TRIP12 as a mediator of human papillomavirus/p16-related radiation enhancement effects.

Patients with human papillomavirus (HPV)-positive head and neck squamous cell carcinoma (HNSCC) have better responses to radiotherapy and higher overall survival rates than do patients with HPV-negative HNSCC, but the mechanisms underlying this phenomenon are unknown. p16 is used as a surrogate marker for HPV infection. Our goal was to examine the role of p16 in HPV-related favorable treatment outcomes and to investigate the mechanisms by which p16 may regulate radiosensitivity. HNSCC cells and xenografts (HPV/p16-positive and -negative) were used. p16-overexpressing and small hairpin RNA-knockdown cells were generated, and the effect of p16 on radiosensitivity was determined by clonogenic cell survival and tumor growth delay assays. DNA double-strand breaks (DSBs) were assessed by immunofluorescence analysis of 53BP1 foci; DSB levels were determined by neutral comet assay; western blotting was used to evaluate protein changes; changes in protein half-life were tested with a cycloheximide assay; gene expression was examined by real-time polymerase chain reaction; and data from The Cancer Genome Atlas HNSCC project were analyzed. p16 overexpression led to downregulation of TRIP12, which in turn led to increased RNF168 levels, repressed DNA damage repair (DDR), increased 53BP1 foci and enhanced radioresponsiveness. Inhibition of TRIP12 expression further led to radiosensitization, and overexpression of TRIP12 was associated with poor survival in patients with HPV-positive HNSCC. These findings reveal that p16 participates in radiosensitization through influencing DDR and support the rationale of blocking TRIP12 to improve radiotherapy outcomes.

Hormone suppression with GnRH antagonist promotes spermatogenic recovery from transplanted spermatogonial stem cells in irradiated cynomolgus monkeys.

Hormone suppression given before or after cytotoxic treatment stimulates the recovery of spermatogenesis from endogenous and transplanted spermatogonial stem cells (SSC) and restores fertility in rodents. To test whether the combination of hormone suppression and transplantation could enhance the recovery of spermatogenesis in primates, we irradiated (7 Gy) the testes of 12 adult cynomolgus monkeys and treated six of them with gonadotropin-releasing hormone antagonist (GnRH-ant) for 8 weeks. At the end of this treatment, we transfected cryopreserved testicular cells with green fluorescent protein-lentivirus and autologously transplanted them back into one of the testes. The only significant effect of GnRH-ant treatment on endogenous spermatogenesis was an increase in the percentage of tubules containing differentiated germ cells (tubule differentiation index; TDI) in the sham-transplanted testes of GnRH-ant-treated monkeys compared with radiation-only monkeys at 24 weeks after irradiation. Although transplantation alone after irradiation did not significantly increase the TDI, detection of lentiviral DNA in the spermatozoa of one radiation-only monkey indicated that some transplanted cells colonized the testis. However, the combination of transplantation and GnRH-ant clearly stimulated spermatogenic recovery as evidenced by several observations in the GnRH-ant-treated monkeys receiving transplantation: (i) significant increases (~20%) in the volume and weight of the testes compared with the contralateral sham-transplanted testes and/or to the transplanted testes of the radiation-only monkeys; (ii) increases in TDI compared to the transplanted testes of radiation-only monkeys at 24 weeks (9.6% vs. 2.9%; p = 0.05) and 44 weeks (16.5% vs. 6.1%, p = 0.055); (iii) detection of lentiviral sequences in the spermatozoa or testes of five of the GnRH-ant-treated monkeys and (iv) significantly higher sperm counts than in the radiation-only monkeys. Thus hormone suppression enhances spermatogenic recovery from transplanted SSC in primates and may be a useful tool in conjunction with spermatogonial transplantation to restore fertility in men after cancer treatment.

SU-E-T-527: Assessment of Field-Junction Dosimetry: Pinnacle V9.0 versus Film (EBT2).

