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  • 1.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Dose to 'water-like' media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate2015In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 60, no 1, p. 309-337Article in journal (Refereed)
    Abstract [en]

    The difference between Monte Carlo Treatment Planning (MCTP) based on the assumption of 'water-like' tissues with densities obtained from CT procedures, or on tissue compositions derived from CT-determined densities, have been investigated. Stopping powers and electron fluences have been calculated for a range of media and body tissues for 6 MV photon beams, including changes in their physical data (density and stopping powers). These quantities have been used to determine absorbed doses using cavity theory. It is emphasized that tissue compositions given in ICRU or ICRP reports should not be given the standing of physical constants as they correspond to average values obtained for a limited number of human-body samples. It has been shown that mass stopping-power ratios to water are more dependent on patient-to-patient composition differences, and therefore on their mean excitation energies (I-values), than on mass density. Electron fluence in different media are also more dependent on media composition (and their I-values) than on density. However, as a consequence of the balance between fluence and stopping powers, doses calculated from their product are more constant than what the independent stopping powers and fluence variations suggest. Additionally, cancelations in dose ratios minimize the differences between the 'water-like' and 'tissue' approaches, yielding practically identical results except for bone, and to a lesser extent for adipose tissue. A priori, changing from one approach to another does not seem to be justified considering the large number of approximations and uncertainties involved throughout the treatment planning tissue segmentation and dose calculation procedures. The key issue continues to be the composition of tissues and their I-values, and as these cannot be obtained for individual patients, whatever approach is selected does not lead to significant differences from a water reference dose, the maximum of these being of the order of 5% for bone tissues. Considering, however, current developments in advanced dose calculation methods, planning in terms of dose-to-tissue should be the preferred choice, under the expectancy that progress in the field will gradually improve some of the crude approximations included in MCTP and numerical transport methods. The small differences obtained also show that a retrospective conversion from dose-to-tissue to dose-to-water, based on a widely used approach, would mostly increase the final uncertainty of the treatment planning process. It is demonstrated that, due to the difference between electron fluence distributions in water and in body tissues, the conversion requires an additional fluence correction that has so far been neglected. An improved expression for the conversion and data for the fluence correction factor are provided. These will be necessary even in a dose-to-tissue environment, for the normalization of the treatment plan to the reference dosimetry of the treatment unit, always calibrated in terms of absorbed dose to water.

  • 2.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics.
    On the clinical spatial resolution achievable with protons and heavier charged particle radiotherapy beams2009In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 54, no 11, p. n205-N215Article in journal (Refereed)
    Abstract [en]

    The 'sub-millimetre precision' often claimed to be achievable in protons and light ion beam therapy is analysed using the Monte Carlo code SHIELD-HIT for a broad range of energies. Based on the range of possible values and uncertainties of the mean excitation energy of water and human tissues, as well as of the composition of organs and tissues, it is concluded that precision statements deserve careful reconsideration for treatment planning purposes. It is found that the range of I-values of water stated in ICRU reports 37, 49 and 73 (1984, 1993 and 2005) for the collision stopping power formulae, namely 67 eV, 75 eV and 80 eV, yields a spread of the depth of the Bragg peak of protons and heavier charged particles (carbon ions) of up to 5 or 6 mm, which is also found to be energy dependent due to other energy loss competing interaction mechanisms. The spread is similar in protons and in carbon ions having analogous practical range. Although accurate depth-dose distribution measurements in water can be used at the time of developing empirical dose calculation models, the energy dependence of the spread causes a substantial constraint. In the case of in vivo human tissues, where distribution measurements are not feasible, the problem poses a major limitation. In addition to the spread due to the currently accepted uncertainties of their I-values, a spread of the depth of the Bragg peak due to the varying compositions of soft tissues is also demonstrated, even for cases which could be considered practically identical in clinical practice. For these, the spreads found were similar to those of water or even larger, providing support to international recommendations advising that body-tissue compositions should not be given the standing of physical constants. The results show that it would be necessary to increase the margins of a clinical target volume, even in the case of a water phantom, due to an 'intrinsic basic physics uncertainty', adding to those margins usually considered in normal clinical practice due to anatomical or therapeutic strategy reasons. Individualized patient determination of tissue composition along the complete beam path, rather than CT Hounsfield numbers alone, would also probably be required even to reach 'sub-centimetre precision'.

