Characterization of the Exradin W1 scintillator for use in radiotherapy.

PURPOSE: To evaluate the main characteristics of the Exradin W1 scintillator as a dosimeter and to estimate measurement uncertainties when used in radiotherapy. METHODS: We studied the calibration procedure, energy and modality dependence, short-term repeatability, dose-response linearity, angular dependence, temperature dependence, time to reach thermal equilibrium, dose-rate dependence, water-equivalent depth of the effective measurement point, and long-term stability. An uncertainty budget was derived for relative and absolute dose measurements in photon and electron beams. RESULTS: Exradin W1 showed a temperature dependence of -0.225% °C(-1). The loss of sensitivity with accumulated dose decreased with use. The sensitivity of Exradin W1 was energy independent for high-energy photon and electron beams. All remaining dependencies of Exradin W1 were around or below 0.5%, leading to an uncertainty budget of about 1%. When a dual channel electrometer with automatic trigger was not used, timing effects became significant, increasing uncertainties by one order of magnitude. CONCLUSIONS: The Exradin W1 response is energy independent for high energy x-rays and electron beams, and only one calibration coefficient is needed. A temperature correction factor should be applied to keep uncertainties around 2% for absolute dose measurements and around 1% for relative measurements in high-energy photon and electron beams. The Exradin W1 scintillator is an excellent alternative to detectors such as diodes for relative dose measurements.

On the suitability of ultrathin detectors for absorbed dose assessment in the presence of high-density heterogeneities.

PURPOSE: The aim of this study was to evaluate the suitability of several detectors for the determination of absorbed dose in bone. METHODS: Three types of ultrathin LiF-based thermoluminescent dosimeters (TLDs)-two LiF:Mg,Cu,P-based (MCP-Ns and TLD-2000F) and a (7)Li-enriched LiF:Mg,Ti-based (MTS-7s)-as well as EBT2 Gafchromic films were used to measure percentage depth-dose distributions (PDDs) in a water-equivalent phantom with a bone-equivalent heterogeneity for 6 and 18 MV and a set of field sizes ranging from 5 x 5 cm2 to 20 x 20 cm2. MCP-Ns, TLD-2000F, MTS-7s, and EBT2 have active layers of 50, 20, 50, and 30 µm, respectively. Monte Carlo (MC) dose calculations (PENELOPE code) were used as the reference and helped to understand the experimental results and to evaluate the potential perturbation of the fluence in bone caused by the presence of the detectors. The energy dependence and linearity of the TLDs' response was evaluated. RESULTS: TLDs exhibited flat energy responses (within 2.5%) and linearity with dose (within 1.1%) within the range of interest for the selected beams. The results revealed that all considered detectors perturb the electron fluence with respect to the energy inside the bone-equivalent material. MCP-Ns and MTS-7s underestimated the absorbed dose in bone by 4%-5%. EBT2 exhibited comparable accuracy to MTS-7s and MCP-Ns. TLD-2000F was able to determine the dose within 2% accuracy. No dependence on the beam energy or field size was observed. The MC calculations showed that a[Formula: see text] thick detector can provide reliable dose estimations in bone regardless of whether it is made of LiF, water or EBT's active layer material. CONCLUSIONS: TLD-2000F was found to be suitable for providing reliable absorbed dose measurements in the presence of bone for high-energy x-ray beams.

Comparison of dose calculation algorithms in slab phantoms with cortical bone equivalent heterogeneities.