PURPOSE: To investigate dose agreement of Pinnacle V9.0 with measurements across the junction between abutting photon and electron fields. METHODS: Photon-photon, photon-electron, and electron-electron field junctions of energies 6 MV and 9 MeV were evaluated with 0, 5, and 10 mm gaps between the field edges at depths of 2, 15, and 30 mm with Gafchromic film (EBT2). Photon and electron fields (10×10 cm) were setup to deliver 300 cGy in their field centers to 30 mm depth. Film optical density was measured every millimeter across the field-junction with a densitometer (light beam 2 mm). Optical Density to dose conversion was accomplished using a calibration curve specific to the batch of this film. For comparison, all field combinations were generated in Pinnacle, and doses were calculated with collapsed-cone convolution algorithm employing calculation-grid sizes 2, 3, and 4 mm. RESULTS: Measured doses at junction are as much as 20% different than Pinnacle calculation. 3 profiles with zero gap, 1 profile with 0.5 cm gap, and 0 profiles with 1.0 cm gap showed a disagreement greater than 12%. Measurement showed a bias towards higher dose in the junction when compared to Pinnacle calculation. With dose-calculation grid reduced below 4 mm, Pinnacle calculations showed improved agreements, but only a few percent at most. Pinnacle versus measured dose disagreements do not show clear trends with field separation or depth. CONCLUSIONS: Doses calculated by Pinnacle V9.0 in the field-junction region may not be accurate for abutting photon and electron fields, particularly when a coarse dose-calculation grid is utilized. Lack of clear trends in dose-agreement with field separation and depth suggests that uncertainties in other factors such as jaw positions and phantom set-ups may be additional contributors. Significant inaccuracies in doses reported by the treatment-planning system in field-junction region should be considered while making clinical decisions.

SU-E-T-235: Dosimetry Study within a Millimeter from Beam Exiting Air-Tissue Interface Using Film and Monte-Carlo Simulation on a Co-60 Beam.

PURPOSE: Objective is to assess dose accuracy and uniformity to cells in a well-plate irradiated on Co-60 beam using film (EBT2) and Monte-Carlo simulation. METHODS: Situations are encountered where dosimetry near beam-exiting air-interface is needed. Typical example is irradiation of cells in well-plate. Small amount of cell emulsion may amount to as small as 1 mm thickness which results in leaving large air space above the cells. Due to contamination concerns, air space above cells can not be avoided. Preferred geometry for irradiation is to place cell-plate on a build-up slab (water-equivalent), and point the beam upward. On Co-60 beam, film EBT2 was employed to measure doses 0.1, 0.4, 0.7, 1.2 mm upstream from air- interface. Film optical-density (OD) was measured with a pocket densitometer, and converted to dose using film-calibration specific to film-batch. To validate film data, Monte-Carlo code DOSXYZNRC was employed for simulation. Doses were calculated employing (i) calculation grid 0.1-1.0 mm along beam direction, and (ii) 15000 million incident particles to achieve < 0.2% precision. RESULTS: Doses measured with film close to beam-exiting air-interface are ∼16% lower than the maximum doses 1.2 mm up-stream and beyond. Monte-Carlo calculations validate the doses measured with film within measurement uncertainty (2%). Dose variation is steep in the first 0.1 mm zone near air-interface, but it is within 10% further up-stream. CONCLUSIONS: In cell irradiations, dose in-homogeneity in the top 1mm zone is significant. Layer close to air-interface receives ∼16% lower dose than the layer 1.2mm up-stream. Dose homogeneity with the stated geometry can be improved by having (i) larger amount of cells to provide emulsion thickness > 3mm, and (ii) non-homogeneous emulsion for cells to settle at the bottom. Since dose in-homogeneity is primarily due to air interface, results are expected to be qualitatively similar for other beams (Cs-137 or 6 MV.

SU-E-T-528: Appraisal of Acorus XB and Convolution Dose Algorithms in Field Junction of Breast Tangential/superclavicular Fields.

