A local diagnostic reference level for velopharyngeal investigations

The purpose of this study was to derive an initial local diagnostic reference level for velopharyngeal investigations carried out as standard radiological practice in the Medical Imaging Department, Queen Victoria Hospital, East Grinstead. This is a specialist video-fluoroscopic radiological technique used to evaluate velopharyngeal dysfunction, especially for paediatric patients. A retrospective analysis over a period of 7 months involving 50 examinations yielded dose-area product values ranging from 0.04 Gy cm(2) (minimum) to 0.37 Gy cm(2) (maximum) with a mean value of 0.11 Gy cm(2) and 3rd quartile value of 0.12 Gy cm(2). The maximum effective dose was estimated as 0.016 mGy. An initial local diagnostic reference level of 0.12 Gy cm(2) has been levied.

A design study for a spiral staircase ionization chamber for the quality control of electron beams

In order to verify that the energies of electron beams used for external beam therapy remain constant, IPEM 81 recommends a constancy check based on the ratio of ionization chamber measurements at two depths along the central axis. Such measurements for a range of electron energies can be a time consuming process. The purpose of this study was to design a device that would use several ion chambers simultaneously to measure electron depth dose curves, and hence the electron energy. A design was developed for a device consisting of ten independent ionization chambers, shaped and arranged in a solid phantom like the steps of a spiral staircase, the axis of the staircase being coincident with the axis of the electron beam. Measurements were carried out to test the design of individual chambers and to optimize the radius of the spiral and both the depth intervals and the lateral spacing between adjacent chambers. For ranges of electron energy from 6-12 MeV and 12-20 MeV the radii of the spirals needed were found to be 36.5 mm and 30.9 mm, the angular separations between edges of the chambers were 52 degrees and 30 degrees and chamber depths were found to be 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 mm and 20, 40, 45, 50, 55, 60, 65, 70, 75, 80 mm, respectively.

A comparison of polyacrylamide gels and radiochromic film for source measurements in intravascular brachytherapy

For intravascular brachytherapy with catheter-based systems, AAPM Task Group 60 has recommended measurements that should be made to characterize the sources. Beta emitters, including (90)Sr/(90)Y are ideal for intravascular brachytherapy, but problems arise in measuring dose distributions in the high dose gradient region at short distances from the source. In this paper, measurements of radial and orthogonal dose distributions and dose profiles for a (90)Sr/(90)Y source train using polyacrylamide gel (PAG) dosimetry and a high-field 4.7 Tesla MRI scanner are presented and compared with measurements made with two types of radiochromic film, MD-55 and HD-810. For the PAG system, the dose distributions were determined with in-plane resolutions of 0.4 mm and 0.2 mm. The measurements of absorbed dose distributions both orthogonal and parallel to the source axis show good agreement between the PAG and radiochromic film. The absolute dose at a radial distance of 2 mm in the central 32 mm of a line parallel to the axis was measured. For the PAG the measured absorbed dose was 1.25% lower, for MD-55 4% higher and for the HD-810 1.6% higher when compared with the value given by the source calibration. These results confirm that both absorbed dose and dose distributions for high gradient vascular brachytherapy sources can be measured using PAG but the disadvantages of gel manufacture and the need for access to a high resolution scanner suggests that the use of radiochromic film is the method of choice.

Recording accelerator monitor units during electronic portal imaging: application to collimator position verification during IMRT

The application of multiple portal image acquisition to collimator position verification during dynamic multileaf collimation (DMLC) using a commercial camera-based electronic portal imaging device (EPID) (Theraview, Cablon Medical BV, Leusden, The Netherlands) mounted on an Elekta SL15i accelerator (Elekta Oncology Systems, Crawley, UK) is described. This is achieved using a custom-built dose acquisition system optically interfaced to both the camera control unit of the EPID and the monitor unit (MU) channel of the accelerator. The method uses the beam blanking camera control signal to trigger the dose acquisition system to read the cumulative accelerator MUs at the beginning and end of each period of image formation. A maximum delay of 15 ms has been estimated for recording of accelerator MUs in the current system. The camera interface was observed to have no effect on the operation of the EPID during normal clinical use and could therefore be left permanently in situ. Use of the system for collimator position verification of a test case is presented. The technique described uses a specific camera-based EPID and accelerator, although the general principle of using an EPID control signal to trigger recording of accelerator MUs may be applicable to other EPIDs/accelerators with suitable knowledge of the accelerator dosimetry system.

