[en] Purpose: In current dosimetry protocols [AAPM TG51, IAEA TRS-389] the beam quality correction factor kQ and the water-to-air restricted mass collision stopping-power ratio SPR are related to beam quality specifiers %dd(10)_x respectively TPR_20,10 Determining kQ and SPR using these regular beam quality specifiers for conventional accelerators (WFF) and flattening filter free accelerators (FFF) similarly could lead to systemic bias.The influence of the flattening filter on the relationship between various beam quality specifiers and SPR respectively k_Q was studied using Monte Carlo simulations with realistic beam sources. Methods: All Monte Carlo simulations were performed using the BEAMnrc/EGSnrc code system. Radiation transport through nine linear accelerator heads modeled according to technical drawings given by the manufactures and a ⁶⁰ Co therapy source was simulated with BEAMnrc and then used as a radiation source for further simulations. FFF beam sources were implemented by removing the fattening filter from the WFF model. SPR was calculated applying the user code SPRRZnrc. The mean photon energy below the accelerator head and the mean energies of photons and electrons at the measuring point within the water phantom were calculated using FLURZnrc. Dose calculations within a small water voxel and the thimble ionization chamber PTW-31010 in a water depth of 10 cm were made using the egs-chamber code. Results: SPR and k_Q as a function of fluence spectra based beam quality specifiers as well as conventional beam quality specifiers differ systematically between FFF and WFF beams. According to the results the specifier %dd(10)_x revealed the smallest deviation (max. 0.4%) between FFF and WFF beams. Conclusion: The results show that %dd(10)_x is an acceptable beam quality specifier for FFF beams. Nevertheless the results confirm the expected bias between FFF and WFF beams which must by further investigated

[en] The application of small photon fields in modern radiotherapy requires the determination of total scatter factors Scp or field factors Ωfclin,fmsrQclin,Qmsr with high precision. Both quantities require the knowledge of the field-size-dependent and detector-dependent correction factor kfclin,fmsrQclin,Qmsr. The aim of this study is the determination of the correction factor kfclin,fmsrQclin,Qmsr for different types of detectors in a clinical 6 MV photon beam of a Siemens KD linear accelerator. The EGSnrc Monte Carlo code was used to calculate the dose to water and the dose to different detectors to determine the field factor as well as the mentioned correction factor for different small square field sizes. Besides this, the mean water to air stopping power ratio as well as the ratio of the mean energy absorption coefficients for the relevant materials was calculated for different small field sizes. As the beam source, a Monte Carlo based model of a Siemens KD linear accelerator was used. The results show that in the case of ionization chambers the detector volume has the largest impact on the correction factor kfclin,fmsrQclin,Qmsr; this perturbation may contribute up to 50% to the correction factor. Field-dependent changes in stopping-power ratios are negligible. The magnitude of kfclin,fmsrQclin,Qmsr is of the order of 1.2 at a field size of 1 × 1 cm2 for the large volume ion chamber PTW31010 and is still in the range of 1.05–1.07 for the PinPoint chambers PTW31014 and PTW31016. For the diode detectors included in this study (PTW60016, PTW 60017), the correction factor deviates no more than 2% from unity in field sizes between 10 × 10 and 1 × 1 cm2, but below this field size there is a steep decrease of kfclin,fmsrQclin,Qmsr below unity, i.e. a strong overestimation of dose. Besides the field size and detector dependence, the results reveal a clear dependence of the correction factor on the accelerator geometry for field sizes below 1 × 1 cm2, i.e. on the beam spot size of the primary electrons hitting the target. This effect is especially pronounced for the ionization chambers. In conclusion, comparing all detectors, the unshielded diode PTW60017 is highly recommended for small field dosimetry, since its correction factor kfclin,fmsrQclin,Qmsr is closest to unity in small fields and mainly independent of the electron beam spot size. (paper)

[en] The use of small photon fields in modern radiotherapy requires the determination of total scatter factors S_cp or field factors Ω with high precision. Therefore, chamber-dependent correction factors for dose measurements in small fields are necessary. In this study Monte Carlo simulations were used to calculate the field factor Ω and chamber response-related correction factors for four different types of detectors in a clinical 6 MV photon beam for a square field size of 1 cm * 1 cm. As a beam source a Monte Carlo-based model of a Siemens KD linear accelerator was applied. The calculations aimed at the investigation of the influence of electron beam spot size on correction factors for small field dosimetry. The results confirm that accurate Monte Carlo calculations of the field factor Ω can only be carried out when the exact electron spot size is known. On the other hand no dependence of the electron beam spot size on the correction factors for the field size of 1 cm * 1 cm was observed. (authors)

