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Pantak Therapax SXT 150: performance assessment and dose determination using IAEA TRS-398 protocol

Servei de Radiofísica i Radioprotecció, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain

	Abstract

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
The performance assessment and beam characteristics of the Therapax SXT 150 unit, which encompass both low and medium-energy beams, were evaluated. Dose determination was carried out by implementing the International Atomic Energy Agency (IAEA) TRS-398 protocol and measuring all the dosimetric parameters in order to have a solid, consistent and reliable data set for the unit. Mechanical movements, interlocks and applicator characteristics agreed with specifications. The timer exhibited good accuracy and linearity. The output was very stable, with good repeatability, long-term reproducibility and no dependence on tube head orientation. The measured dosimetric parameters included beam first and second half-value layers (HVLs), absorbed dose rate to water under reference conditions, central axis depth dose distributions, output factors and beam profiles. Measured first HVLs agreed with comparable published data, but the homogeneity coefficients were low in comparison with typical values found in the literature. The timer error was significant for all filters and should be taken into consideration for the absorbed dose rate determination under reference conditions as well as for the calculation of treatment times. Percentage depth-dose (PDD) measurements are strongly recommended for each filter–applicator combination. The output factor definition of the IAEA TRS-398 protocol for medium-energy X-ray qualities involves the use of data that is difficult to measure. Beam profiles had small penumbras and good symmetry and flatness except for the lowest energy beam, for which a heel effect was observed.

	Introduction

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
The use of kilovoltage treatment units has decreased, largely because of the availability of electron beams provided by linear accelerators. Nevertheless, there has been a revival of these units during recent years.

A Therapax SXT 150 unit, manufactured by Pantak (Pantak Inc., Branford, CT), was installed in our centre in 2003. Performance assessment and beam characteristics of other kilovoltage units have been published by several authors: the Therapax DXT 300 (Pantak Inc.) has been studied both by Gerig et al [1] and by Aukett et al [2], and the Gulmay D3300 (Gulmay Ltd., Chertsey, UK) by Evans et al [3], but there are no published data for the Therapax SXT 150. The aim of this work was to study the performance and beam characteristics of the Therapax SXT 150, in order to provide a basis for comparison with other users and a data reference for potential new users.

Dose determination was carried out in accordance with the International Atomic Energy Agency (IAEA) TRS-398 protocol [4], based on standards of absorbed dose to water. Although some parameters can be found in the literature (e.g. percentage depth-dose (PDD) data in The British Journal of Radiology (BJR) Supplement 25 [5]), all the dosimetric parameters were measured in order to have a solid, consistent and reliable data set for this unit.

Before proceeding with this study, some tests were performed, including the performance of the mechanical movements, interlocks and applicator characteristics. All the elements tested complied with specifications.

	Material

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
The Therapax SXT 150
The Therapax SXT 150 is a kilovoltage therapy unit that encompasses low and medium energy X-ray beams as defined in the IAEA TRS-398 protocol [4]. The components of the unit include a microprocessor based control console, a HT generator and control system, a cooling water system, a mobile tube stand, a metal ceramic X-ray tube, a set of filters and a set of applicators.

The water-cooled metal ceramic X-ray tube has an inherent filtration of 2 mm beryllium. It provides accelerating potentials between 10 kVp and 150 kVp in 0.1 kVp steps. The tube current covers a range of between 1 mA and 50 mA in 0.1 mA steps with the restriction of never exceeding 3 kW of output power.

Additional filtration can be introduced to modify beam quality. Only eight different combinations of tube potential, tube current and added filtration are allowed. An extra specific filter is provided for warm-up. Filters are recognized by the system when the added filtration is inserted into the tube head, setting the tube potential and tube current automatically. At our centre, filter specific characteristics (listed in Table 1) were chosen to provide beam qualities capable of covering a wide range of situations found in clinical practice.

Radiation detection equipment
A PTW 10001 UNIDOS electrometer (PTW, Freiburg, Germany) was used connected to the following chambers:

• A PTW M23342 plane-parallel chamber (PTW, Freiburg, Germany) suitable for low-energy X-ray dosimetry. The chamber was calibrated together with the electrometer and a PMMA phantom, described below, at Physikalisch-Technische Bundesanstalt (PTB) laboratory. The calibration was performed in standards of absorbed dose to water at four different radiation qualities covering the range of low-energy X-rays. The calibration factors were provided at the phantom surface for a field size of 3 cm diameter. According to these calibration factors, the energy response of the chamber was within 2% over the quality range. Although it is recommended in the IAEA TRS-398 protocol [4] and by Aukett et al [6], a build-up foil was not used in the calibration because PTB did not perform calibrations with these foils.
  