To evaluate the dose values predicted by several calculation algorithms in two treatment planning systems, Monte Carlo (MC) simulations and measurements by means of various detectors were performed in heterogeneous layer phantoms with water- and bone-equivalent materials. Percentage depth doses (PDDs) were measured with thermoluminescent dosimeters (TLDs), metal-oxide semiconductor field-effect transistors (MOSFETs), plane parallel and cylindrical ionization chambers, and beam profiles with films. The MC code used for the simulations was the PENELOPE code. Three different field sizes (10 x 10, 5 x 5, and 2 x 2 cm2) were studied in two phantom configurations and a bone equivalent material. These two phantom configurations contained heterogeneities of 5 and 2 cm of bone, respectively. We analyzed the performance of four correction-based algorithms and one based on convolution superposition. The correction-based algorithms were the Batho, the Modified Batho, the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system (TPS), and the Helax-TMS Pencil Beam from the Helax-TMS (Nucletron) TPS. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. All the correction-based calculation algorithms underestimated the dose inside the bone-equivalent material for 18 MV compared to MC simulations. The maximum underestimation, in terms of root-mean-square (RMS), was about 15% for the Helax-TMS Pencil Beam (Helax-TMS PB) for a 2 x 2 cm2 field inside the bone-equivalent material. In contrast, the Collapsed Cone algorithm yielded values around 3%. A more complex behavior was found for 6 MV where the Collapsed Cone performed less well, overestimating the dose inside the heterogeneity in 3%-5%. The rebuildup in the interface bone-water and the penumbra shrinking in high-density media were not predicted by any of the calculation algorithms except the Collapsed Cone, and only the MC simulations matched the experimental values within the estimated uncertainties. The TLD and MOSFET detectors were suitable for dose measurement inside bone-equivalent materials, while parallel ionization chambers, applying the same calibration and correction factors as in water, systematically underestimated dose by 3%-5%.

Dose evaluation in lung-equivalent media in high-energy photon external radiotherapy.

In high-energy photon external radiotherapy treatment planning systems (TPSs) are used to calculate the dose to the target volume and the dose distribution around it. Commonly used TPSs include algorithms based on measurements in water and often fail in the estimate of dose in the presence of heterogeneities. In this study TL detectors were used to study the reliability of the Cadplan (Varian) TPS in the presence of low-density heterogeneities such as the lung for 6 and 18 MV photon beams at different field sizes. TL measurements were compared with TPS calculations and Monte Carlo simulations performed with the PENELOPE MC code. In a phantom with lung heterogeneity, TL measurements and MC simulations agreed, with an average deviation inside the lung of 2%. In contrast, TPS results overestimated the dose inside the lung, with a maximum deviation of 39% for the 18 MV photon beam and a field size of 2 x 2 cm(2).

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

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.

Comparison of dose calculation algorithms in phantoms with lung equivalent heterogeneities under conditions of lateral electronic disequilibrium.

An extensive set of benchmark measurement of PDDs and beam profiles was performed in a heterogeneous layer phantom, including a lung equivalent heterogeneity, by means of several detectors and compared against the predicted dose values by different calculation algorithms in two treatment planning systems. PDDs were measured with TLDs, plane parallel and cylindrical ionization chambers and beam profiles with films. Additionally, Monte Carlo simulations by means of the PENELOPE code were performed. Four different field sizes (10 x 10, 5 x 5, 2 x 2, and 1 x 1 cm2) and two lung equivalent materials (CIRS, p(w)e=0.195 and St. Bartholomew Hospital, London, p(w)e=0.244-0.322) were studied. The performance of four correction-based algorithms and one based on convolution-superposition was analyzed. The correction-based algorithms were the Batho, the Modified Batho, and the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system and the TMS Pencil Beam from the Helax-TMS (Nucletron) treatment planning system. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. The only studied calculation methods that correlated successfully with the measured values with a 2% average inside all media were the Collapsed Cone and the Monte Carlo simulation. The biggest difference between the predicted and the delivered dose in the beam axis was found for the EqTAR algorithm inside the CIRS lung equivalent material in a 2 x 2 cm2 18 MV x-ray beam. In these conditions, average and maximum difference against the TLD measurements were 32% and 39%, respectively. In the water equivalent part of the phantom every algorithm correctly predicted the dose (within 2%) everywhere except very close to the interfaces where differences up to 24% were found for 2 x 2 cm2 18 MV photon beams. Consistent values were found between the reference detector (ionization chamber in water and TLD in lung) and Monte Carlo simulations, yielding minimal differences (0.4%+/-1.2%). The penumbra broadening effect in low density media was not predicted by any of the correction-based algorithms, and the only one that matched the experimental values and the Monte Carlo simulations within the estimated uncertainties was the Collapsed Cone Algorithm.

Comparison study of MOSFET detectors and diodes for entrance in vivo dosimetry in 18 MV x-ray beams.