PURPOSE: Dose accuracy injunction regions of breast-tangential therapy is a challenge, and inaccurate dose predictions may lead to unreal hot/cold spots. Availability of the novel deterministic radiation transport method Acuros XB (AXB) provides a potential for more accurate dose predictions. This study assesses relative dose accuracies of this and the widely used other algorithms: collapsed cone convolution (CCC) and anisotropic analytical algorithm (AAA) against film measurements. METHODS: A typical tangential and superclav fileld combination was planned for an anthropomorphic body phantom using Pinnacle-9.0 treatment-planning system (TPS). The created plan employing 6MV beam was delivered to the phantom on a Varian linac. In region of the field junction of tangentials & superclav, films (EBT2) were placed in coronal planes at two depths (∼ 2 and 4 cm). Optical density was measured along and ± 5mm away from the field-match line, and converted to dose using film-calibration curve specific to the batch of film. The same plan was also imported to Eclipse TPS using an import filter written in MATLAB. Algorithms Pinnacle CCC 9.0, Eclipse AAA 10.0.24 and AXB 11.0.3 were used for calculations. Comparison of the measured doses (assumed as gold standard) against doses calculated from planning-systems were preformed in a MATLAB platform. RESULTS: In general, dose distributions from all three TPS algorithms are found to agree closely with film data. Agreements between AXB and CCC dose calculations were found to be reasonable. AXB appears to be better in modeling the backscatter effects in the heterogeneous regions. AAA calculations gives acceptable results, but with less accuracy compared to CCC and AXB. CONCLUSIONS: The novel deterministic algorithm AXB in Eclipse is found to provide better agreement with measured data in breast-tangential therapy. Benefits of using Acuors XB algorithm in tangential fields planning requires further investigation. This work was funded by National Institutes of Health through grant 2R44CA105806-02 and MD Anderson’s Cancer Center Support Grant CA0 16672.

Contact lens-related microbial keratitis in deployed British military personnel.

AIMS: To quantify the effect of contact lens-related microbial keratitis (CLMK) in the British Defence personnel particularly those in active service in the Arabian Gulf and Afghanistan between June 2001 and January 2007. METHODS: A retrospective review of all British military personnel who developed contact lens-related keratitis during deployment. RESULTS: A total of 27 cases (27, eyes, 23, male; median age 26 (range 19-41) years) were identified, of whom 19 cases were evacuated from Iraq alone. Twenty cases were associated with soft contact lens wear. Seven cases were culture positive, of which five grew Pseudomonas aeruginosa. The overall incidence of CLMK in contact lens wearer in the British military in Iraq was 35 per 10,000. There was an increased incidence during the summer months. Seventeen eyes (63%) lost more than one line of visual acuity with a resultant permanent medical downgrading in duty capability in nine cases. CONCLUSIONS: CLMK has a poorer outcome in a deployed military environment when compared to the civilian setting. Increased awareness of the health risks of contact lens wear together with standardised treatment regimens based on improved pathogen detection using molecular diagnostics have improved outcomes.

Isolated central nervous system involvement in adult T-cell lymphoma/leukaemia.

Central nervous system (CNS) presentation of adult T-cell lymphoma/leukaemia is rare, and almost invariably associated with systemic disease. We report an unusual manifestation of adult T-cell lymphoma/leukaemia, with isolated CNS involvement and unusual imaging findings. We also describe objective response to antiviral therapy. To our knowledge, this is the first report of such presentation and response.

Calibration of low-energy electron beams from a mobile linear accelerator with plane-parallel chambers using both TG-51 and TG-21 protocols.

A new approach to intraoperative radiation therapy led to the development of mobile linear electron accelerators that provide lower electron energy beams than the usual conventional accelerators commonly encountered in radiotherapy. Such mobile electron accelerators produce electron beams that have nominal energies of 4, 6, 9 and 12 MeV. This work compares the absorbed dose output calibrations using both the AAPM TG-51 and TG-21 dose calibration protocols for two types of ion chambers: a plane-parallel (PP) ionization chamber and a cylindrical ionization chamber. Our results indicate that the use of a 'Markus' PP chamber causes 2-3% overestimation in dose-output determination if accredited dosimetry-calibration laboratory based chamber factors (N(60Co)(D,w,) Nx) are used. However, if the ionization chamber factors are derived using a cross-comparison at a high-energy electron beam, then a good agreement is obtained (within 1%) with a calibrated cylindrical chamber over the entire energy range down to 4 MeV. Furthermore, even though the TG-51 does not recommend using cylindrical chambers at the low energies, our results show that the cylindrical chamber has a good agreement with the PP chamber not only at 6 MeV but also down to 4 MeV electron beams.

TG-51: experience from 150 institutions, common errors, and helpful hints.

