Tray Factors Research Articles

Effect of an integral quality monitor on 4-, 6-, 10-MV, and 6-MV flattening filter-free photon beams.

PurposeTo investigate the effect of an integral quality monitor (IQM; iRT Systems GmbH, Koblenz, Germany) on 4, 6, 10, and 6‐MV flattening filter‐free (FFF) photon beams.MethodsWe assessed surface dose, PDD20,10, TPR20,10, PDD curves, inline and crossline profiles, transmission factor, and output factor with and without the IQM. PDD, transmission factor, and output factor were measured for square fields of 3, 5, 10, 15, 20, 25, and 30 cm and profiles were performed for square fields of 3, 5, 10, 20, and 30 cm at 5‐, 10‐, and 30‐cm depth.ResultsThe differences in surface dose of all energies for square fields of 3, 5, 10, 15, 20, and 25 cm were within 3.7% whereas for a square field of 30 cm, they were 4.6%, 6.8%, 6.7%, and 8.7% for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV, respectively. Differences in PDD20,10, TPR20,10, PDD, profiles, and output factors were within ±1%. Local and global gamma values (2%/2 mm) were below 1 for PDD beyond d max and inline/crossline profiles in the central beam region, respectively. The gamma passing rates (10% threshold) for PDD curves and profiles were above 95% at 2%/2 mm. The transmission factors for 4‐MV, 6‐MV, 6‐MV‐FFF, and 10‐MV for field sizes from 3 × 3 to 30 × 30 cm2 were 0.926–0.933, 0.937–0.941, 0.937–0.939, and 0.949–0.953, respectively.ConclusionsThe influence of the IQM on the beam quality (in particular 4‐MV X‐ray has not verified before) was tested and introduced a slight beam perturbation at the surface and build‐up region and the edge of the crossline/inline profiles. To use IQM in pre‐ and intra‐treatment quality assurance, a tray factor should be put into treatment planning systems for the dose calculation for the 4‐, 6‐, 10‐, and 6‐MV flattening filter‐free photon beams to compensate the beam attenuation of the IQM detector.

Gamma Putty shielding effect in megavoltage photon beam

Purpose: Traditionally, lead and Cerrobend have been employed for field shaping in radiation therapy. Lately, another shielding material called Gamma Putty has emerged. The objective of this report is to examine its dosimetric and shielding characteristics in megavoltage photon beam. Methods: All measurements were carried out in a dual energy linac. Data were collected using a calibrated ionization chamber. Percent transmission, linear attenuation, and field size dependence were evaluated for open square fields (4 × 4 cm 2 to 10 × 10 cm 2 ) defined by collimator jaws and for different Gamma Putty thicknesses (t = 0, 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm) at 6 and 18 MV photon beams. The measurements were performed both in air using appropriate acrylic buildup cap and in solid water. Results: The Gamma Putty tray factor (GPTF) increased steadily with field size for both 6 and 18 MV. It was characterized by a half value thickness (HVT) of 2.513 ± 0.101 and 2.855 ± 0.024 cm for 6 and 18 MV, respectively. The reduction in surface dose was about 6%, 14.5%, 22%, 36.37%, and 54% for 6 MV and 2.75 %, 9.36 %, 16.25 %, 28.95 %, and 44.47 % for 18 MV for Gamma Putty thicknesses of 0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 cm. Conclusion: The result of Gamma Putty shielding on the photon beam output increases with thickness, beam energy, and field size. Therefore, clinical use of Gamma Putty tray factors should be tailored for all thicknesses, beam energies, and field sizes.

Influence of the Integral Quality Monitor transmission detector on high energy photon beams: A multi-centre study

PurposeThe influence of the Integral Quality Monitor (IQM) transmission detector on photon beam properties was evaluated in a preclinical phase, using data from nine participating centres: (i) the change of beam quality (beam hardening), (ii) the influence on surface dose, and (iii) the attenuation of the IQM detector. MethodsFor 6 different nominal photon energies (4 standard, 2 FFF) and square field sizes from 1×1cm2 to 20×20cm2, the effect of IQM on beam quality was assessed from the PDD20,10 values obtained from the percentage dose depth (PDD) curves, measured with and without IQM in the beam path. The change in surface dose with/without IQM was assessed for all available energies and field sizes from 4×4cm2 to 20×20cm2. The transmission factor was calculated by means of measured absorbed dose at 10cm depth for all available energies and field sizes. Results(i) A small (0.11–0.53%) yet statistically significant beam hardening effect was observed, depending on photon beam energy. (ii) The increase in surface dose correlated with field size (p<0.01) for all photon energies except for 18MV. The change in surface dose was smaller than 3.3% in all cases except for the 20×20cm2 field and 10MV FFF beam, where it reached 8.1%. (iii) For standard beams, transmission of the IQM showed a weak dependence on the field size, and a pronounced dependence on the beam energy (0.9412 for 6MV to 0.9578 for 18MV and 0.9440 for 6MV FFF; 0.9533 for 10MV FFF). ConclusionsThe effects of the IQM detector on photon beam properties were found to be small yet statistically significant. The magnitudes of changes which were found justify treating IQM either as tray factors within the treatment planning system (TPS) for a particular energy or alternatively as modified outputs for specific beam energy of linear accelerators, which eases the introduction of the IQM into clinical practice.

