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.

In the last few years there has been considerable interest in the use of polymer gels to measure complex dose distributions in radiotherapy. Despite considerable advantages they are still not widely used in clinical situations. This is due primarily to the difficulty in manufacture, particularly the need to exclude oxygen both from the gel and the manufacturing process, the limited number of suitable phantom materials and the need for easy access to an MRI facility. The purpose of this paper is to report on an investigation of the basic properties of MAGIC gels namely: linearity of response, effects of temperature and stability.

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.

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 90Sr/90Y 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 90Sr/90Y 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.

Electronic portal imaging (EPI) has been used in the assessment of the accuracy of radiotherapy treatments in the pelvis. The daily reproducibility of any treatment is primarily dependent on the effectiveness of the set-up method. Treatments of radical planned volumes within the pelvis are subject to field placement errors (FPE) which could potentially compromise the successful outcome of radiotherapy treatment. Increasing use of shaped treatment fields to limit the dose delivered to surrounding normal tissues has prompted a more detailed examination of set-up methods.Within the radiotherapy department at the Leicester Royal Infirmary it was noted that tattooed marks on the anterior or posterior and the lateral skin surface, marked at simulation, could not always be aligned for daily radiotherapy treatment.An assessment of the relative merit of the anterior/posterior or the lateral tattoos in determining the isocentre position in the superior-inferior (cranio-caudal) plane is presented. This study showed that changes in the use of tattoo marks during set-up would have reduced the incidence of FPE >5 mm for a small sample group of patients. Implementation of changes in clinical practice, or research using a larger sample group is now needed to verify any improvement in accuracy using a modified treatment set-up technique.

The phosphor Gd2O2S: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 µs duration) was observed to decay with a dominant lifetime of 558 µs. 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.

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.

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.

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.

s: Abstracts of the British Nuclear Medicine Society Autumn Meeting. Nottingham, 9-10 September 1999

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.

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.

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 ‘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 '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.