Measurement of the response of Gd2O2S:Tb phosphor to 6 MV x-rays

The phosphor GdO2S:Tb is widely used in camera-based electronic portal imaging devices (EPIDs). There is considerable interest in the application of EPIDs to dosimetry and the verification of intensity modulated radiation therapy produced by dynamic multileaf collimation (DMLC). This paper presents direct measurement of Gd2O2S:Tb phosphor luminescence under 6 MV x-ray irradiation from a linear accelerator using a photomultiplier tube. The luminescence following each radiation pulse (3 micros duration) was observed to decay with a dominant lifetime of 558 micros. Using a specialized electrometer, the temporal variation of the optical signal has been compared with the dose rate incident on the phosphor measured using a semiconductor diode detector. Under dose rates typical of those used in the clinic (1.2 Gy min(-1) to the phosphor), measurements at beam-start confirmed that the optical signal is linear with dose per radiation pulse. Measurements at beam termination following phosphor doses up to 4.4 Gy showed no residual signal associated with long-lived luminescence (afterglow) from the phosphor above the noise level of the optical signal (0.17% standard deviation). This measurement demonstrates that afterglow from Gd2O2S:Tb is not of significance for its application to DMLC verification. Additionally, it was confirmed that the accelerator pulse repetition frequency has no effect on the optical signal from the phosphor in the range 25-400 Hz.

A method for controlling image acquisition in electronic portal imaging devices

Certain types of camera-based electronic portal imaging devices (EPIDs) which initiate image acquisition based on sensing a change in video level have been observed to trigger unreliably at the beginning of dynamic multileaf collimation sequences. A simple, novel means of controlling image acquisition with an Elekta linear accelerator (Elekta Oncology Systems, Crawley, UK) is proposed which is based on illumination of a photodetector (ORP-12, Silonex Inc., Plattsburgh, NY, USA) by the electron gun of the accelerator. By incorporating a simple trigger circuit it is possible to derive a beam on/off status signal which changes at least 100 ms before any dose is measured by the accelerator. The status signal does not return to the beam-off state until all dose has been delivered and is suitable for accelerator pulse repetition frequencies of 50-400 Hz. The status signal is thus a reliable means of indicating the initiation and termination of radiation exposure, and thus controlling image acquisition of such EPIDs for this application.

Dosimetric properties of the Theraview fluoroscopic electronic portal imaging device

Electronic portal imaging devices (EPIDs) can be used for non-imaging applications in radiotherapy such as patient dosimetry. Of the systems available, the fluoroscopic camera-based EPID Theraview (InfiMed Inc.) has not been studied to date, and a review of the dosimetric properties of the system is presented here. In the "single set-up" mode of image acquisition, pixel intensity increases sublinearly with applied dose. The response was dependent on the system's video signal gain and showed a threshold dose to the detector in the range 0.05-0.35 cGy, and pixel saturation at detector doses in the range 1.2-1.6 cGy. Repeated exposures of the EPID were observed to be extremely reproducible (standard deviation 0.5%). The sensitivity of the system showed a linear decline of 0.04% day-1 over a 68-day period, during which time the relative off-axis response within 10 x 10 cm2 field was constant to within a standard deviation of 0.56%. The system shows spatial non-uniformity, which requires correction for application to dose measurements in two-dimensions. Warm-up of the camera control unit required a period of at least 40 min and was associated with an enhancement in pixel intensity of up to 12%. A radiation dose history effect was observed at doses as low as 0.2 Gy. Camera dark current was shown to be negligible at normal accelerator operation. No discernible image distortion was found. Mechanical stability on gantry rotation was also assessed and image displacement of up to 5 mm at the isocentre was observed. It was concluded that the device could be used for dosimetry provided necessary precautions were observed and corrections made.

