British Journal of Radiology 74 (2001),537-547 © 2001 The British Institute of Radiology
Performance assessment of the Gulmay D3300 kilovoltage X-ray therapy unit
P A Evans, BSc, MSc
A J Moloney, BSc
and
P J Mountford, PhD, FIPEM
Medical Physics Directorate, North Staffordshire Hospital, Royal Infirmary, Princes Road, Hartshill, Stoke-on-Trent, Staffordshire ST4 7LN, UK
 |
Abstract
|
|---|
A performance assessment was made of the Gulmay D3300 kilovoltage (combined superficial and orthovoltage) X-ray therapy unit. Results are presented for the key dosimetric beam parameters required for routine patient treatment. This unit is relatively new to the UK market and displayed similar properties to other existing equipment. Beam half-value layers were different from comparable published data, but were consistent with the actual values of external tube filtration employed. The applicator, system interlocks and dose monitor performance were satisfactory and the tube leakage was below the UK recommended maximum (air kerma rate 300 mGy h-1 at 5 cm from the tube head). The variation of absorbed dose with stand-off distance from the applicator base followed the inverse-square law for all tested combinations of beam tube potential (kVp) and applicator, and the measured focus-to-surface distances were in acceptable agreement with the nominal values. A significant beam profile asymmetry was seen for field sizes greater than 10 cm at the upper tube potential (kVp) range (maximum ionization quotient 1.08), but this was an inherent property of the X-ray tube. The difficulties of obtaining percentage depth dose measurements are discussed, and it was concluded that the use of published data (appropriately verified) was acceptable. The methodology followed could form the basis of an acceptance and commissioning protocol. To address the relative lack of agreed standards for this type of equipment, performance test tolerances are proposed that are recommended for new installations.
|
Introduction
|
|---|
There are many applications of kilovoltage X-rays in radiotherapy [1, 2], including the treatment of basal or squamous cell carcinomas of the skin and the palliative irradiation of bone metastases. Kilovoltage therapy beams have been conventionally separated into low energy or superficial X-rays, typically generated at tube voltages of 50–160 kV, and medium energy or orthovoltage X-rays, generated over the range 160–300 kV [3].Until recently, it was common to use two separate items of equipment for these different beam energy ranges, i.e. dedicated superficial and orthovoltage treatment units. However, beams encompassing the whole quality range can now be obtained from a single unit [4, 5]. The advantages of such a combined unit are cost savings involved in halving the number of shielded treatment rooms required, and reductions in capital costs, spares, preventative maintenance and quality control resources. The potential drawbacks to this approach include the compromise in beam characteristics necessitated by the use of a single X-ray tube to generate beams at all these energies and the possible loss of all kilovoltage treatment capacity if the combined unit should fail in operation.
The purpose of this investigation was to summarize the assessment process of such a unit for which there are no published data (Gulmay D3300; Gulmay Ltd., Chertsey, UK). This is similar in design to the Therapax DXT-300 (Pantak Inc., Branford, CT), which has been extensively investigated by Aukett et al [4] and Gerig et al [5]. The main differences between the units are in the high tension generator and in the control and interlock systems.
Data are presented to address the lack of published guidance on the performance to be expected from combined kilovoltage units, with particular regard to applicator specifications, beam profile and depth dose characteristics, and the variation of dose with applicator stand-off distance. This investigation should also inform other kilovoltage unit users, or potential purchasers, of recent technical developments in this field and enable a comparison to be made with existing units, with reference to beam data and equipment characteristics.
|
Description of the unit
|
|---|
The Gulmay D3300 is a combined superficial and orthovoltage X-ray therapy unit able to produce tube generating potentials in the range 40–300 kV. It comprises a ceiling or floor mounted tubestand, a bipolar oil-cooled metal ceramic X-ray tube (Comet MXR 321; Comet AG, Liebefeld-Berne, Switzerland), a high stability generator (Gulmay CP320) and a software-based user interface and controller (Gulmay TP1). The system is fully user-configurable across its range of operation with regard to tube potential (kVp), tube current (mA) and external tube filtration, making it possible to obtain a wide range of beam qualities. A range of circular open-ended applicators (30 cm focus-to-surface distance (FSD)), and square and rectangular closed-ended applicators (50 cm FSD) were available. They comprised an upper, circular mounting ring containing lead collimation and the primary beam aperture, to which was attached a rectangular or circular section copper-walled cone, which terminated in an acrylic end-piece to facilitate patient set-up. The end-piece was engraved with red marker lines to indicate the beam centralaxis position (Figure 1
). Novel features include a binary encoded array of proximity switches in the applicator mounting, which allows unique identification of each applicator, with a select and confirm interlock.