Behaviour of wedges for different field sizes and depths

Background: The relative dosimetry plays vital part in treatment planning of patients. Factors such as percent depth doses, tissue maximum ratios, tray factors, wedge factors, etc., determined from the relative dosimetry, affects the patient dose. The current study intended at measuring and evaluating the wedge factor for different field sizes and depths for 60Co teletherapy unit GWXJ80 of NPIC China. Methods: The measurements for 15o, 30o, 45o and 60o wedges for different field sizes and depths on 60Co teletherapy unit GWXJ80 of NPIC China installed at Nuclear Institute of Medicine and Radiotherapy (NIMRA), Jamshoro, Pakistan, were done in water phantom of 30x30x30 cm3 dimension at 80 cm Source-to-Surface Distance (SSD) by using calibrated Farmer’s NE 2570 electrometer with NE 2571 0.6 cc ionization chamber. Results: The evaluation of data showed that there was no significant difference in factor of each of wedge being analyzed for different field sizes and depths. Conclusion: The current study suggests that wedge factor for a particular wedge is approximately a constant value irrespective of field size and depth. The measurement for only one field size at one depth is sufficient to calculate the wedge factor for a particular wedge.

SU‐F‐T‐326: Diode Array Transmission Detector Systems Evaluation

Purpose:A new transmission detector, Delta4 Discover, developed by Scandidos (Uppsala, Sweden) was evaluated for external photon beam verification and quality assurance. The device is an array of 4040 diodes designed to be mounted on the linac accessory tray to measure photon field shape, position and fluence during patient treatment. Interfractional measurements are compared to a baseline measurement made during delivery quality assurance. The aim of this work is to evaluate the stability of the device and its effect on the shape and magnitude of the treatment beam.Methods:Beam profiles, percent depth dose, and beam attenuation was measured for 6, 10, and 15 MV photon beams with and without the device in place for 1×1 and 30×30 cm2 fields. Changes in profile and percent depth dose was quantified to evaluate the need to recommission the treatment beam, or account for the device with a tray factor. The stability of the radiation measurements was evaluated by measuring the deviation of each diode measurement during repeated prostate VMAT treatment delivery.Results:Photon beam profiles changed by &lt; 1.25% in the nonpenumbra regions of the 30×30 cm2 beam. Percent depth dose curves show a 5–7% increased dose at depths &lt; 2.5cm, but agreed within 1% at depths &gt; 2.5cm. This indicates increased skin dose, similar to the use of a physical beam wedge. The device attenuated 6, 10, and 15 MV photon beams by 1.71±0.02%, 1.36±0.03%, and 1.17±0.03%, respectively. The diode array reproduced dosimetric measurements within 0.5% standard deviation for repeated prostate VMAT measurement.Conclusion:The device demonstrated stabile radiation measurements, while not changing the treatment beam shape in a clinically significantly manner. Use of this device can be accounted for with a tray factor, as opposed to recommissioning the treatment beam.

SU‐F‐T‐551: Beam Hardening and Attenuation of Photon Beams Using Integral Quality Monitor in Radiotherapy

Purpose:Aim of the multi‐centre study was to analyse beam hardening effect of the Integral Quality Monitor (IQM) for high energy photon beams used in radiotherapy with linear accelerators. Generic values for attenuation coefficient k(IQM) of IQM system were additionally investigated.Methods:Beam hardening effect of the IQM system was studied for a set of standard nominal photon energies (6 MV–18 MV) and two flattening filter free (FFF) energies (6 MV FFF and 10 MV FFF). PDD curves were measured and analysed for various square radiation fields, with and without IQM in place. Differences between PDD curves were statistically analysed through comparison of respective PDD_20,10 values. Attenuation coefficients k(IQM) were determined for the same range of photon energies.Results:Statistically significant differences in beam qualities for all evaluated high energy photon beams were found, comparing PDD_20,10 values derived from PDD curves with and without IQM in place. Significance of beam hardening effect was statistically proven with high confidence (p &lt; 0,01) for all analysed photon beams except for 15 MV (p = 0,078), although relative differences in beam qualities were minimal, ranging from 0,1 % to 0,5 %. Attenuation of the IQM system showed negligible dependence on radiation field size. However, clinically important dependence of kIQM versus TPRs20,10 was found: 0,941 for 6 MV photon beams, to 0,959 for 18 MV photon beams, with highest uncertainty below 0,006. k(IQM) versus TPRs were tabulated and polynomial equation for the determination of k(IQM) is suggested for clinical use.Conclusion:There was no clinically relevant beam hardening, when IQM system was on linear accelerators. Consequently, no additional commissioning is needed for the IQM system regarding the determination of beam qualities. Generic values for k(IQM) are proposed and can be used as tray factors for complete range of examined photon beam energies.