The Radiological Physics Center (RPC) is a resource to the medical physics community for assistance regarding dosimetry procedures. Since the publication of the AAPM TG-51 calibration protocol, the RPC has responded to numerous phone calls raising questions and describing areas in the protocol where physicists have had problems. At the beginning of the year 2000, the RPC requested that institutions participating in national clinical trials provide the change in measured beam output resulting from the conversion from the TG-21 protocol to TG-51. So far, the RPC has received the requested data from approximately 150 of the approximately 1300 institutions in the RPC program. The RPC also undertook a comparison of TG-21 and TG-51 and determined the expected change in beam calibration for ion chambers in common use, and for the range of photon and electron beam energies used clinically. Analysis of these data revealed two significant outcomes: (i) a large number (approximately 1/2) of the reported calibration changes for photon and electron beams were outside the RPC's expected values, and (ii) the discrepancies in the reported versus the expected dose changes were as large as 8%. Numerous factors were determined to have contributed to these deviations. The most significant factors involved the use of plane-parallel chambers, the mixing of phantom materials and chambers between the two protocols, and the inconsistent use of depth-dose factors for transfer of dose from the measurement depth to the depth of dose maximum. In response to these observations, the RPC has identified a number of circumstances in which physicists might have difficulty with the protocol, including concerns related to electron calibration at low energies (R50<2 cm), and the use of a cylindrical chamber at 6 MeV electrons. In addition, helpful quantitative hints are presented, including the effect of the prescribed lead filter for photon energy measurements, the impact of shifting the chamber depth for photon depth-dose measurements, and the impact of updated stopping-power data used in TG-51 versus that used in TG-21, particularly for electron calibrations.

Predictability of electron cone ratios with respect to linac make and model.

In the past, the Radiological Physics Center (RPC) has developed standard sets of photon depth-dose and wedge-factor data, specific to the make, model, and wedge design of the linear accelerator (linac). In this paper, the RPC extends the same concept to electron-cone ratios. Since 1987, the RPC has measured and documented cone-ratio (CR) values during on-site dosimetry review visits to institutions participating in National Cancer Institute cooperative clinical trials. Data have been collected for approximately 500 electron beams from a wide spectrum of linac models. The analysis presented in this paper indicates that CR values are predictable to 2% to 3% (two standard deviations) for a given make and model of linac with a few exceptions. The analysis also revealed some other interesting systematics. For some models, such as the Varian Clinac 2500 and the Elekta/Philips SL18, SL20, and SL25, CR values were nearly identical for cone sizes 15 cm x 15 cm (or 14 cm x 14 cm) and 20 cm x 20 cm across the range of available energies. Certain models of the same make of linac, such as the Mevatron MD, KD, and 6700 series models or the Clinac 2100 and 2300 models, exhibited indistinguishable CRs. Irrespective of linac model, two consistent general trends were observed: namely, an increase in CR value with incident beam energy for cone sizes smaller than 10 cm x 10 cm and a decrease with energy for cone sizes larger than 10 cm x 10 cm. These data are valuable to the RPC as a quality assurance remote-monitoring tool to identify potential dosimetry errors. The physics community will also find the data useful in several ways: as a redundant check for clinical values in use, to validate the values measured during commissioning of new machines or to ensure consistency of values measured during annual quality assurance procedures.

Calculated absorbed-dose ratios, TG51/TG21, for most widely used cylindrical and parallel-plate ion chambers over a range of photon and electron energies.