View larger version (23K):
[in this window]
[in a new window]
|
Figure 1. Schematic diagram of applicator design and geometrical alignment testing. FSD, focus-to-surface distance.
|
|
The dose monitoring system comprises an unsealed parallel plate ionization chamber and electrometer (Gammex RMI Ltd., Nottingham, UK). Compensation for the monitor chamber response owing to ambient temperature and pressure conditions are made automatically via software corrections. Temperature data are obtained from a copper–constantan (type T) thermocouple sensor, in close proximity to the monitor chamber. Ambient atmospheric pressure data are derived from a pressure transducer (type PX222; Omega Engineering Inc., Stamford, CT).
Dose output calibration of the unit is achieved by determining the absorbed dose delivered to a reference point for a fixed time using one reference applicator, which must be defined for each beam energy in use. Software corrections are then stored, which set the output at 1 unit of absorbed dose per monitor unit (MU) set, at a nominal dose rate in MU per min. When any other applicator is used with a particular beam energy, the monitor chamber signal is corrected in software for the effect of the change in the magnitude of radiation backscattered into the monitor chamber. This correction results in the same displayed dose rate for any beam, irrespective of the applicator selected.
The system also includes a dose rate interlock, which monitors the steady-state beam dose rate against the stored nominal value (obtained at calibration), and terminates the beam for values outside ±3% of the nominal value. This interlock feature has the advantage of detecting significant changes in beam dose rate owing to, for example, target material deposition on the tube exit window, any high tension cable/generator fault or any failure in the monitor chamber. To guard against primary dose monitor failure, an independent back-up timer is automatically set by the software system to terminate treatment at a time limit that is 5% greater than that calculated on the basis of the stored nominal dose rate and the number of MUs set.
|
Performance assessment procedures
|
|---|
Applicator assessment
The geometry of each applicator acrylic end-piece was tested by rotating it through 360° about its axis, using the mount on the tube head and an externally positioned dial gauge. In addition, movement of the inscribed beam central axis on the base plate of each rectangular applicator was checked by a similar 360° rotation but with reference to a fixed pointer. For every applicator, a height gauge was used to check the maximum and minimum distance of the acrylic end-piece edges or base plate above the applicator mounting ring. The orthogonality of the inscribed lines indicating the beam axis was checked using graph paper. A schematic applicator diagram and an illustration of these tests are given in Figure 1
.
To investigate any distortion of the applicator construction, thin solder wire was attached to thebase, coincident with the engraved lines indicating the beam axis. A therapy level film (Kodak X-Omat V; Eastman Kodak Company, New York, NY) was exposed to an approximate optical density of 1.0. The films were examined to determine the displacement between the solder crosswire image and the geometrical centre of the radiation field, and also the difference between the actual radiation field dimensions and the nominal indicated sizes. In each case the field shape and size was also visually inspected for any significant (larger than 2 mm) deviations from the stated geometry. The films were also examined for any evidence of detectable radiation leakage outside the geometrical field, other than that due to scattering out of the main beam.
Beam and applicator rotational axis alignment
Alignment of the beam and applicator rotational axes was performed in accordance with theprocedure specified by Massey [6]. The 10 cm x 12 cm applicator with a 0.5 mm diameter lead pinhole collimator (thickness 5 mm) positioned in the primary aperture of the mounting ring was used with the beam orientated vertically. Thin solder wire (0.5 mm diameter) was taped over the engraved cross on the acrylic base of the applicator. A Kodak X-Omat V film was positioned below the applicator and irradiated so as to produce an optical density of about 1.0, then the applicator was rotated 180° about its axis and the exposure repeated. After allowance for magnification, the processed film was used to determine the displacement between the centre of the two focal spot images and the centre of the two crosswire images (Figure 2). This was repeated for each beam quality in clinical use.
