol.

BACKGROUND AND PURPOSE: A new protocol for calibration of proton beams was established by the ICRU in report 59 on proton dosimetry. In this paper we report the results of an international proton dosimetry intercomparison, which was held at Loma Linda University Medical Center. The goals of the intercomparison were, first, to estimate the level of consistency in absorbed dose delivered to patients if proton beams at various clinics were calibrated with the new ICRU protocol, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration factors. MATERIALS AND METHODS: Eleven institutions participated in the intercomparison. Measurements were performed in a polystyrene phantom at a depth of 10.27 cm water equivalent thickness in a 6-cm modulated proton beam with an accelerator energy of 155 MeV and an incident energy of approximately 135 MeV. Most participants used ionization chambers calibrated in terms of exposure or air kerma. Four ionization chambers had 60Co-based calibration in terms of absorbed dose-to-water. Two chambers were calibrated in a 60Co beam at the NIST both in terms of air kerma and absorbed dose-to-water to provide a comparison of ionization chambers with different calibrations. RESULTS: The intercomparison showed that use of the ICRU report 59 protocol would result in absorbed doses being delivered to patients at their participating institutions to within +/-0.9% (one standard deviation). The maximum difference between doses determined by the participants was found to be 2.9%. Differences between proton doses derived from the measurements with ionization chambers with N(K)-, or N(W) - calibration type depended on chamber type. CONCLUSIONS: Using ionization chambers with 60Co calibration factors traceable to standard laboratories and the ICRU report 59 protocol, a distribution of stated proton absorbed dose is achieved with a difference less than 3%. The ICRU protocol should be adopted for clinical proton beam calibration. A comparison of proton doses derived from measurements with different chambers indicates that the difference in results cannot be explained only by differences in 60Co calibration factors.

Proton-beam radiotherapy for early-stage lung cancer.

STUDY OBJECTIVE: A prospective study was undertaken to assess the efficacy and toxicity of conformal proton-beam radiotherapy for early-stage, medically inoperable non-small cell lung cancer. DESIGN: Eligible patients had clinical stage I to IIIa non-small cell lung cancer and were not candidates for surgical resection for medical reasons or because of patient refusal. Patients with adequate cardiopulmonary function received 45 Gy to the mediastinum and gross tumor volume with photons with a concurrent proton boost to the gross tumor volume of an additional 28.8 cobalt gray equivalents (CGE). Total tumor dose was 73.8 CGE given over 5 weeks. Patients with poor cardiopulmonary function received proton-beam radiotherapy to the gross tumor volume only, with 51 CGE given in 10 fractions over a 2-week period. RESULTS: Thirty-seven patients were treated in the study from July 1994 to March 1998. Clinical staging of patients was as follows: stage I, 27 patients; stage II, 2 patients; and stage IIIa, 8 patients. Eighteen patients received a combination of protons and x rays, while 19 patients received proton-beam radiation only. Follow-up of evaluable patients ranged from 3 to 45 months, with a median of 14 months. Two patients in the proton and photon arm developed pneumonitis that resolved with oral steroids; otherwise, no significant toxicities were encountered. The actuarial disease-free survival at 2 years for the entire group was 63%; for stage I patients, disease-free survival at 2 years was 86%. Local disease control was 87%. CONCLUSION: Preliminary results from this study indicate that proton-beam radiotherapy can be used safely in this group of patients. Disease-free survival and local control appear to be good and compare favorably with published reports utilizing conventional photon irradiation.

Magnetic resonance imaging as a monitor of changes in the irradiated rat brain. An aid in determining the time course of events in a histologic study.

RATIONALE AND OBJECTIVES: Optimal dose schedules and total dose of ionizing radiation for human central nervous system malignancy are not known. An animal model has been developed for the investigation of rat central nervous system response to proton irradiation using magnetic resonance imaging (MRI). A clinical MRI device was used to monitor the response of the rat brain after irradiation as a possible indicator for histologic injury as a function of time and dose. METHODS: Single-dose fractions of protons were delivered to the left brain of 25 adult Sprague-Dawley rats. T1- and T2-weighted images were obtained using a 1.5-T MRI device via a 12-cm diameter coil at 4- to 6-week intervals after irradiation. Coronal images were evaluated by visual inspection and relaxation maps comparing the control and irradiated hemispheres. Histologic review was conducted on all rats' brains after death. RESULTS: Proton irradiation was delivered successfully to only the left brain of the animals. Histologic review confirms the location and extent of tissue damage demonstrated on MRIs obtained in vivo. Statistically significant differences were seen in the T2-weighted relaxation times in the irradiated cerebral hemisphere compared with the unirradiated hemisphere. CONCLUSION: The proton hemibrain rat model can be used to test treatment schedules of irradiation for central nervous system response using MRI to noninvasively document early and late effects within the same animal.

Comparisons of LET distributions for protons with energies between 50 and 200 MeV determined using a spherical tissue-equivalent proportional counter (TEPC) and a position-sensitive silicon spectrometer (RRMD-III).

Experiments have been performed to measure the response of a spherical tissue-equivalent proportional counter (TEPC) and a silicon-based LET spectrometer (RRMD-III) to protons with energies ranging from 50-200 MeV. This represents a large portion of the energy distribution for trapped protons encountered by astronauts in low-Earth orbit. The beam energies were obtained using plastic polycarbonate degraders with a monoenergetic beam that was extracted from a proton synchrotron. The LET spectrometer provided excellent agreement with the expected LET distribution emerging from the energy degraders. The TEPC cannot measure the LET distribution directly. However, the frequency mean value of lineal energy, y(-)(f), provided a good approximation to LET. This is in contrast to previous results for high-energy heavy ions where y(-)(f) underestimated LET, whereas the dose-averaged lineal energy, y(-)(D), provided a good approximation to LET.