The feasibility of dual bias dual metal oxide semiconductor field effect transistors (MOSFETs) for entrance in vivo dose measurements in high energy x-rays beams (18 MV) was investigated. A comparison with commercially available diodes for in vivo dosimetry for the same energy range was performed. As MOSFETs are sold without an integrated build-up cap, different caps were tested: 3 cm bolus, 2 cm bolus, 2 cm hemispherical cap of a water equivalent material (Plastic Water) and a metallic hemispherical cap. This metallic build-up cap is the same as the one that is mounted on the in vivo diode used in this study. Intrinsic precision and response linearity with dose were determined for MOSFETs and diodes. They were then calibrated for entrance in vivo dosimetry in an 18 MV x-ray beam. Calibration included determination of the calibration factor in standard reference conditions and of the correction factors (CF) when irradiation conditions differed from those of reference. Correction factors for field size, source surface distance, wedge, and temperature were determined. Sensitivity variation with accumulated dose and the lifetime of both types of detectors were also studied. Finally, the uncertainties of entrance in vivo measurements using MOSFET and diodes were discussed. Intrinsic precision for MOSFETs for the high sensitivity mode was 0.7% (1 s.d.) as compared to the 0.05% (1 s.d.) for the studied diodes. The linearity of the response with dose was excellent (R2 = 1.000) for both in vivo dosimetry systems. The absolute values of the studied correction factors for the MOSFETs when covered by the different build-up caps were of the same order of those determined for the diodes. However, the uncertainties of the correction factors for MOSFETs were significantly higher than for diodes. Although the intrinsic precision and the uncertainty on the CF was higher for MOSFET detectors than for the studied diodes, the total uncertainty in entrance dose determination, once they were calibrated, was of 2.9% (1 s.d.) while for diodes it was 2.0% (1 s.d.). MOSFETs showed no sensitivity variation with accumulated dose or temperature. When used in the high sensitivity mode, after approximately 50 Gy of accumulated dose MOSFETs could no longer be used as radiation dosimeters. In conclusion, MOSFETs can be used for entrance in vivo dosimetry in high energy x-rays beams if covered by an appropriate build-up cap. Metallic build-up caps, such as those used for in vivo diodes, have the advantage of greater patient comfort and less perturbation of the treatment field than the other build-up caps tested, while keeping the correction factors of the same order.

In vivo dosimetry: intercomparison between p-type based and n-type based diodes for the 16-25 MV energy range.

This paper compares two different types of diodes designed to cover the energy range from 16 to 25 MV, one n-type (diode-A) and the other p-type (diode-B). A 18 MV x-ray beam has been used for all tests. Signal stability postirradiation, intrinsic precision and linearity of response with dose, front-back symmetry, and dose decrease under the diode were studied. Also, the water equivalent thickness of the build up caps was determined. Both types of diodes were calibrated to give entrance dose. Entrance correction factors for field size, tray, source skin distance, angle, and wedge were determined. Finally, the effect of dose rate, temperature and accumulated dose on the diode's response were studied. Only diode-A had full build-up for 18 MV x rays and standard irradiation conditions. Field size correction factor was about 2%-4% for field sizes bigger than 20 x 20 cm2 for both diodes. Tray correction factor was negligible for diode-A while diode-B would overestimate the dose by a 2% for a 40 x 40 cm2 field size if the correction factor was not applied. Wedge correction factors are only relevant for the 60 degrees wedge, being the correction factor for diode-A significantly higher than for diode-B. Diode-A showed less temperature dependence than diode-B. Sensitivity dependence on dose per pulse was a 1.5% higher for diode-A than for diode-B and therefore a higher SSD dependence was found for diode-A. The loss of sensitivity with accumulated radiation dose was only about 0.3% for diode-A, after 300 Gy, while it amounted to 8% for diode-B. Weighing the different correction factors for both types of diodes no conclusions about which type is better can be driven. From these results it can be also seen that the dependence of the diode response on dose rate in a pulsed beam does not seem to be associated with the fact of being n-type or p-type but could be related to the doping level of the diodes.

Thermoluminescence dosimetry applied to in vivo dose measurements for total body irradiation techniques.