The potential use of polymer gel dosimetry in boron neutron capture therapy

Polymer gels with and without 60 ppm of 10B were exposed to an epithermal neutron beam produced by the Dynamitron at the University of Birmingham on two separate occasions. Eight vials containing the gel, four with and four without boron, were irradiated in pairs in a water phantom for 5 h. The maximum dose was calculated to be 9 Gy in A-150 tissue equivalent plastic, 4 cm deep in the phantom. Measurements were made of the variation of relaxation rates of the gels with depth in a phantom. These were compared with calculations using the MCNP Monte Carlo program and the gel response followed the general trend of the results of the calculations. The calculations showed that the absence of boron gave 66.1% and 44.3% of the absorbed dose with boron and the measurements showed the response of the gel without boron to give 65+/-2% and 41+/-6% of the response with boron for the two halves of the first vial. All the gel measurements showed an enhancement in absorbed dose when boron was added. These results indicate that polymer gels may have a role in measuring the enhancement of absorbed dose due to boron in an epithermal or thermal neutron.

An investigation into the use of polymer gel dosimetry in low dose rate brachytherapy

An investigation has been carried out into the properties of the BANG polymer gel and its use in the dosimetry of low dose rate brachytherapy. It was discovered that the response of the gel was reproducible and linear to 10 Gy. The gel was found to be tissue equivalent with a response independent of energy to within experimental accuracy (standard error of measurement +/- 5%). The slope of the calibration curve was found to increase from 0.28 +/- 0.01 s-1 Gy-1 to 0.50 +/- 0.02 s-1 Gy-1 for an increase in monomer concentration from 6 to 9%. Absorbed dose distributions for a straight applicator containing 36 137Cs sources were measured using the gel and the results compared with measurements made with thermoluminescent dosemeters (TLDs) and calculated values. Good agreement was found for the relative measurements. The root mean square residual percentage errors were 3%, 1% and 4% for the gel and the two groups of TLDs, respectively. There were some significant differences in absolute values of absorbed dose in the gel, possibly owing to the effects of oxygen. Measurements of a complex gynaecological insert were also made and compared with isodose curves from a planning system (Helax TMS), and in areas unaffected by oxygen diffusion the isodose levels from 100 to 50% agreed to within less than 0.5 mm.

An evaluation of the potential and limitations of three-dimensional reconstructions from intravascular ultrasound images

Three-dimensional (3D) reconstructions of arteries can be produced using two-dimensional (2D) intravascular ultrasound (IVUS) images. Any artefact that affects 2D images has the potential to limit the quality of a 3D reconstruction. Using a catheter withdrawal technique, a range of test rigs were used to assess: (i) the effect of rotation of the probe orientation; (ii) the ability to reconstruct the true path of a tortuous vessel; (iii) the effect of image distortion on diameter measurements; (iv) the number of images per unit length used to produce a 3D reconstruction; and (v) the quality of the IVUS 3D reconstruction of a stent. These investigations show that 3D IVUS imaging is prone to artefacts. For 3D IVUS images to be used to quantify the vessel path or to make accurate measurements of vessel dimensions, more information about the catheter tip position and orientation is required than is currently available with the pullback technique.

Supplement to the code of practice for clinical proton dosimetry. ECHED (European Clinical Heavy Particle Dosimetry Group)

The 'Code of Practice for Clinical Proton Dosimetry' (Vynckier, S., Bonnett, D.E. and Jones, D.T.L. Code of practice for clinical proton dosimetry. Radiother. Oncol. 20: 53-63, 1991) was published in 1991, but since then new data for mass stopping powers have been reported and consideration has been given to the specification of absorbed dose in water instead of the original recommendation of absorbed dose in tissue. This supplement summarises the basic recommendations of the original Code of Practice and incorporates the new stopping power data for dose specification in water.

The development of an interdepartmental audit as part of a physics quality assurance programme for external beam therapy

A cost-effective audit system has been developed that will both detect systematic error in data and procedures and evaluate the quality assurance programme provided by a physics department for radiotherapy. The audit has been developed for external beam radiotherapy and assesses one modality and one treatment machine per year. The audit is carried out on an interdepartmental basis and can be undertaken by two physicists from each department in one working day. The method of assessing the quality assurance programme and the schedule of measurements are described. The process is illustrated using the results of trial audits between the medical physics departments at Coventry and Leicester.