• A waterproof PTW M30013 Farmer-type cylindrical chamber (PTW, Freiburg, Germany). The chamber was calibrated together with the electrometer at the PTB laboratory. The calibration was performed in terms of absorbed dose to water at four different radiation qualities covering the range of medium-energy X-rays. The calibration factors were given for a field size of 10 cm of diameter. According to these calibration factors, the energy response of the chamber was within 2% over the whole quality range.
  
• A waterproof PTW TW31002 flexible cylindrical chamber (PTW, Freiburg, Germany).
  

A NE PDM 1A portable dose rate meter (Bicron-NE; latterly Saint-Gobain Crystals and Detectors, Paris, France), with 600 cm³ of cavity volume, calibrated in standards of ambient dose equivalent rate at the ¹³⁷Cs energy at Institut de Tècniques Energètiques (INTE) laboratory, certified by the Entidad Nacional de Calibración (ENAC).

Kodak X-Omat V therapy verification films (Eastman Kodak Co., Rochester, NY) were used and read using the Scanditronix RFA300 film densitometer (Scanditronix Medical AB, Uppsala, Sweden; latterly IBA, Lovain-la-Neuve, Belgium).

Phantoms
A PMMA slab phantom of 13 cm x 13 cm x 12 cm, with slab thicknesses of 1 mm, 5 mm and 10 mm was used. One of the slabs had a special insert for the PTW M23342 chamber in such a way that the outside surface of the chamber window was flush with the slab surface.

A water phantom of 35 cm x 45 cm x 35 cm was used. It had a depth adjustable support (0.5 mm precision) for different chambers.

HVL measurement material
A special device was developed in our centre for half-value layer (HVL) measurements (Figure 1). It was made of PMMA and designed to be attached to the tube head and locate all the elements involved in the measurements, guaranteeing a fixed and reproducible geometry relative to the source.

View larger version (103K): [in this window] [in a new window]   Figure 1. Half value layer measurement device.

	Methods

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
Tube head leakage
Measurements were carried out at the highest tube voltage and current (150 kVp and 20 mA) using the warm-up filter to minimize any scattered radiation. The areas of maximum leakage were identified using therapy verification films wrapped around the tube head and performing a 10 min exposure. The tube head leakage was measured using the NE PDM 1A chamber at distances of 5 cm and 1 m from these points of maximum leakage.

Timer check
Timer accuracy and linearity with dose were checked. The timer response is independent of the filter and the applicator. Therefore, filter 4 and the 3 cm diameter applicator were chosen to perform the measurements.

Accuracy was checked using a digital chronometer (Tag Heuer SA, Marin, Switzerland). Exposures with a timer selection of 1.50 min were performed, using the chronometer to measure the time elapsed between the console time display showing 0.50 min and 1.50 min. These measurements were carried out starting the chronometer when the irradiation was running in order to avoid the delay from when the start button is pushed until the timer starts counting.

Linearity of the timer with dose was assessed by performing exposures with a timer selection of 0.50 min, 1.00 min, 2.00 min and 4.00 min, measuring the output with the PTW M23342 chamber in the PMMA phantom. Results were plotted using a linear regression fit.

Output repeatability and long-term reproducibility
The repeatability of the unit output was estimated for each filter. A series of 10 consecutive exposures with a timer selection of 1.00 min was performed for each filter, measuring the output with the PTW M23342 chamber. The repeatability of the dosimetric equipment was checked using a ⁹⁰Sr check source and found to be less than 0.05% (1 SD).

The long-term reproducibility of tube output was estimated for each filter by measuring absorbed dose rate to water under reference conditions over a period of several months. The long-term reproducibility of the dosimetric equipment was checked using the ⁹⁰Sr check source and was less than 0.3% (1 SD) over the same period of time.

Output reproducibility with tube angle
The output reproducibility with tube angle was checked for filters 1, 4 and 8. Measurements were performed using the PTW M23342 chamber, fixed to a 1 mm thick PMMA sheet attached to the 10 cm diameter applicator. For tube head angles around the anode–cathode axis, measurements were performed at –90°, –45°, 0°, 45° and 90°, while for tube head angles around the axis perpendicular to the anode–cathode axis and parallel to the floor, measurements were performed at –70°, –35°, 0°, 35° and 70°.