BACKGROUND AND PURPOSE: In total body irradiation (TBI) treatments in vivo dosimetry is recommended because it makes it possible to ensure the accuracy and quality control of dose delivery. The aim of this work is to set up an in vivo thermoluminescence dosimetry (TLD) system to measure the dose distribution during the TBI technique used prior to bone marrow transplant. Some technical problems due to the presence of lung shielding blocks are discussed. MATERIALS AND METHODS: Irradiations were performed in the Hospital de la Santa Creu i Sant Pau by means of a Varian Clinac-1800 linear accelerator with 18 MV X-ray beams. Different TLD calibration experiments were set up to optimize in vivo dose assessment and to analyze the influence on dose measurement of shielding blocks. An algorithm to estimate midplane doses from entrance and exit doses is proposed and the estimated dose in critical organs is compared to internal dose measurements performed in an Alderson anthropomorphic phantom. RESULTS: The predictions of the dose algorithm, even in heterogeneous zones of the body such as the lungs, are in good agreement with the experimental results obtained with and without shielding blocks. The differences between measured and predicted values are in all cases lower than 2%. CONCLUSIONS: The TLD system described in this work has been proven to be appropriate for in vivo dosimetry in TBI irradiations. The described calibration experiments point out the difficulty of calibrating an in vivo dosimetry system when lung shielding blocks are used.

Midplane dose determination during total body irradiation using in vivo dosimetry.

BACKGROUND AND PURPOSE: During TBI techniques an accurate determination of the dose distribution is very difficult when using commercial treatment planning systems. In order to determine the midplane dose, an algorithm was developed based on the use of in vivo dosimetry. MATERIALS AND METHODS: Scanditronix EDP-30 diodes were placed at the entrance and the exit surface for in vivo dosimetry. The proposed algorithm was validated firstly in a regular and homogeneous phantom of different thickness with an ionization chamber and TL dosimeters and secondly in an Alderson anthropomorphic phantom with TL dosimeters. In this study, in vivo measurements were evaluated in 60 patients and furthermore, in 20 of them, the midplane dose calculated with this algorithm was compared with the method described by Rizzotti A, Compri C, Garusi GF. Dose evaluation to patients irradiated by 60Co beams, by means of direct measurement on the incident and on the exit surfaces. Radiother. Oncol. 1985;3:279-283. RESULTS: No differences were found between the two methods. The differences between dose values calculated with both methods and dose values measured with the ionization chamber and TL dosimeters were within +/-22% and +/-4%, respectively, in the regular and homogeneous phantom and within +/-2% in the Alderson phantom. The algorithm was useful in calculating the midplane dose when heterogeneities as lungs were present. Even when partial transmission blocks were used to reduce the dose to the lungs, the algorithm with modified correction factors gave a midplane lung dose in the Alderson phantom within 1.3% of the measurements with TL dosimeters. For 360 patients' measurements in each A-P and P-A field, the relative deviations were analyzed between the measured and calculated entrance, exit dose and midplane dose and the prescribed dose, always applying the temperature correction factor. These deviations at the entrance dose were within +/-4%. Greater deviations were found for the exit dose measurements. Deviations larger than +/-10% corresponded in general to obese patients, with a thickness over 25 cm. The relative deviations between the total received and prescribed midplane doses in 60 patients were within +/-3%. CONCLUSIONS: The results indicate excellent correspondence between the total prescribed and calculated midplane doses using this algorithm while also no significant differences were found when the Rizzotti method was used. Comparison between doses measured with TL dosimeters in the core of Alderson phantom lungs and doses calculated from in vivo measurements showed that the proposed algorithm could be used in the presence of heterogeneities even when partial transmission blocks were used. The temperature correction factor must be applied in order to avoid a 2-3% dose overestimation.

Calibration of semiconductor detectors for dose assessment in total body irradiation.

The aim of this paper is to discuss the measurements carried out to implement 'in vivo dosimetry' with EDP-30 diodes in total body irradiation (TBI) techniques. Exit calibrations and calibrations behind cerrobend protection blocks showed the importance of calibrating diodes in all relevant clinical conditions. Special attention was given to calibration of diodes behind cerrobend blocks. Dependence of the calibration factors on the thickness of the shielding blocks was, therefore, studied. This dependence was again studied after adding a wax cap to the diode and when the ionisation chamber was placed at the same depth as the measuring point of the diode. Temperature dependence in diode sensitivity and dependence on accumulated dose for diodes response and for temperature correction factors were also examined.