Task Group 51 (TG51), of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM), has developed a calibration protocol for high-energy photon and electron therapy beams based on absorbed dose standards. This protocol is intended to replace the air-kerma based protocol developed by an earlier AAPM task group (TG21). Conversion to the newer protocol introduces a change in the determined absorbed dose. In this work, the change in dose is expressed as the ratio of the doses (TG51/TG21) based on the two protocols. Dose is compared at the TG-51 reference depths of 10 cm for photons and d(ref) for electrons. Dose ratios are presented for a variety of ion chambers over a range of photon and electron energies. The TG51/TG21 dose ratios presented here are based on the dosimetry factors provided by the two protocols and the chamber-specific absorbed dose and exposure calibration factors (N60Co(D,w) and Nx) provided by the Accredited Dosimetry Calibration Laboratory (ADCL) at The University of Texas, M. D. Anderson Cancer Center (MDACC). As such, the values presented here represent the expected discrepancies between the two protocols due only to changes in the dosimetry parameters and the differences in chamber-specific dose and air-kerma standards. These values are independent of factors such as measurement uncertainties, setup errors, and inconsistencies arising from the mix of different phantoms and ion chambers for the two protocols. Therefore, these ratios may serve as a guide for institutions performing measurements for the switch from TG21-to-TG51 based calibration. Any significant deviation in the ratio obtained from measurements versus those presented here should prompt a review to identify possible errors and inconsistencies. For all cylindrical chambers included here, the TG51/TG21 dose ratios are the same within +/-0.6%, irrespective of the make and model of the chamber, for each photon and electron beam included. Photon beams show the TG51/TG21 dose ratios decreasing with energy, whereas electrons exhibit the opposite trend. The dose ratio for photons is near 1.00 at 18 mV increasing to near 1.01 at 4 mV while the dose ratio for electrons is near 1.02 at 20 MeV decreasing only 0.5% to near 1.015 at 6 MeV. For parallel-plate chambers, the situation is complicated by the two possible methods of obtaining calibration factors: through an ADCL or through a cross-comparison with a cylindrical chamber in a high-energy electron beam. For some chambers, the two methods lead to significantly different calibration factors, which in turn lead to significantly different TG51/TG21 results for the same chamber. Data show that if both N60Co(D,w) and Nx are obtained from the same source, namely an ADCL or a cross comparison, the TG51/TG21 results for parallel-plate chambers are similar to those for cylindrical chambers. However, an inconsistent set of calibration factors, i.e., using N60Co(D,w) x k(ecal) from an ADCL but Ngas from a cross comparison or vice versa, can introduce an additional uncertainty up to 2.5% in the TG51/TG21 dose ratios.

Verification of the accuracy of 3D calculations of breast dose during tangential irradiation: measurements in a breast phantom.

This report specifically describes the use of a unique anthropomorphic breast phantom to validate the accuracy of three-dimensional dose calculations performed by a commercial treatment-planning system for intact-breast tangential irradiation. The accuracy of monitor-unit calculations has been corroborated using ionization chamber measurements made in this phantom. Measured doses have been compared to those calculated from a variety of treatment plans. The treatment plans utilized a 6-MV x-ray beam and incorporated a variety of field configurations and wedge combinations. Dose measurements at several clinically relevant points within the breast phantom have confirmed the accuracy of calculated doses generated from the variety of treatment plans. Overall agreement between measurements and calculations averaged 0.998+/-0.009. These results indicate that the dose per monitor-unit calculations performed by the treatment-planning system can be confidently utilized in the fulfillment of clinical dose prescriptions.

An empirical relationship for determining photon beam quality in TG-21 from a ratio of percent depth doses.

A key component of the Radiological Physics Center's (RPC) on-site dosimetry review visits are photon beam calibrations for which determination of the energy of the x ray is a key element. The ratio of ionizations, TPR20/TPR10, for a 10 cm x 10 cm field at depths of 20 and 10 cm for a constant SCD is used as a quantitative measure of beam quality in the Task Group 21 protocol. The RPC has measured both TPR20/TPR10 and the corresponding ratio of percent depth dose (D20/D10) at a constant SSD for 685 photon beams (4-25 MV) for most makes and models if accelerators. A strong correlation between TPR20/TPR10 and D20/D10 is presented which allows the determination of the TPR ratio from the measurement of the ratio of percent depth doses. An analysis of the uncertainty introduced in the TG-21 factors (L/rho, Pwall, Prepl) caused by the spread in the measured data and translated into the determination of the TPR ratio results in an insignificant error (< 0.3%). This empirical relationship provides an alternate technique for quantifying the beam quality defined in the TG-21 protocol without surrendering any loss of precision in output calibration. This technique may be found by those who calibrate at a fixed SSD to be an easier and quicker method.

A generic off-axis energy correction for linac photon beam dosimetry.

Cooperative clinical trial group protocols frequently require off-axis point dose calculations. The Radiological Physics Center uses the calculative technique developed by Hanson et al. [Med. Phys. 7, 145-146 (1980); 7, 147-150 (1980)] to verify these calculations. In order to correct for off-axis energy changes, this technique requires off-axis half-value layer data, HVL, as a function of off-axis ray angle for the specific beam. This paper presents a formulism based on HVL mesurements on a limited number of therapy beams, which allows the calculation of an off-axis energy-correction factor for any clinical photon beam created by a linear accelerator using conventional flattening filters.

Equilibration of air temperature inside the thimble of a Farmer-type ion chamber.