[en] Purpose: The impact of removing the flattening filter on absolute dosimetry based on IAEA’s TPR-398 and AAPM’s TG-51 was investigated in this study using Monte Carlo simulations. Methods: The EGSnrc software package was used for all Monte Carlo simulations performed in this work. Five different ionization chambers and nine linear accelerator heads have been modeled according to technical drawings. To generate a flattening filter free radiation field the flattening filter was replaced by a 2 mm thick aluminum layer. Dose calculation in a water phantom were performed to calculate the beam quality correction factor k_Q as a function of the beam quality specifiers %dd(10)_x, TPR_2_0_,_1_0 and mean photon and electron energies at the point of measurement in photon fields with (WFF) and without flattening filter (FFF). Results: The beam quality correction factor as a function of %dd(10)_x differs systematically between FFF and WFF beams for all investigated ionization chambers. The largest difference of 1.8% was observed for the largest investigated Farmer-type ionization chamber with a sensitive volume of 0.69 cm"3. For ionization chambers with a smaller nominal sensitive volume (0.015 – 0.3 cm"3) the deviation was less than 0.4% between WFF and FFF beams for %dd(10)_x > 62%. The specifier TPR_2_0_,_1_0 revealed only a good correlation between WFF and FFF beams (< 0.3%) for low energies. Conclusion: The results confirm that %dd(10)_x is a suitable beam quality specifier for FFF beams with an acceptable bias. The deviation depends on the volume of the ionization chamber. Using %dd(10)_x to predict k_Q for a large volume chamber in a FFF photon field may lead to not acceptable errors according to the results of this study. This bias may be caused by the volume effect due to the inhomogeneous photon fields of FFF linear accelerators.

[en] Purpose: Changing the characteristic of a photon beam by mechanically removing the flattening filter may impact the dose response of ionization chambers. Thus, perturbation factors of cylindrical ionization chambers in conventional and flattening filter free photon beams were calculated by Monte Carlo simulations. Methods: The EGSnrc/BEAMnrc code system was used for all Monte Carlo calculations. BEAMnrc models of nine different linear accelerators with and without flattening filter were used to create realistic photon sources. Monte Carlo based calculations to determine the fluence perturbations due to the presens of the chambers components, the different material of the sensitive volume (air instead of water) as well as the volume effect were performed by the user code egs-chamber. Results: Stem, central electrode, wall, density and volume perturbation factors for linear accelerators with and without flattening filter were calculated as a function of the beam quality specifier TPR_2_0_/_1_0. A bias between the perturbation factors as a function of TPR_2_0_/_1_0 for flattening filter free beams and conventional linear accelerators could not be observed for the perturbations caused by the components of the ionization chamber and the sensitive volume. Conclusion: The results indicate that the well-known small bias between the beam quality correction factor as a function of TPR20/10 for the flattening filter free and conventional linear accelerators is not caused by the geometry of the detector but rather by the material of the sensitive volume. This suggest that the bias for flattening filter free photon fields is only caused by the different material of the sensitive volume (air instead of water)

[en] Purpose: Developing a fast and accurate calculation model to reconstruct the applied photon fluence from an external photon radiation therapy treatment based on an image recorded by an electronic portal image device (EPID). Methods: To reconstruct the initial photon fluence the 2D EPID image was corrected for scatter from the patient/phantom and EPID to generate the transmitted primary photon fluence. This was done by an iterative deconvolution using precalculated point spread functions (PSF). The transmitted primary photon fluence was then backprojected through the patient/phantom geometry considering linear attenuation to receive the initial photon fluence applied for the treatment.The calculation model was verified using Monte Carlo simulations performed with the EGSnrc code system. EPID images were produced by calculating the dose deposition in the EPID from a 6 MV photon beam irradiating a water phantom with air and bone inhomogeneities and the ICRP anthropomorphic voxel phantom. Results: The initial photon fluence was reconstructed using a single PSF and position dependent PSFs which depend on the radiological thickness of the irradiated object. Appling position dependent point spread functions the mean uncertainty of the reconstructed initial photon fluence could be reduced from 1.13 % to 0.13 %. Conclusion: This study presents a calculation model for fluence reconstruction from EPID images. The"R"e"s"u"l"t show a clear advantage when position dependent PSF are used for the iterative reconstruction. The basic work of a reconstruction method was established and further evaluations must be made in an experimental study

[en] Purpose: Today the majority of radiation therapy treatments are performed at medical electron linear accelerators (linacs). The accelerated electrons are used for the generation of bremsstrahlung photons. The use of higher electron respectively photon energies has some advantages over lower energies such as the longer dose build-up. However photons with energies higher than ∼7 MeV can additionally to the interaction with bound electrons undergo inelastic reactions with nuclei. These photonuclear reactions lead to the emission of fast neutrons which contaminate the primary photon field. The neutrons might penetrate through the collimators and deliver out-of-field dose to the patient. Furthermore the materials inside the linac head as well as the air inside the treatment room get activated which might deliver dose to the medical employees even when the linac is not in operation. A detailed knowledge of these effects is essential for adequate radiation protection of the employees and an optimal patient treatment. Methods: It is a common method to study the radiation fields of such linacs by means of Monte Carlo simulations. For the investigation of the effects caused by photonuclear reactions a typical linac in high energy mode (Varian Clinac 18 MV-X) as well as the surrounding bunker were modelled and simulated using the Monte Carlo code FLUKA which includes extensive nuclear reaction and neutron transport models additional to electron-photon transport as well as capabilities for a detailed study of effective dose distributions and activation yields. Results: Neutron spectra as well as neutron effective dose distributions within the bunker were obtained, reaching up to some mSv/Gy in the patient’s plane. The results are normalized per Gy in the depth dose maximum at 10×10 cm"2 field size. Therefore an absolute interpretation is possible. Conclusion: The obtained data gives a better understanding of the photonuclear reaction caused effects