The 62 MeV proton beam for the treatment of ocular melanoma at Clatterbridge

A second treatment room and beam line has been constructed at the Cyclotron Unit at Clatterbridge for the purpose of using 62 MeV protons for the treatment of ocular melanoma. A uniform beam is produced by a double foil scattering system. The initial Bragg peak is spread across the target volume by the use of beam modulators. These are rotating four-vaned stepped absorbers made from Perspex. Two beam lines can be configured with different positions of modulators and range limiters. The first has a maximum penetration of 31.9 +/- 0.2 mm in water and the second a penetration of 31.2 +/- 0.2 mm. The second configuration has the advantage of less variation in beam penumbra, with a typical value of 1.7 +/- 0.1 mm for the 90% to 10% decrement lines. The patients are treated with individually shaped collimators. Beam output varies by less than 2% over the range of collimator areas used. The resulting whole-body dose equivalent to patient has also been assessed. In the first three years of operation over 250 patients have been treated.

Current developments in proton therapy: a review

The use of high-energy protons in radiotherapy was first proposed in 1946. In the last decade there has been a significant growth in the number of centres using protons in the treatment of malignant and non-malignant disease. To date (January 1993) a total of more than 11,500 patients have been treated world-wide. Encouraging clinical results have been reported in the literature. The purpose of this article is to outline the advantages of proton beams and to review current developments in physics and engineering applied to the field of proton therapy with particular emphasis on proton accelerator technology and the development of proton therapy facilities. The production of clinically useful beams is discussed and the relative merits of different treatment systems compared. Reference is also made to the factors affecting the absorbed dose in a patient and to proton radiobiology together with the results of studies of comparisons of treatment planning with protons with that using conventional photon therapy. The dosimetry of proton beams is also reviewed.

An investigation of the dose equivalent to radiographers from a high-energy neutron therapy facility

The external radiation hazard to radiographers from the use of high-energy neutrons in radiotherapy has been investigated. The contributions from neutron-induced activity in the therapy gantry, the treatment room, the patient and ancillary equipment have been analysed as has the whole-body dose equivalent to radiographers. It was found that there are significant levels of gamma radiation throughout the treatment room, which increase both in the vicinity of the walls in line with the beam axis and in close proximity to the neutron therapy gantry. The mean dose equivalent to radiographers per field treated was found to be 5.1 +/- 1.8 microSv. The dose per field also varied considerably with the particular site being treated but it was found that the dose equivalent per field per minute of set-up time was approximately constant. It was also found that the dose per field increased with the number of patients treated per day commensurate with a build-up of induced activity. The studies also showed that the dose equivalent to radiography staff was comparable to that at other high-energy neutron facilities but significantly greater than that recorded at facilities with low-energy beams.

High energy neutron treatment for pelvic cancers: study stopped because of increased mortality

OBJECTIVE

To compare high energy fast neutron treatment with conventional megavoltage x ray treatment in the management of locally advanced pelvic carcinomas (of the cervix, bladder, prostate, and rectum).

DESIGN

Randomised study from February 1986; randomisation to neutron treatment or photon treatment was unstratified and in the ratio of 3 to 1 until January 1988, when randomisation was in the ratio 1 to 1 and stratified by site of tumour.

SETTING

Mersey regional radiotherapy centre at Clatterbridge Hospital, Wirral.

PATIENTS

151 patients with locally advanced, non-metastatic pelvic cancer (27 cervical, 69 of the bladder, seven prostatic, and 48 of the rectum).

INTERVENTION

Randomisation to neutron treatment was stopped in February 1990.

MAIN OUTCOME MEASURES

Patient survival and causes of death in relation to the development of metastatic disease and treatment related morbidity.

RESULTS

In the first phase of the trial 42 patients were randomised to neutron treatment and 14 to photon treatment, and in the second phase 48 to neutron treatment and 47 to photon treatment. The relative risk of mortality for photons compared with neutrons was 0.66 (95% confidence interval 0.40 to 1.10) after adjustment for site of tumour and other important prognostic factors. Short term and long term complications were similar in both groups.

CONCLUSIONS

The trial was stopped because of the increased mortality in patients with cancer of the cervix, bladder, or rectum treated with neutrons.