Tube head leakage
The areas of maximum tube head leakage were identified using therapy verification film (CEA TVS EP; CEA AB, Strängnäs, Sweden) wrapped around the tube head so as to give full coverage. A 6 mm thick lead run-up filter, acting as a beam stop, was used to minimize any scattered radiation reaching the films. An exposure of 15 min at 280 kVp and 10 mA was made, and the areas around the head with detectable leakage were identified. The leakage air kerma rate for the maximum selectable tube voltage of 280 kVp was measured at these positions using a Keithley 35050A dosemeter and type 96035, 15 cm3 parallel plate chamber (Keithley Instruments Inc., Cleveland, OH). This instrument was subject to a recent calibration (traceable to the National Physical Laboratory (NPL), Teddington, UK). The leakage measurements were repeated with an NE 2571 chamber (graphite thimble, 0.6 cm3 Farmer-type chamber; NE Technology Ltd., Reading, UK) and Unidos electrometer (PTW GmbH, Freiburg, Germany). This meter had an independent therapy level air kerma calibration at the appropriate tube potential (kVp) setting, traceable to the NPL national standard [3]. Both sets of data were corrected for ambient pressure and temperature conditions.
Testing of interlocks
Each of the 18 clinical applicators was mounted in turn on the tube head and the interlock system was tested to verify unique applicator confirmation. A similar procedure was used for the four clinical external filters. Software pressure and temperature data were set using calibrated instruments (traceable to the NPL). The dose monitor correction system was tested for response to ambient conditions by entering incorrect data into the software system, such that dose output changes of ±1% were induced (about 10 mbar pressure and 3 °C temperature errors). The dose rate interlock was similarly tested by entry of increasing pressure or temperature errors until the interlock operated.
Beam half-value layer
Beam half-value layer (HVL) was measured in accordance with the method given by Massey [6] and Klevenhagen and Thwaites [7], using a horizontal beam axis and thin filters of high purity aluminium (99.99%) or copper (99.9%) (Goodfellow Ltd., Cambridge, UK). Corrections were made to account for the change in air kerma calibration factor with beam quality [3]. First and second half-value layers were derived by linear interpolation.
Beam profile symmetry
Beam profile symmetry was evaluated using ionization chamber scans across the tube anode–cathode and perpendicular axes. For each beam energy, the largest, smallest and one intermediate applicator size were examined. Scans were made in water across the base of the applicators, at a depth of 2 cm for the 180 kVp and 250 kVp beams, and at 1 cm depth for the 70 kVp and 100 kVp beams. RK chambers (0.12 cm3 cylindrical design; Scanditronix Medical AB, Uppsala, Sweden) were selected as the scanning and reference detectors [8]. Beam profile symmetry quotient Q80 was evaluated in each axis over 80% of the full field size at depth and was expressed as a maximum ionization quotient Q80, max evaluated at points equidistant (x+, x-) from the true beam central axis. Ix+ and Ix- were the respective ionization values obtained over the range stated. The parameter is defined in Equation (1).
|
Percentage depth doses
Central axis percentage depth doses for the 180 kVp and 250 kVp beams were determined by ionization measurements at depths of 1 cm, 2 cm, 5 cm and 10 cm using the NE 2571 chamber. This was protected by a Perspex sheath of wall thickness 1 mm, in an open water phantom of dimensions 30 cm x 30 cm x 30 cm. In all cases, the readings at 2 cm depth were normalized to published data [9], and any difference between the measured and published values was calculated as a local percentage at the other depths. Published data for closed-ended applicators at a HVL of 0.5 mm Cu and 2.0 mm Cu were interpolated by field size using the equivalent field technique [9]. Applicator sizes of 4 cm x 4 cm, 10 cm x 12 cm and 20 cm x 20 cm were investigated with the base in contact with the water surface.
For the 70 kVp and 100 kVp beams, a similar technique was used, but 10 cm and 5 cm diameter circular applicators were chosen to ensure full beam coverage of the chamber volume, with readings taken at depths of 1 cm, 2 cm and 5 cm, with 1 cm being the depth selected for normalization. The published depth dose data were linearly interpolated using the measured HVLs of each beam (Table 1).
Applicator output measurements
For the 180 kVp and 250 kVp beams, output measurements were made according to the 1996 IPEMB code of practice [3], in the open water phantom described above. Initially, this method was applied to the reference applicator defined within the Gulmay unit (10 cm x 12 cm at 50 cm FSD). The measurements were repeated in a WT1 phantom (Radiation Physics Department, St Bartholomew's Hospital, London, UK) of similar dimensions. Appropriate phantom factors, to convert absorbed dose determined in the WT1 phantom to that obtained at the same depth in pure water, were derived as a function of beam quality. The relative output of the remaining 50 cm FSD applicators was measured in the same manner using the open water phantom, in accordance with Klevenhagen et al [3] and using depth dose data from BJR Supplement 25 [9].