  • 3.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics.
    On the p(dis) correction factor for cylindrical chambers2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 5, p. l9-L16Article in journal (Refereed)
    Abstract [en]

    The authors of a recent paper (Wang and Rogers 2009 Phys. Med. Biol. 54 1609) have used the Monte Carlo method to simulate the 'classical' experiment made more than 30 years ago by Johansson et al (1978 National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) on the displacement (or replacement) perturbation correction factor p(dis) for cylindrical chambers in Co-60 and high-energy photon beams. They conclude that an 'unreasonable normalization at dmax' of the ionization chambers response led to incorrect results, and for the IAEA TRS-398 Code of Practice, which uses ratios of those results, 'the difference in the correction factors can lead to a beam calibration deviation of more than 0.5% for Farmer-like chambers'. The present work critically examines and questions some of the claims and generalized conclusions of the paper. It is demonstrated that for real, commercial Farmer-like chambers, the possible deviations in absorbed dose would be much smaller (typically 0.13%) than those stated by Wang and Rogers, making the impact of their proposed values negligible on practical high-energy photon dosimetry. Differences of the order of 0.4% would only appear at the upper extreme of the energies potentially available for clinical use (around 25 MV) and, because lower energies are more frequently used, the number of radiotherapy photon beams for which the deviations would be larger than say 0.2% is extremely small. This work also raises concerns on the proposed value of p(dis) for Farmer chambers at the reference quality of Co-60 in relation to their impact on electron beam dosimetry, both for direct dose determination using these chambers and for the cross-calibration of plane-parallel chambers. The proposed increase of about 1% in p(dis) (compared with TRS-398) would lower the k(Q) factors and therefore D-w in electron beams by the same amount. This would yield a severe discrepancy with the current good agreement between electron dosimetry based on an electron cross-calibrated plane-parallel chamber (against a Farmer) or on a directly Co-60 calibrated plane-parallel chamber, which is not likely to be in error by 1%. It is suggested that the influence of the Co-60 source spectrum used in the simulations may not be negligible for calculations aimed at an uncertainty level of 0.1%.

  • 4.
    Andreo, Pedro
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Burns, David T.
    Salvat, Francesc
    On the uncertainties of photon mass energy-absorption coefficients and their ratios for radiation dosimetry2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 8, p. 2117-2136Article in journal (Refereed)
    Abstract [en]

    A systematic analysis of the available data has been carried out for mass energy-absorption coefficients and their ratios for air, graphite and water for photon energies between 1 keV and 2 MeV, using representative kilovoltage x-ray spectra for mammography and diagnostic radiology below 100 kV, and for Ir-192 and Co-60 gamma-ray spectra. The aim of this work was to establish 'an envelope of uncertainty' based on the spread of the available data. Type A uncertainties were determined from the results of Monte Carlo (MC) calculations with the PENELOPE and EGSnrc systems, yielding mean values for mu(en)/rho with a given statistical standard uncertainty. Type B estimates were based on two groupings. The first grouping consisted of MC calculations based on a similar implementation but using different data and/or approximations. The second grouping was formed by various datasets, obtained by different authors or methods using the same or different basic data, and with different implementations (analytical, MC-based, or a combination of the two); these datasets were the compilations of NIST, Hubbell, Johns-Cunningham, Attix and Higgins, plus MC calculations with PENELOPE and EGSnrc. The combined standard uncertainty, u(c), for the mu(en)/rho values for the mammography x-ray spectra is 2.5%, decreasing gradually to 1.6% for kilovoltage x-ray spectra up to 100 kV. For Co-60 and Ir-192, u(c) is approximately 0.1%. The Type B uncertainty analysis for the ratios of mu(en)/rho values includes four methods of analysis and concludes that for the present data the assumption that the data interval represents 95% confidence limits is a good compromise. For the mammography x-ray spectra, the combined standard uncertainties of (mu(en)/rho)(graphite,air) and (mu(en)/rho)(graphite,water) are 1.5%, and 0.5% for (mu(en)/rho)(water,air), decreasing gradually down to u(c) = 0.1% for the three mu(en)/rho ratios for the gamma-ray spectra. The present estimates are shown to coincide well with those of Hubbell (1977 Rad. Res. 70 58-81), except for the lowest energy range (radiodiagnostic) where it is concluded that current databases and their systematic analysis represent an improvement over the older Hubbell estimations. The results for (mu(en)/rho)(graphite,air) for the gamma-ray dosimetry range are moderately higher than those of Seltzer and Bergstrom (2005 private communication).