Beam characterization
Measurements of the dosimetric quantities were carried out following the IAEA TRS-398 protocol [4] definitions and methodology, which differs for low and medium-energy X-ray beams.

Beam quality
Beam quality is specified by the HVL. Beam HVL was measured for each filter following the method proposed in the IAEA TRS-398 protocol [4] and by Klevenhagen and Thwaites [7]. Both first HVL and second HVL were measured, and the homogeneity coefficient (1st HVL/2nd HVL) calculated.

Measurements were performed using the special device for HVL measurement, the PTW M23342 chamber and sheets of either aluminium or copper. To guarantee narrow beam geometry, the smallest applicator (=1 cm) and the 1.5 cm diameter collimating aperture were used (Figure 1). The correct alignment of the source, the collimating aperture and the chamber was checked with a therapy verification film. There was no scattering material within 1 m from the chamber. HVL determination was repeated with the PTW M30013 cylindrical chamber for medium-energy filters.

Absorbed dose rate to water under reference conditions
Absorbed dose rate to water _w,Q was determined for each filter (of quality Q) in reference conditions as:

where M_Q is the reading of the dosemeter for an exposure of time t in reference conditions corrected for k_TP (temperature and pressure), k_pol (polarity) and k_elec (electrometer calibration); N_D,w,Q0 is the calibration factor in terms of absorbed dose to water at a reference quality Q₀; k_Q,Q0 is the chamber-specific factor which corrects for differences between Q₀ and Q; and is the timer error.

Reference conditions are different for low and for medium-energy beams, as is shown in Table 3.

The value of the k_Q,Q0 factor of each filter was calculated using the calibration data.

The timer error was measured in reference conditions for each filter following the Orton and Seibert method [8], recommended in the Worksheet of the IAEA TRS-398 protocol [4]. The measurement of was repeated several times on different days to verify its stability.

Central axis depth-dose measurements
PDD measurements were carried out for each filter–applicator combination. PDD normalization was performed at the depth of maximum dose, coinciding with the surface for these beam qualities.

For low-energy X-ray beams, measurements were made using the PTW M23342 chamber and the PMMA slab phantom. Measurements were performed from the surface to a depth with a PDD value of about 10–15%, in steps of 1 mm, 2 mm or 5 mm (smaller steps in the higher dose gradient zone). According to the IAEA TRS-398 protocol [4], depth-dose distributions were considered to be the same as measured depth–ionization distributions because the response of the chamber with beam quality was within 5%.

For medium-energy X-ray beams, measurements were made using the PTW M30013 Farmer-type chamber, the PTW TW31002 chamber and the water phantom. The choice of the chamber depended on the particular relation of field size and chamber dimensions. In order to establish this relationship, it was verified that depth–ionization distributions obtained with both chambers were the same for the 3 cm and 5 cm diameter applicator and different for the 2.5 cm diameter applicator. The PTW M30013 chamber was thus used for the applicators with diameters ranging from 3 cm to 15 cm, and the PTW TW31002 chamber for the applicators with diameters ranging from 1 cm to 2.5 cm. Measurements were performed from 5 mm depth to approximately 11 cm depth (PDD value of about 10–15%), in 5 mm or 10 mm steps (smaller steps in the higher dose gradient zone). The value of the reading at the surface was obtained by means of a polynomial extrapolation of the depth–ionization distribution. According to the IAEA TRS-398 protocol [4], depth–dose distributions were considered to be the same as measured depth–ionization distributions as measurements were made either with a Farmer-type chamber or with a chamber whose response was checked against this.

Output factors
Output factors were measured for each filter–applicator combination.

For low-energy X-ray beams, the output factor is defined as the ratio of the corrected dosemeter reading M_Q at the surface for a given focus-to-surface distance (FSD) and field size, to that for the reference conditions (surface, 3 cm diameter applicator, FSD=15 cm). Measurements were carried out with the same material and arrangement used for the absorbed dose rate determination under reference conditions.