These methods were repeated for the low X-ray beam energies of 70 kVp and 100 kVp, although here the NE 2571 chamber was positioned in air with a 10 cm diameter circular reference applicator. Relative outputs for smaller applicators were determined using an NE 2515/3, 0.22 cm3 graphite Farmer chamber to minimize the ionization gradient along the chamber axis with the smallest (2.5 cm diameter) applicator. Corrections were made for the variation of backscatter factor with field size as given in Klevenhagen et al [3]. For both low and medium energies, the reference applicator output variation with tube head orientation was also assessed. This was expressed as a maximum percentage deviation from the value obtained with the standard vertical beam orientation routinely used for output measurements.
Stand-off measurements
Stand-off measurements were made for the smallest, largest and one medium sized applicator for each beam quality. For the 70 kVp and 100 kVp beams, the NE 2515/3 chamber was positioned in air with the effective measurement point on the beam axis at distances of 0.43 cm (stem radius) to 5.43 cm from the base of the applicator. This chamber was selected for the reasons stated above. Ionization readings were obtained for a fixed number of MUs set.
For the 180 kVp and 250 kVp beams, the NE 2571 chamber was placed at a depth of 2 cm in the WT1 phantom. Stand-off distances of 0–5 cm between the phantom surface and applicator base were used. Corrections were made to the readings to account for the change in depth dose at 2 cm depth with applicator stand-off distance from the phantom surface [9]. All results were checked by linear regression analysis for the validity of the inverse-square law (ISL) by plotting the square root of the reciprocal mean reading against stand-off distance. Where the ISL was valid, the true applicator FSD was obtained from the distance axis intercept [7].
Performance of the dose monitor
At each beam quality, the performance of the dose monitor was tested for reproducibility, linearity and the effect of treatment interruptions. Dose reproducibility was determined by taking 10 consecutive readings (100 MU set) with a reference applicator output set-up, and calculating the standard deviation expressed as a percentage of the mean. Linearity was assessed from a linear regression fit to subsequent mean chamber readings for monitor settings of 20 MU, 50 MU, 100 MU, 200 MU, 500 MU and 1000 MU. Linearity performance was also expressed as a maximum percentage and dose variation from exact linearity, with reference to the 100 MU mean, as specified in IPSM Report 54 [10]. The effect of interruptions in treatment was examined by taking several 100 MU chamber readings at each beam quality, with each exposure having three additional stop and start actions included. The results were expressed as a maximum percentage and dose difference from the mean values obtained for the reproducibility test. The reproducibility and linearity of the measurement equipment, in the range of use, had been previously established using a 90Sr check source (NE 2503/3; NE Technology Ltd., Reading, UK; approximate activity 210 MBq). Deviation from an ideal response was shown to be negligible relative to the IPSM specification [10].
Long-term stability of the Gulmay dose monitor for each beam in clinical use was determined from 50 output measurements taken over 1 year, during which the stability of the measuring equipment was also verified using the 90Sr check source. The maximum change in response recorded was 0.7%.
|
Results and discussion
|
|---|
Applicator assessment
All 30 cm FSD applicators were found to be satisfactorily regular, with end-piece rotational movement less than 1 mm and base height differences less than 0.2 mm. The engraved beam axis markers were orthogonal within 0.5° and indicated the true beam axis within ±1.5 mm. The measured beam diameters were within ±2 mm of those stated and no visible field defect or leakage outside the main beam was seen.
For the 50 cm FSD applicators, the engraved beam axis and the rotational axis positions were coincident within 1 mm, and the acrylic end-plate height differences were less than 0.6 mm. The engraved beam axis markers were orthogonal within 0.5° and indicated the true radiation axis within ±1 mm. The measured beam dimensions were within ±1.5 mm of those stated and again no visible field defect or leakage outside the main beam was seen.
Beam and applicator rotational axis alignment
A consistent difference or alignment error of 1.2 mm was found, which was independent of beam quality. In these circumstances, an asymmetry of the beam penumbra may become apparent owing to the asymmetric X-ray incidence on the acrylic end-piece walls of the applicator. The alignment error was minimized by relocating the applicator mount on the tube head until the error was reduced to less than 0.2 mm.