[en] Purpose: National and international dosimetry protocols assume a position accuracy for ionization chambers of less than 0.2mm. To follow this precept the manufacturer PTW-Freiburg introduced a positioning assistance system (TRUFIX) for their particular ion chambers. Aim of this study is an experimental investigation of the positioning uncertainties for ROOS-type ionization chambers. Methods: For all measurements a linear accelerator Elekta Synergie was used. The experiments were performed in a water-phantom. To collimate the electron beam a 10×10cm"2 applicator was installed. All measured depth dose curves were normalized to their maximum. In all cases the TRUFIX system was applied for chamber positioning. For the first measurement series, to determine the positioning reproducibility of a ROOS chamber, one person placed the chamber three times in a 6 MeV electron beam. The mean value of this three measurements was the reference for further six random persons who repeated this procedure. The results were compared for different depths (R_5_0, z_r_e_f and R_p). To investigate the impact of different individual chambers of the same type 10 different ROOS chambers were placed by the same person in a 6, 12 and 18MeV electron beam and the measured reference depths z_r_e_f were compared. Results: The absolute positioning reproducibility is less than 0.1mm for the same person. The positioning uncertainties are increasing up to +/−0.3mm if different persons perform the chamber’s positioning within the water phantom. The comparison of the 10 different ROOS chambers resulted in reference depths z_r_e_f with deviations in the range of +/−0.45mm for all energies. Conclusion: The position accuracy of 0.2mm can be fulfilled with the TRUFIX system. The comparison of the 10 different ROOS ionization chambers showed noticeable deviations in the determined reference depth. The impact of a positioning uncertainty of about 0.3–0.4mm on the total perturbation correction will be considered.

[en] Purpose: This paper aims to determine the effective point of measurement and the total perturbation correction p of parallel-plate chambers for clinical photon dosimetry. Methods: The effective point of measurement (EPOM) was calculated using the EGSnrc Monte Carlo code system with the EGSnrc user code egs- chamber. Depth dose curves of the ionization chambers were calculated in a water phantom for several high energy photon spectra (4, 6, 10, 15, 18 MV-X). Different normalization criterions (normalization to the maximum of the depth dose curve and normalization to the value in 10 cm depth) have been applied. The EPOM was determined by shifting the normalized depth dose curve of a small water voxel against the depth ionization curve until the disagreement (calculated by the root mean square deviation) reaches a minimum. In addition, the total perturbation correction p was calculated by the ratio of the dose to water and the product of the dose determined in the chamber and the water to air stopping power ratio. Results: The EPOM varied slightly depending on the chosen normalization criterion. For all chambers the necessary shift of the EPOM decreased linearly with increasing beam quality specifier TPR_2_0_/_1_0. For the Roos and NACP chamber, the results were positive suggesting that the chambers need to be shifted towards the focus. For the Markus chamber, the required shift was negative and for the Advanced Markus chamber partly negative and partly positive. The total perturbation correction p was almost independent of the depth. Only for regions below 1 cm the perturbation correction deviated significantly from unity. Conclusion: In the present study, the effective point of measurement and the total perturbation correction p was determined for four parallel-plate ionization chambers and five clinical relevant photon spectra. Applying the calculated EPOM, the residual perturbation correction p was mostly depth independent

[en] Purpose: Verifying an algorithm to reconstruct relative initial photon fluence for clinical use. Clinical EPID and CT images were acquired to reconstruct an external photon radiation treatment field. The reconstructed initial photon fluence could be used to verify the treatment or calculate the applied dose to the patient. Methods: The acquired EPID images were corrected for scatter caused by the patient and the EPID with an iterative reconstruction algorithm. The transmitted photon fluence behind the patient was calculated subsequently. Based on the transmitted fluence the initial photon fluence was calculated using a back-projection algorithm which takes the patient geometry and its energy dependent linear attenuation into account. This attenuation was gained from the acquired cone-beam CT or the planning CT by calculating a water-equivalent radiological thickness for each irradiation direction. To verify the algorithm an inhomogeneous phantom consisting of three inhomogeneities was irradiated by a static 6 MV photon field and compared to a reference flood field image. Results: The mean deviation between the reconstructed relative photon fluence for the inhomogeneous phantom and the flood field EPID image was 3% rising up to 7% for off-axis fluence. This was probably caused by the used clinical EPID calibration, which flattens the inhomogeneous fluence profile of the beam. Conclusion: In this clinical experiment the algorithm achieved good results in the center of the field while it showed high deviation of the lateral fluence. This could be reduced by optimizing the EPID calibration, considering the off-axis differential energy response. In further progress this and other aspects of the EPID, eg. field size dependency, CT and dose calibration have to be studied to realize a clinical acceptable accuracy of 2%.