For medium-energy X-ray beams, the output factor is defined as the ratio of the absorbed dose at the surface of a water phantom for a given FSD and field size to that for the reference conditions (2 cm depth, 10 cm diameter applicator, FSD=25 cm). Since no measurements could be performed directly at the surface, absorbed dose to water at the surface was obtained from absorbed dose measurements at 2 cm depth, using the depth–dose distribution of the filter–applicator combination. Measurements were performed using the water phantom and the two different cylindrical chambers (the PTW M30013 and the PTW TW31002) according to the same field size/chamber dimensions relation stated in the previous section. These measurements were carried out with the same arrangement used for the absorbed dose rate determination under reference conditions.

Beam profiles
Beam profiles were measured for each applicator and filters 1, 5 and 8 using therapy verification films in contact with the end of the applicator and exposing them to an optical density (OD) of about 1.8. Relative optical density profiles were read using the Scanditronix RFA-300 film densitometer in the anode–cathode axis direction (Y) and in the perpendicular direction (X), normalizing at the central axis point.

The field size, defined as the full width at 50% relative OD value, and the penumbra, defined as the lateral distance between the 80% and the 20% of maximum OD, were determined for each profile. The flatness and the symmetry were obtained from the Organismo Internacional de Energía Atómica (OIEA) protocol [9] definitions as:

where OD_max and OD_min are the maximum and minimum optical densities within 80% of the field size.

where OD₁ and OD₂ are the optical densities at off-axis distances of ±25% of the field size, and OD₃ and OD₄ are the optical densities at off-axis distances of ±40% of the field size.

	Results and discussion

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
Tube head leakage
Two areas of maximum leakage were identified from the film survey, one around the cathode high tension cable input and the other at the front of the filter insert slot. The maximum dose rate at 5 cm from the surface of the tube was 3 mSv h^–1 and the maximum dose rate at 1 m was 80 µSv h^–1. These values are two orders of magnitude less than the limits established in the International Commission on Radiological Protection (ICRP) 33 [10].

Timer check
Timer accuracy was better than 0.2% (1 SD). Accuracy measurement uncertainty took different factors into account: the timer uncertainty, the precision of the chronometer (0.2 s) and the response time of the operator to start and stop the timer. The timer accuracy itself was therefore better than 0.2%.

Timer performance can be considered linear with dose according to a regression coefficient of unity.

Output repeatability and long-term reproducibility
The output repeatability was better than 0.3% (1 SD) for all filters. The long-term output reproducibility was better than 1.5% (1 SD) in measurements of absorbed dose rate to water under reference conditions over a 9 month period.

Output reproducibility with tube angle
No output dependence with tube angle was observed. For the three measured filters (1, 4 and 8), the output reproducibility with tube angle was better than 0.2% for all the measured angles of the two rotational axis possibilities.

Beam characterization
Beam quality
The values of the first and second HVL as well as the homogeneity coefficient for each filter are shown in Table 4. These values allowed us to classify the filters either in the low or in the medium-energy X-ray beam category in accordance with the IAEA TRS-398 protocol [4] classification. Filter 4 is in the overlapping region between the two categories, but we decided to consider it as a low-energy filter. Therefore, filters 1, 2, 3 and 4 belonged to low-energy kilovoltage X-ray beams and filters 5, 6, 7 and 8 to medium-energy kilovoltage X-ray beams.

It is difficult to compare our HVL values with those found in the literature due to the difficulties in finding the same tube potential and the same added and inherent filtration in both cases. However, first HVLs of some filters have been compared with values given in Table 1.1.1 of BJR Supplement 25 [5], although they correspond to tubes with an inherent filtration of 1 mm Be equivalent. These comparisons are summarized in Table 4, showing that measured values and those from the BJR Supplement 25 [5] are in good agreement.

The values of second HVLs were not compared, but the homogeneity coefficients were lower than typical values found in the literature (e.g. Table C.1 and Table C.2 of BJR Supplement 25 [5], Gerig et al [1] and Aukett et al [2] for the Therapax DXT 300, Evans et al [3] for the Gulmay D3300).

Absorbed dose rate to water under reference conditions
The values of the timer error, needed for the determination of absorbed dose rate, are listed in Table 5. These values correspond to the mean value of obtained over several days and were time reproducible within a standard deviation of 5%. The values of were of the same order as the timer precision and were also significant compared with usual treatment times. Therefore, values were not negligible and should be taken into consideration for absorbed dose rate determination as well as for treatment time calculations.