Code of practice for clinical proton dosimetry

The objective of this document is to make recommendations for the determination of absorbed dose to tissue for clinical proton beams and to achieve uniformity in proton dosimetry. A Code of Practice has been chosen, providing specific guidelines for the choice of the detector and the method of determination of absorbed dose for proton beams only. This Code of Practice is confined specifically to the determination of absorbed dose and is not concerned with the biological effects of proton beams. It is recommended that dosimeters be calibrated by comparison with a calorimeter. If this is not available, a Faraday cup, or alternatively, an ionization chamber, with a 60Co calibration factor should be used. Physical parameters for determining the dose from tissue-equivalent ionization chamber measurements are given together with a worksheet. It is recommended that calibrations be carried out in water at the centre of the spread-out-Bragg-peak and that dose distributions be measured in a water phantom. It is estimated that the error in the calibrations will be less than +/- 5% (1 S.D.) in all cases. Adoption and implementation of this Code of Practice will facilitate the exchange of clinical information.

A comparison of two methods of in vivo dosimetry for a high energy neutron beam

Two methods of in vivo dosimetry have been compared in a high energy neutron beam. These were activation dosimetry and thermoluminescence dosimetry (TLD). Their suitability was determined by comparison with estimates of total dose, obtained using a tissue equivalent ionization chamber. Measurements were made on the central axis and a profile of a 10 x 10 cm square field and also behind a shielding block in order to simulate conditions of clinical use. The TLD system was found to provide the best estimate of total dose.

Changes in biological effectiveness of the neutron beam at Clatterbridge (62 MeV p on Be) measured with cells in vitro

Chinese hamster V79 cells have been used to assess changes in RBE of the p(62)Be neutron beam at the Clatterbridge Hospital with depth in a phantom and with use of a hydrogenous filter. The cells were exposed at depths of 2 and 12 cm and at a depth of 2 cm with a hydrogenous filter. Two groups of experimenters each conducted two experiments. The ratios of relative biological effectiveness (RBE) at a depth of 12 cm to that at 2 cm were found by the two groups to be 0.99 +/- 0.04 and 0.96 +/- 0.02 (standard errors). The effect of a polythene filter 4.5 cm thick was measured at a depth of 2 cm and the ratio of RBE with and without the filter was found by both groups to be 0.99 +/- 0.02. All the experiments suggest that there may be small effects of beam hardening by depth and filtration but these results are in marked contrast with those obtained using an in vivo system.

Changes in relative biological effectiveness with depth of the Clatterbridge neutron therapy beam

We have measured the biological equivalence of the Clatterbridge neutron therapy beam [p(62)-Be] and the Hammersmith neutron therapy beam [d(16)-Be] using the mouse intestinal crypt assay. The ratio (NDR) of Clatterbridge neutron (n + gamma) dose relative to Hammersmith neutron dose (n + gamma) was found to be 1.2-1.13 over a dose/fraction range of 1.8-9 Gy at 2 cm deep in a Perspex phantom. It is shown that the effectiveness of the Clatterbridge beam was reduced with penetration into the phantom because of hardening of the beam to a maximum reduction of 11% at 12 cm deep in the phantom. The hardening of the beam with depth of penetration will need to be taken into account by clinicians in assessing the tumour dose and tissue tolerance. Relative biological effectiveness values for the Clatterbridge and Hammersmith neutron beams were also measured. All neutron doses for both Hammersmith and Clatterbridge beams are total doses (n + gamma) which comply with the European protocol for neutron dosimetry and include the gamma-ray component of dose.

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%.

The Clatterbridge high-energy neutron therapy facility: measurements of beam parameters for clinical use

Measurements have been performed on the 62 MeV proton cyclotron at the Douglas Cyclotron Centre, Clatterbridge Hospital, to determine the variation in beam parameters necessary for clinical use of the neutron therapy facility. These measurements are of total (neutron and gamma) doses, and include: depth doses for wedged and unwedged fields at various treatment distances; profile measurements and the production of associated isodose charts; calibration of the dosimetry system of the cyclotron; and determination of variations in calibration associated with changes in field size, wedge and focus-skin distance. Measurements have also been performed to estimate the degree of long-term stability of both calibration and field uniformity.