Acute effects of whole-body proton irradiation on the immune system of the mouse.

The acute effects of proton whole-body irradiation on the distribution and function of leukocyte populations in the spleen and blood were examined and compared to the effects of photons derived from a (60)Co gamma-ray source. Adult female C57BL/6 mice were exposed to a single dose (3 Gy at 0.4 Gy/min) of protons at spread-out Bragg peak (SOBP), protons at the distal entry (E) region, or gamma rays and killed humanely at six different times thereafter. Specific differences were noted in the results, thereby suggesting that the kinetics of the response may be variable. However, the lack of significant differences in most assays at most times suggests that the RBE for both entry and peak regions of the Bragg curve was essentially 1.0 under the conditions of this study. The greatest immunodepression was observed at 4 days postexposure. Flow cytometry and mitogenic stimulation analyses of the spleen and peripheral blood demonstrated that lymphocyte populations differ in radiosensitivity, with B (CD19(+)) cells being most sensitive, T (CD3(+)) cells being moderately sensitive, and natural killer (NK1.1(+)) cells being most resistant. B lymphocytes showed the most rapid recovery. Comparison of the T-lymphocyte subsets showed that CD4(+) T helper/inducer cells were more radiosensitive than the CD8(+) T cytotoxic/suppressor cells. These findings should have an impact on future studies designed to maximize protection of normal tissue during and after proton-radiation exposure.

Response of thyroid follicular cells to gamma irradiation compared to proton irradiation. I. Initial characterization of DNA damage, micronucleus formation, apoptosis, cell survival, and cell cycle phase redistribution.

The RBE of protons has been assumed to be equivalent to that of photons. The objective of this study was to determine whether radiation-induced DNA and chromosome damage, apoptosis, cell killing and cell cycling in organized epithelial cells was influenced by radiation quality. Thyroid-stimulating hormone-dependent Fischer rat thyroid cells, established as follicles, were exposed to gamma rays or proton beams delivered acutely over a range of physical doses. Gamma-irradiated cells were able to repair DNA damage relatively rapidly so that by 1 h postirradiation they had approximately 20% fewer exposed 3' ends than their counterparts that had been irradiated with proton beams. The persistence of free ends of DNA in the samples irradiated with the proton beam implies that either more initial breaks or a quantitatively different type of damage had occurred. These results were further supported by an increased frequency of chromosomal damage as measured by the presence of micronuclei. Proton-beam irradiation induced micronuclei at a rate of 2.4% per gray, which at 12 Gy translated to 40% more micronuclei than in comparable gamma-irradiated cultures. The higher rate of micronucleus formation and the presence of larger micronuclei in proton-irradiated cells was further evidence that a qualitatively more severe class of damage had been induced than was induced by gamma rays. Differences in the type of damage produced were detected in the apoptosis assay, wherein a significant lag in the induction of apoptosis occurred after gamma irradiation that did not occur with protons. The more immediate expression of apoptotic cells in the cultures irradiated with the proton beam suggests that the damage inflicted was more severe. Alternatively, the cell cycle checkpoint mechanisms required for recovery from such damage might not have been invoked. Differences based on radiation quality were also evident in the alpha components of cell survival curves (0.05 Gy(-1) for gamma rays, 0.12 Gy(-1) for protons), which suggests that the higher level of survival of gamma-irradiated cells could be attributed to the persistence of nonlethally irradiated thyrocytes and/or the capacity to repair damage more effectively than cells exposed to equal physical doses of protons. The final assessment in this study was radiation-induced cell cycle phase redistribution. Gamma rays and protons produced a similar dose-dependent redistribution toward a predominantly G(2)-phase population. From our cumulative results, it seems likely that a majority of the proton-irradiated cells would not continue to divide. In conclusion, these findings suggest that there are quantitative and qualitative differences in the biological effects of proton beams and gamma rays. These differences could be due to structured energy deposition from the tracks of primary protons and the associated high-LET secondary particles produced in the targets. The results suggest that a simple dose-equivalent approach to dosimetry may be inadequate to compare the biological responses of cells to photons and protons.

Neutron insensitive depolymerization of polyacrylamide for californium-252 photon dosimetry.

A photon dosimeter based on degradation of polyacrylamide (PAA) in dilute aqueous solution has been found to be insensitive to neutrons from Cf-252, independent of photon energy from 34 keVeff to 1.33 MeV, and useful over a dose range of 1 to 100 Gy. Because this dilute polymer solution is virtually 100% water, it is nearly tissue equivalent for photon absorption as well as neutron scatter and attenuation. Radiation response was determined by measuring decreases in viscosity three days after irradiation. The measured response is dose rate independent from 10 to over 300 Gy/h but increases rapidly at lower dose rates. Long-term, radiation-induced viscosity changes were also observed. The many advantages of this tissue equivalent system for photon/neutron dosimetry suggest the need for further studies of the observed over-response at low dose rates.

Determination of the neutron and photon spectra of a clinical fast neutron beam.

A simple technique to determine the neutron and photon spectra of a clinical fast neutron beam is described. This technique involves making narrow beam attenuation measurements with a pair of ionization chambers and an iterative fitting program to analyze the data. A method is also described for determining the first-guess neutron spectrum for input into the iterative program. The results of the analysis yield spectra suitable for use in dose calculation algorithms and dosimetry protocols. Presented here is the first-known published photon spectrum from a clinical machine.