Evaluation of MOSFETs for entrance dose dosimetry for 6 and 10 MV photons with a custom made build up cap

Commercially available MOSFETs, Thomson and Nielsen TN502-RD, were evaluated for suitability as an entrance dose in vivo dosimeter for 6MV and 10MV. Detector response was normally distributed around a mean (skewness = -0.01 +/- 0.24, kurtosis = -0.09 +/- 0.48) with a mean of 110.6 mV/Gy, with a standard deviation of 2.4% at 0.86 Gy. The standard deviation of readings increased with decreasing dose and increased at a rate greater than inverse square. The linearity coefficient was 0.9999. No significant dependence on angle, field size, dose rate, energy or time was observed. As such, they would be useful for entrance dose in vivo dosimetry. With a custom made build up cap corrections were required for field size, wedge, beam energy and tray factors, showing that build up cap design is an important consideration for entrance dose in vivo dosimetry using MOSFETs.

Dependence of the tray transmission factor on collimator setting and source-surface distance.

When blocks are placed on a tray in megavoltage x-ray beams, generally a single correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assumed to be independent of field size and source-surface distance (SSD). Analysis of a set of measurements performed in beams of 13 different linear accelerators demonstrates that there is, however, a slight variation of the tray transmission factor with field size and SSD. The tray factor changes about 1.5% for collimator settings varying between 4x4 cm and 40 x 40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The variation with field size is smaller if the source-surface distance is increased. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerating potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source-surface distance can easily be taken into account in the dose calculation by considering the volume of the irradiated tray material and the position of the tray in the beam. A relation is presented which can be used to calculate the numerical value of the tray transmission factor directly. These calculated values can be checked with only a few measurements using a cylindrical beam coaxial miniphantom.

Berechnung und Messung der spezifischen Dosis bei irregulärer Feldformung durch Lamellenblende und Individualblende

Output factors of irregularly shaped beams created by multileaf collimator (MLC) or individually moulded blocks were measured and compared with the results of a commercial treatment planning system. Rhombs and triangles of various sizes were created by blocking half of the area of the corresponding open fields. Taking into account the tray factor, the measured output factors using individually moulded blocks were between 2 % and 5 % higher than the corresponding values for the MLC. In most cases deviations less than 2 % were found when comparing output factors calculated by the treatment planning system with the measured values. However, rhomb–shaped fields with diagonals of 7 cm or 5 cm showed deviations of 3 % or 8%, respectively.

Changes in incident photon fluence of 6 and 18 MV x rays caused by blocks and block trays.

When a block and tray are placed in a x-ray beam the dose to a point in a phantom is changed by the following factors: (1) attenuation of photon and electron fluence from the head of the accelerator by the tray and the block, (2) decrease in the scatter in the phantom by a reduction in the phantom volume that receives radiation, and (3) generation of scatter off the tray and block. This third factor is generally ignored in dosimetry calculation but has been measured in this work. Measurements of incident photon fluence for 6 and 18 MV x rays were made with a columnar miniphantom of 10 cm depth. The tray factor for a 9 mm thick Lexan tray is found to be variable and to increase by 1.8% due to scatter off the tray when the field size is increased from a 3cm x 3 cm to 40cm x 40 cm field. Also, it was found that scatter off a block could increase the incident photon fluence by as much as 2%. The magnitude of this block scatter depends on the length of the inner edge of the opening in the block and on amount of block that is being irradiated, the overlap of the block by the radiation field. The total block-tray factor can be as much as 3% larger than the single-value tray factor measured with a 10cm x 10cm field that is traditionally used. An analytical equation is developed that accurately models the block-tray factor.

Physical characteristics of photon and electron beams from a dual energy linear accelerator

Clinically pertinent data of the photon beams of nominal energy 6 MV and 18 MV from a dual photon energy linear accelerator are measured using a radiation field analyser with semiconductor diodes and ionisation chambers. Percentage depth dose values are compared with available data from other linacs and BJR-17. Measurements made in the buildup region using a parallei plate chamber show a marked increase in the surface dose and d max shifts by 3 mm and 14 mm for 6 MV and 18 MV photons, respectively as the field size is increased from 4 × 4 cm 2 to the maximum. Variations of wedge angles with energy and field size are also determined up to wedge widths of 15 cm for all the four available wedge angles. Isodose plots of both the energies are plotted using semiconductor diodes in a RFA-3 system. Output factors, wedge transmission factors, and shielding tray factors were also measured in clear polystyrene phantom for both the photon energies at the depths of maximum ionisation. Beam characteristics of the electron beams of energies 6, 9, 12, 16, and 20 MeV produced by the Clinac-1800 have also been studied. The characteristics include percentage depth dose, isodose distribution, depth of maximum dose, surface dose, photon contamination, uniformity index, and penumbra. Most of the measurements were carried out using semiconductor detectors, whereas small volume ionization chambers and a plane-parallel chamber were kept as standards for comparison. Isodose distributions were drawn from the film densitometry method. Range-energy parameters are obtained from the observed depth dose data. These parameters vary from machine to machine and must be ascertained for individual units. The parameters differ to a considerable extent from their theoretically predicted values but generally follow the trend, experimentally observed by others, for similar types of units.