The Clatterbridge high-energy neutron therapy facility: specification and performance

A new high-energy neutron therapy facility has been installed at the Douglas Cyclotron Centre, Clatterbridge Hospital, Merseyside, in order to extend the clinical trials of fast neutrons initiated by the Medical Research Council. The neutron beam is produced by bombarding a beryllium target with 62 MeV protons. The target is isocentrically mounted with the potential for 360 degrees rotation and has a fully variable collimator. This gives a range of rectilinear field sizes from 5 cm x 5 cm to 30 cm x 30 cm. Basic neutron beam data including output, field flatness, penumbra and depth-dose data have been measured. For a 10 cm x 10 cm field, the 50% depth dose occurs at 16.2 cm in water and the output is 1.63 cGy microA-1 min-1 at the depth of dose maximum. The effectiveness of the target shielding and the neutron-induced radioactivity in the treatment head have also been measured. It is concluded that the equipment meets both the design specifications and also fully satisfies criticisms of earlier neutron therapy equipment. A full radiation survey of the centre was also carried out and it was found that radiation levels are low and present no significant hazard to staff.

Fast neutron therapy at Edinburgh: staff protection

The major hazards encountered by staff using neutrons for radiotherapy are discussed. Specific reference is made to the experience gained at the MRC Cyclotron Unit at the Western General Hospital, Edinburgh, using neutrons generated by the d(15 MeV) + Be reaction. The neutron therapy facility consists of a cyclotron and both a fixed horizontal and an isocentric therapy beam, and staff protection during five years' operation is reviewed. Levels of induced activity in the cyclotron and therapy equipment are reported and problems of radioactive contamination discussed. Summaries of whole-body and finger dose equivalents received by engineering staff, and of whole-body dose equivalents received by physics and radiography staff, are presented and analysed. It is shown that, although doses received by staff are higher than for staff in an X-ray facility, they are all well below the maximum permissible levels, and it is also concluded that radioactive contamination of staff is minimal.

Effect of variation in the energy spectrum of a cyclotron-produced fast neutron beam in a phantom relevant to its application in radiotherapy

The fast neutron spectrum of the beam produced by the MRC cyclotron at the Hammersmith Hospital has been measured in air and at several depths in a water phantom using three field sizes. The neutron spectra were determined both by a set of six threshold detectors and by a liquid scintillator spectrometer. Where a direct comparison of the two methods was possible the agreement was satisfactory. It was found that the shape of the spectrum, above 3 MeV, is unchanged with depth in the phantom, but for each field size the neutron fluence between 0.5 and 3.5 MeV increases significantly and reaches a maximum and the mean energy a minimum. This minimum decreases with increase in field size. Values of the kerma ratio in ICRU muscle to both that in A--150 plastic and bone were calculated and found to vary by less than 0.5% with depth in the phantom: that of carbon to ICRU muscle varied by 5%, and values of W for methane-based tissue-equivalent gas were found to change by less than 1%. Variations in biological response with changes in neutron spectrum are also discussed, together with the clinical significance of the results.

The isocentric fast neutron therapy facility at Edinburgh

A second fast neutron beam for radiotherapy has been brought into operation at the Western General Hospital, Edinburgh. This beam is isocentrically mounted and can be rotated through +/- 120 deg from the vertical. Neutrons are produced by bombarding a thick beryllium target with 15 MeV deuterons. The provision of an isocentric facility enables some of the problems associated with poor beam penetration to be alleviated. The physical features of the isocentric facility are described with particular reference to the shielding and the methods employed to overcome problems of neutron activation. Both the prompt radiation leakage and the induced activity have been measured. It has been found that for the first eight months of operation the whole body dose-equivalent to radiographers was approximately 6 microSv (0.6 mrem) per patient treated. Dosimetry associated with the facility is discussed and an example of a seven field treatment plan for a bladder is presented.

The fixed horizontal neutron therapy beam at Edinburgh: dosimetry and radiation protection

A compact cyclotron producing 15 MeV deuterons has been installed at the Western General Hospital, Edinburgh, to extend the Medical Research Council's clinical trials of fast neutrons. Two treatment rooms are available one of which has an isocentric unit and the other a fixed horizontal beam which is the subject of this paper. A radiation protection survey has demonstrated safe levels of radiation throughout the building although there is some activation of the fixed horizontal beam cone resulting in doses to radiographers of 10 mrem per week. The calibration of neutron dose is discussed and the measurements of dose distribution described. Isodose and depth-dose data for both the neutron and photon components of the field are presented.