View this table: [in this window] [in a new window]   Table 5. Timer error

View this table: [in this window] [in a new window]   Table 6. Absorbed dose rate to water ( w) in reference conditions

The value of the reading at the surface for medium-energy X-rays was determined by a polynomial (6th order) extrapolation of the measured depth–ionization distribution, with a polynomial regression coefficient better than 0.9999 in all cases.

Although a direct comparison with the values found in the literature was difficult, several PDD curves were compared with those found in the BJR Supplement 25 [5] making some approximations.

Data sets of filters 1, 2, 3 and 4 were compared for some field sizes. Data in the BJR Supplement 25 [5] have the first HVL as the sole specifier of radiation quality. Therefore, the curves corresponding to our HVLs were derived by performing a linear interpolation of the data sets of the two HVLs closest to our HVL, with the same FSD and field size as our applicator. When depths were not the same, a linear interpolation was performed to obtain PDD values at common depths. PDDs were then compared at all depths. The sign of these comparisons was positive if the PDD measured at our centre was higher, and negative if it was lower. For filter 1, the 1.5 cm, 3 cm and 5 cm diameter applicators were compared and differences were within –4%. For filter 2, the 3 cm and 5 cm diameter applicators were compared, with differences within +1 and –2%. For filters 3 and 4, the 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, 10 cm and 15 cm diameter applicators were compared and differences were within +3 and –5%. These differences were expected, as PDD data given in BJR Supplement 25 [5] come from average values of data from different centres, tubes, tube potential, added filtration, detectors and phantoms, with discrepancies up to 10% at 50% depth.

Data sets of filters 5, 6, 7 and 8 were not suitable for comparison because tabulated values in BJR Supplement 25 [5] belong to a different FSD (50 cm) and also because the tube potential used with these filters was at the limit of applicability of the BJR Supplement 25 [5] data.

Owing to the lack of published values and the differences found in the performed comparisons, specific measurements of PDDs for each filter–applicator combination are strongly recommended.

Output factors
The measured output factors are listed in Table 7. As expected, output factors increased with field size for a fixed FSD, except in the case of filter 5 for the 10 cm and 15 cm diameter applicators. This is because for medium-energy X-ray filters, the output factors include a PDD value so as to obtain absorbed doses at the surface from absorbed dose measurements performed at 2 cm depth. Therefore, although the ratio of absorbed doses at 2 cm depth always increased with field size, the PDD values of the applicators may lead output factors to decrease with field size.

Beam profiles
Relative optical density profiles along the x and y axes are shown in Figure 2 for filter 1 and the 1 cm, 3 cm, 5 cm, 10 cm and 15 cm diameter applicators. As can be observed, these profiles were quite different between the x and y axes. For filters 5 and 8 the profiles were symmetrical and almost identical for both axes. This is attributable to a heel effect present in filter 1 that disappears for filters 5 and 8. This heel effect becomes more evident when the field size increases, reaching differences between both axes of up to 10% of OD for the 15 cm diameter applicator. The dependence of this effect with kilovoltage is expected due to the fact that the same tube is used over a wide range of voltages [7]. This dependence, although the X-ray tubes were different, was also reported for the Therapax DXT 300 by Gerig et al [1] and Aukett et al [2], and for the Gulmay D3300 by Evans et al [3].

View larger version (14K): [in this window] [in a new window]   Figure 2. Relative optical density (OD) beam profiles along the x (dotted line) and y axes (solid line) for filter 1: (a) 1 cm, 3 cm and 5 cm diameter applicators; (b) 10 cm and 15 cm diameter applicators.

For filter 1, flatness and symmetry values were affected by the heel effect present in this filter. Flatness values for the x axis ranged from 0.3% to 3.1% while for the y axis ranged from 0.7% to 6.1%. Symmetry values for the x axis ranged from 97.7% to 101.3% while for the y axis ranged from 92.6% to 99.6%. For a fixed applicator, the flatness value was higher for the y axis than for the x axis, and the symmetry value was worse for the y axis than for the x axis, which was close to 100%. Furthermore, these differences in flatness and symmetry values between the two axes were accentuated with the size of the applicator. On the other hand, for filters 5 and 8, symmetry values were close to 100% in all cases (between 98.9% and 102.8%). For these filters, flatness values for the x axis ranged from 0.3% to 4.2% and for the y axis ranged from 0.4% to 4.8%. The lowest values were found for the 1 cm diameter applicator and increased with field size due to the effect of the beam divergence. For a fixed applicator, flatness values were similar between both axes but the y axis value was slightly higher than the x axis value (differences about 0.2%).