Tube head leakage
The positions of maximum leakage, as determined from the film survey, were the areas around the cathode high tension cable input, the front of the tube near the filter insert slot and adjacent to the focal spot position indicator. The current UK guidance [11] specifies a maximum air kerma rate of 300 mGy h-1 at 5 cm from the tube head and 10 mGy h-1 at 1 m from the focus. The results at 5 cm from the tube housing are given in Table 2. These results are about an order of magnitude less than the specified limit. Measurements at 1 m from the tube focus could not be distinguished from leakage currents. If the ISL is assumed to apply to this leakage radiation, it can be shown that the air kerma rates at 1 m from the focus will be substantially less than the above limit.
The calibration factors (true air kerma rate/instrument reading) quoted for the Keithley chamber/electrometer only applied to X-rays generated from 50–120 kVp, and the change in response over this range was approximately 5%. The factor for 120 kVp was used for these measurements, since above 50 kVp the calibration factors are less than 1.0, which means that uncorrected air kerma rate measurements above this quality would overestimate the true values. The differences between the Keithley and NE 2571 chambers are probably related to their volumes (approximately 15 cm3 and 0.6 cm3, respectively). The survey films showed that the leakage radiation had an irregular spatial distribution, and therefore the air kerma rate measured with each chamber will be very dependent upon its position. The specification [11] allows for averaging over an area of 10 cm2 for the measurements at 5 cm from the tube housing, which would suggest that the Keithley chamber was the more reliable estimate since its surface area was similar to that specified.
Testing of interlocks
The applicator and filter interlocks were tested and found to perform as specified by the manufacturer. Reference applicator output measurements demonstrated that the dose monitor system is configured to deliver the same absorbed dose per set MU, irrespective of pressure and temperature conditions, as it would under standard conditions (1013 mbar and 20.0 °C). The dose rate interlock was found to activate at the boundaries of the ±3% window specified.
Beam half-value layer
The HVL results obtained are given in Table 1. They differ from those given for similar tube potential (kVp) values by Aukett et al [4] and Gerig et al [5] for the Pantak DXT-300, and with those stated in Table C.1 of BJR Supplement 25 [9], because different external filtration was required by local practice.
Beam profile symmetry
Maximum symmetry quotient values are given in Table 3, and a representative sample of beam profiles, illustrating some of the salient features, are shown in Figure 3. The beam asymmetry was generally greatest along the tube anode–cathode axis and tended to increase with applicator size and tube potential (kVp) setting. The results for the smallest 2.5 cm circular and 4 cm square applicators were probably affected by the pronounced dome-shaped profile seen at these beam sizes. This effect produces a steeper ionization profile gradient at the beam edges, which may then, with detector positional uncertainties, exaggerate any beam asymmetry present.
View larger version (34K):
[in this window]
[in a new window]
|
Figure 3. Beam profile example data. 250 kVp along (a) the anode–cathode axis and (b) the perpendicular axis; and 70 kVp along (c) the anode–cathode axis and (d) the perpendicular axis.
|
|
The pattern of results was consistent with the expected tube heel effect and the corresponding data obtained by Aukett et al [4] and Gerig et al [5]. The use of a single tube over a wide kilovoltage range with a fixed target angle must be expected to exhibit such beam profile characteristics [7].
Percentage depth doses
For medium energy (160–300 kVp) beams, the Gulmay dose monitor was calibrated to deliver 1 cGy per MU applied (100%) dose, yet the appropriate dosimetry code of practice [3] gave the absorbed dose at a depth of 2 cm in water. Thus, the determination of percentage depth dose data to relate the two points is vital for accurate dosimetry. A common UK practice is to use published depth dose data [9], whose applicability has been verified by relative depth measurements, and where the data have been selected on the basis of the measured first HVL. Measurements for the Gulmay unit are compared with published data [9] in Table 4b. The effective point of measurement of the NE 2571 ionization chamber was taken to be on the axis of the thimble [8]. The maximum difference in local dose between measured and published data was about 6% at 10 cm depth and 1% at 5 cm depth in the phantom. In the absence of definitive results to the contrary, it was considered justified to use the published depth dose data for the determination of applied doses.
View this table:
[in this window]
[in a new window]
|
Table 4. (b) Measured (Meas) percentage depth doses normalized to BJR Supplement 25 [9] at depth 2 cm, and the percentage difference (%Diff) in local dose at depths 1 cm, 5 cm and 10 cm
|
|
The equivalent depth dose comparison for the low energy 70 kVp and 100 kVp beams are listed in Table 4a. The maximum difference in local dose at 5 cm depth was approximately 6%. This was accepted as satisfactory agreement since the accuracy of the data did not directly affect output dosimetry at these qualities [3], and the data are used clinically for guidance only.