Ionization chambers are frequently moved from one environment to another, sometimes with significant differences in temperature between the chamber and measurement phantom. To obtain reliable ionization data, the temperature of the air in the chamber must be allowed to equilibrate with the measuring phantom. The air temperature inside a thimble of a Farmer-type ion chamber was measured as a function of time for various phantom materials (air, water, and plastic). Equilibration rates for the various conditions are presented. Heat-diffusion theory is presented to explain the characteristics of the measured data. Waiting times for temperature equilibration down to 10% of the initial temperature difference ranges from 1 to 18 min, depending on the phantom material and use of bare or covered thimble. Radiation measurements confirm the temperature data.

A first order approximation of field-size and depth dependence of wedge transmission.

The Radiological Physics Center, through its dosimetry review visits to participating institutions, is aware that many institutions ignore the field-size and depth dependence of wedge transmission values. Reference wedge transmission values are normally measured by the Radiological Physics Center for a 10 cm x 10 cm field at the calibration depth of 5 or 7 cm. Recently, additional measurements (1) for a 10 cm x 10 cm field at 20-cm depth and (2) for a 20 cm x 20 cm field at the calibration depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transmission values for 138 photon beams from 88 separate linear accelerators (4-25 MV) and 60Co units were measured. Our data suggest that the dependence of the wedge transmission on field-size and depth, in the first approximation, depends on the absolute value of the transmission under reference conditions. For wedges with a transmission value greater than 0.65%, field-size dependence and change in depth dose are typically less than 2%. However, for wedges with transmission values less than 0.65%, field-size dependence increases with decreasing reference wedge transmission. The change in wedge transmission with depth is significant (> 2%) only for photon energies less than or equal to 10 MV and can exceed 5% for thick wedges. Failure to include the depth and field-size dependencies of wedge transmission in patient dosimetry calculations can result in significant tumor-dose discrepancies.

A novel family of small cysteine-rich antimicrobial peptides from seed of Impatiens balsamina is derived from a single precursor protein.

Four closely related peptides were isolated from seed of Impatiens balsamina and were shown to be inhibitory to the growth of a range of fungi and bacteria, while not being cytotoxic to cultured human cells. The peptides, designated Ib-AMP1, Ib-AMP2, Ib-AMP3, and Ib-AMP4, are 20 amino acids long and are the smallest plant-derived antimicrobial peptides isolated to date. The Ib-AMPs (I. balsamina antimicrobial peptides) are highly basic and contain four cysteine residues which form two intramolecular disulfide bonds. Searches of protein data bases have failed to identify any proteins with significant homology to the peptides described here. Characterization of isolated cDNAs reveals that all four peptides are encoded within a single transcript. The predicted Ib-AMP precursor protein consists of a prepeptide followed by 6 mature peptide domains, each flanked by propeptide domains ranging from 16 to 35 amino acids in length. Such a primary structure with repeated alternating basic mature peptide domains and acidic propeptide domains has, to date, not been reported in plants.

Depth of ionization chamber in water.

The device developed by the authors and described here enables the user to measure the depth from the water surface to the point of measurement for a cylindrical ion chamber with a waterproof plastic cap in a water phantom, free of surface-tension error with a high precision. The device seeks vertical orientation and provides the convenience of hands-free operation. The measurement process is simple and quick with a precision of 0.1 mm. (The device is currently available as a 'water phantom depth gauge' from Nuclear Associates, Division of Victoreen Inc., Clare Place, NY, USA.)

How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams?

The water equivalency of five "water-equivalent" solid phantom materials was evaluated in terms of output calibration and energy characterization over a range of energies for both photon (Co-60 to 24 MV) and electron (6-20 MeV) beams. Evaluations compared absorbed doses calculated from ionization measurements using the same dosimeter in the solid phantom materials and in natural water (H2O). Ionization measurements were taken at various calibration depths. The Radiological Physics Center's standard dosimetry system, a Farmer-type ion chamber in a water phantom, was used. Complying with the TG-21 calibration protocol, absorbed doses were calculated using eight measurement and calculational techniques for photons and five for electrons. Results of repeat measurements taken over a period of 2 1/2 years were reproducible to within a +/- 0.3% spread. Results showed that various combinations of measurement techniques and solid phantom materials caused a spread of 3%-4% in the calculation of dose relative to the dose determined from measurements in water for all beam energies on both modalities. An energy dependence of the dose ratios was observed for both photons and electrons.