	Conclusion

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 
Performance and dose characterization of the Therapax SXT 150 were studied. The dose characterization was performed by applying the IAEA TRS-398 protocol [4] to these beam qualities, which provides a solid, consistent and reliable way of measuring the dosimetric parameters.

Mechanical movements, interlocks and applicator characteristics agreed with specifications. The tube head leakage was two orders of magnitude less than the limits established in the ICRP 33 [10]. Timer accuracy was better than 0.2% and linearity with dose was good. The output was stable, with a repeatability better than 0.3%, a long-term reproducibility better than 1.5% and no dependence with tube head orientation.

Measured first HVLs agreed with those of the BJR Supplement 25 [5] when comparison was possible, although the homogeneity coefficient was low in comparison with typical values found in the literature.

The timer error was significant for all filters and should be taken into consideration for the absorbed dose rate determination under reference conditions as well as for the calculation of treatment times.

As small field sizes are of common use in these therapy units, attention should be paid to chamber dimensions in relation to field size.

Measurements of PDD data sets for each tube potential-added filtration combination and each applicator (field size and FSD) are strongly recommended, because in some cases the information cannot be found in the literature. As we could not perform measurements at the surface for medium-energy X-ray qualities, the value of the reading at the surface was obtained by using a mathematical extrapolation of measured data at other depths.

The output factors of medium-energy X-ray filters, as defined in the IAEA TRS-398 protocol [4], include a PDD value so as to obtain absorbed doses at the surface from absorbed dose measurements performed at 2 cm depth. This PDD value involves the use of measurements at the surface, which are difficult to perform for these qualities. To avoid this, the output factors used in our centre for these beam qualities were re-defined to take into account only measurements performed at 2 cm depth.

Beam optical density profiles had small penumbras and field sizes agreed with the applicator specifications. These profiles had good symmetry and flatness except for the lowest energy beam, for which a heel effect was observed.

Measurements of all the parameters of the unit are strongly recommended. The data presented here may provide a base for comparison and a reference for other or potential new users of the unit.

	Appendix 1. Percentage depth doses

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 

Received for publication May 12, 2004. Revision received January 4, 2005. Accepted for publication February 3, 2005.

	References

Top Abstract Introduction Material Methods Results and discussion Conclusion Appendix 1. Percentage depth... References

 

1. Gerig L, Soubra M, Salhani D. Beam characteristics of the Therapax DXT300 orthovoltage therapy unit. Phys Med Biol 1994;39:1377–92.[CrossRef][Medline]
2. 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]
3. Evans PA, Moloney AJ, Mountford PJ. Performance assessment of the Gulmay D3300 kilovoltage X-ray therapy unit. Br J Radiol 2001;74:537–47.[Abstract/Free Full Text]
4. International Atomic Energy Agency. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water, IAEA TRS-398. Vienna, Austria: IAEA, 2001.
5. Central axis depth dose data for use in radiotherapy: 1996, British Journal of Radiology Supplement 25. London: British Institute of Radiology, 1996.
6. Aukett RJ, Harrison RM, Rosser KE. The Characteristics of ionization chambers for measurements with X-rays in the kilovoltage range. In: Ma CM, Seuntjens JP, editors. Kilovoltage X-ray beam dosimetry for radiotherapy and radiobiology. Madison: Medical Physics Publishing, 1999:195–211.
7. Klevenhagen SC, Thwaites DI. Kilovoltage X-rays. In: Williams JR, Thwaites DI, editors. Radiotherapy physics. Oxford: Oxford University Press, 1993:97–8.
8. Orton CG, Seibert JB. The measurement of teletherapy unit timer errors. Phys Med Biol 1972;17:198–205.[CrossRef][Medline]
9. Organismo Internacional de Energía Atómica. Aspectos físicos de la garantía de calidad en radioterapia: Protocolo de control de calidad, IAEA-TECDOC-1151. Vienna, Austria: OIEA, 2000.
10. International Commission on Radiological Protection. Protection against ionizing radiation from external sources used in medicine, ICRP publication 33. New York: ICRP, 1981.

This Article Abstract Figures Only Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Jurado, D Articles by Ribas, M Search for Related Content PubMed PubMed Citation Articles by Jurado, D Articles by Ribas, M