View this table:
[in this window]
[in a new window]
|
Table 4. (a) Measured (Meas) percentage depth doses normalized to BJR Supplement 25 [9] at depth 1 cm, and the percentage difference (%Diff) in local dose at depths 2 cm and 5 cm
|
|
The NE 2571 chamber was selected on the basis of its uniform energy response over this beam quality range [4, 8, 12]. It was impossible to obtain an accurate surface measurement with thisdetector, restricting its use to relative depth measurements. In addition, particularly for beam dimensions less than 4–5 cm, there will be a significant dose gradient across the chamber axis that will vary with depth, thus distorting the measurement of depth doses. This feature may explain, at least in part, the pattern of 180 kVp and 250 kVp depth doses where the smallest (4 cm x 4 cm) applicator yielded consistently greater depth dose values than the published data [9] compared with the larger applicators, where the differences were smaller.
Applicator output measurements
The phantom factors, as defined above, for each of the medium energy X-ray beams are given in Table 5. On the basis of these results, the difference in radiation absorption and scattering properties between WT1 and water became progressively greater as the beam quality was reduced. Therefore, this material cannot be used for output determination in the medium X-ray energy range [3] without the application of appropriate measured correction factors. It must also be noted that these factors only relate to an applicator size of 12 cm x 10 cm at a depth of 2 cm, and must be expected to change for other field sizes or measurement depths.
The effects of tube orientation on output were negligible (Table 6). Relative applicator output data are given in Table 7. The data are normalized to the reference applicators and are given units of MU cGy-1 at the beam axis, on the surface of a water phantom.
Stand-off measurements
Stand-off data were consistent with the ISL, yielding regression coefficients of 0.998 to 0.999 and measured applicator FSDs in good agreement with their respective nominal values (Table 8). It was inferred from these data that corrections for stand-off from all clinical applicators may be made according to the ISL, using the nominal applicator FSD. On this basis, for applicator-to-surface stand-off distances of up to 2 cm, and theapplicator whose measured FSD was most different from its nominal value, an acceptable error in delivered dose of 0.5% would be made.
View this table:
[in this window]
[in a new window]
|
Table 8. Stand-off measurements, showing a comparison of measured and nominal focus-to-surface distance (FSD) values for a range of different applicator and tube potential (kVp) combinations
|
|
Performance of the dose monitor
An analysis of dose monitor performance for each beam is given in Table 9, and the requirements for acceptable performance stated in IPSM Report 54 [10] were satisfied. A summary of the long-term performance of the dose monitor is given in Table 10. This level of dose output consistency over a 1-year period was satisfactory in relation to the comparable performance of a linear accelerator. The threshold for re-calibration in our existing quality control programme is set at four consecutive output values outside the 1 cGy MU-1±2% range, or 1 value outside the ±3% range, and would be invoked only on a timescale of several months.
|
Conclusion
|
|---|
The performance of the Gulmay D3300 unit was broadly similar in many respects to that of alternative available equipment [4, 5]. The characteristics of the applicator set were satisfactory for clinical use. The purpose of these applicator tests was to ensure that the beam characteristics would not be unduly influenced by geometrical misalignment of any applicator. Furthermore, the alignment of the beam and applicator rotational axes, measured using a single applicator, would then be valid for the full set. By comparison with megavoltage X-ray beam specifications [10], true applicator beam dimensions and the indication of the beam axis should be accurate within ±2 mm of the stated values.
The symmetry of the 70 kVp and 100 kVp X-ray beams was acceptable over their full applicator range. For the 180 kVp and 250 kVp beam profiles, symmetry quotients of up to 1.08 were obtained and the significance for patient dosimetry must be considered. Symmetry quotients greater than 1.10 have been reported for comparable units with an identical X-ray tube operated at 300 kVp [4, 5]. Thus, this level of uncorrected beam profile asymmetry is a limitation of tube design and operation. A possible means of reducing this beam asymmetry would involve the use of an aluminium flattening filter in addition to the existing external filtration [13]. Perhaps the most practical solution is to ensure that the staff operating the unit are fully aware of the profile asymmetry and can take appropriate action, such as orientating the longest field dimension along the tube perpendicular axis, thus minimizing the impact on delivered dose uniformity.
Measured depth dose data with notable differences have been reported, depending upon the selected ionization chamber, external filtration and tube potential (kVp) employed, even for beams of similar HVL [4, 5, 8, 9]. Using a detector with a uniform energy response, it was concluded that the agreement between measured percentage depth doses and BJR Supplement 25 data [9] was reasonable. There are clearly significant practical problems associated with depth dose measurements for kilovoltage beams, with possible causes due to beam spectral differences and detector characteristics. Any consensus move away from the use of published data must be based on a satisfactory resolution of these difficulties.
The performance of the dose monitoring system over both the short- and long-term was satisfactory with respect to such variables as beam orientation, interruption, consistency, reproducibility and linearity.
There is currently a relative absence of published evaluation techniques and permissible tolerances for the assessment of kilovoltage units. The methods described in this paper could form the basis of one such approach. A summary of the suggested desirable performance characteristics of a combined kilovoltage unit is given in Table 11. This is an attempt to summarize our view of the scope of assessment and performance testing required, and the present levels of performance that can be expected from modern units. The table may be of some assistance in the drafting of a tendering specification. It should also help to identify some of the current performance limitations of such equipment, which will hopefully be addressed in future technical developments.
|
Acknowledgments
|
|---|
The authors would like to thank the members of the Radiotherapy Physics section at this hospital for assistance with data collection and processing, and also the equipment manufacturer for the supply of technical data included in thiswork. Assistance with preparation of the diagrams by Mr T Buttery is also gratefully acknowledged.
Received for publication August 23, 2000.
Revision received January 2, 2001.
Accepted for publication January 24, 2001.
|
References
|
|---|
-
Bratherton DG. Skin malignancy. In: Hope-Stone HF, editor. Radiotherapy in clinical practice. London: Butterworths, 1986:280–92.
-
McLelland J, Chu AC. Skin. In: Sikora K, Halnan KE, editors. Treatment of cancer. London: Chapman and Hall, 1990:611–8.
-
Klevenhagen SC, Aukett RJ, Harrison RM, Moretti C, Nahum AE, Rosser KE. The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al–4 mm Cu HVL; 10–300 kV generating potential). Phys Med Biol 1996;41:2605–25.[Medline]
-
Aukett RJ, Thomas DW, Seaby AW, Gittins JT. Performance characteristics of the Pantak DXT-300 kilovoltage x-ray treatment machine. Br J Radiol 1996;69:726–34.[Abstract/Free Full Text]
-
Gerig L, Soubra M, Salhani D. Beam characteristics of the Therapax DXT300 orthovoltage therapy unit. Phys Med Biol 1994;39:1377–92.[Medline]
-
Massey JB. Manual of dosimetry in radiotherapy. Vienna, Austria: IAEA, 1970:69–75, 81–3.
-
Klevenhagen SC, Thwaites DI. Kilovoltage X-rays. In: Williams JR, Thwaites DI, editors. Radiotherapy physics in practice. Oxford: Oxford University Press, 1993:95–112.
-
Li XA, Ma C-M, Salhani D. Measurement of percentage depth dose and lateral beam profile for kilovoltage x-ray therapy beams. Phys Med Biol 1997;42:2561–8.[Medline]
-
Central axis depth dose data for use in radiotherapy: 1996, British Journal of Radiology Supplement 25. London: British Institute of Radiology, 1996.
-
Institute of Physical Sciences in Medicine. Commissioning and quality assurance of linear accelerators, Report No. 54. York: IPSM, 1988.
-
National Radiological Protection Board. Guidance notes for the protection of persons against ionising radiations arising from medical and dental use. London: HMSO, 1988.
-
Rosser KE. Investigation of the chamber correction factor (kch) for the UK secondary standard ionization chamber (NE2561/NE2611) using medium energy x-rays. Phys Med Biol 1998;43:3195–206.[Medline]
-
Meyer J, Mills JA. Demonstration of a combined filter to improve the field uniformity of a 90 kV superficial X-ray therapy machine for different treatment field sizes. Br J Radiol 1997;70:201–6.[Abstract]
This article has been cited by other articles:
|
|
|
|
 
D Jurado, T Eudaldo, P Carrasco, N Jornet, A Ruiz, and M Ribas
Pantak Therapax SXT 150: performance assessment and dose determination using IAEA TRS-398 protocol
Br. J. Radiol.,
August 1, 2005;
78(932):
721 - 732.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