  • 5.
    Andreo, Pedro
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wulff, Joerg
    Burns, David T.
    Palmans, Hugo
    Consistency in reference radiotherapy dosimetry: resolution of an apparent conundrum when Co-60 is the reference quality for charged-particle and photon beams2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 19, p. 6593-6621Article in journal (Refereed)
    Abstract [en]

    Substantial changes in ion chamber perturbation correction factors in Co-60 gamma-rays, suggested by recent Monte Carlo (MC) calculations, would cause a decrease of about 1.5% in the reference dosimetry of all types of charged particles (electrons, protons and heavier ions) based on calculated k(Q) values. It has gone largely unnoticed that the ratio of calibration coefficients N-D,N-w,N-Co60 and N-K,N-air,N-Co60 yields an experimental value of F-ch,F-Co60 = (s(w-air) pch)(Co60) through N-D,N-air,N-Co60. Coefficients provided by the IAEA and traceable to the BIPM for 91 NE-2571 chambers result in an average F-ch,F-Co60 which is compared with published (and new) MC simulations and with the value in IAEA TRS-398. It is shown that TRS-398 agrees within 0.12% with the experimental F-ch,F-Co60. The 1.5% difference resulting from MC calculations (1.1% for the new simulations) cannot be justified using current fundamental data and BIPM standards if consistency in the entire dosimetry chain is sought. For photons, MC k(Q) factors are compared with TRS-398. Using the same uncertainty for W-air, the two sets of data overlap considerably. Experimental k(Q) values from standards laboratories lie between the two sets of calculated values, showing no preference for one set over the other. Observed chamber-to-chamber differences, that include the effect of waterproof sleeves (also seen for Co-60), justify the recommendation in TRS-398 for k(Q) values specifically measured for the user chamber. Current developments on I-values for the stopping powers of water and graphite are presented. A weighted average I-water = 78 +/- 2 eV is obtained from published experimental and DRF-based values; this would decrease sw-air for all types of radiotherapy beams between 0.3% and 0.6%, and would consequently decrease the MC derived F-ch,F-Co60. The implications of a recent proposal for I-graphite = 81 eV are analysed, resulting in a potential decrease of 0.7% in N-K,N-air,N-Co60 which would raise the experimental F-ch,F-Co60; this would result in an increase of about 0.8% in the current TRS-398 value when referred to the BIPM standards. MC derived F-ch,F-Co60 using new stopping powers would then agree at a level of 0.1% with the experimental value, confirming the need for consistency in the dosimetry chain data. Should world average standards be used as reference, the figures would become +0.4% for TRS-398 and -0.3% for the MC calculation. F-ch,F-Q calculated for megavoltage photons using new stopping powers would decrease by between 0.2% and 0.5%. When they enter as a ratios in k(Q), differences with MC values based on current key data would be within 0.2% but their discrepancy with k(Q) experimental photon values remains unresolved. For protons the new data would require an increase in W-air,W-Q of about 0.6%, as this is inferred from a combination of calorimetry and ionometry. This consistent scenario would leave unchanged the current TRS-398 k(Q) (NE-2571) data for protons, as well as for ions heavier than protons unless new independent W-air,W-Q values become available. Also in these advanced radiotherapy modalities, the need for maintaining data consistency in an analysis that unavoidably must include the complete dosimetry chain is demonstrated.

  • 6.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Ten years after: Impact of recent research in photon and electron beam dosimetry on the IAEA TRS-398 Code of Practice2011In: Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS): Proceedings of an International Symposium. V. 1 / [ed] Benmakhlouf, H.; Andreo, P., Vienna: International Atomic Energy Agency, 2011, , p. 14p. 139-152Conference paper (Refereed)
  • 7.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Bouchard, Hugo
    Fransson, Annette
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry2011In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 56, no 22, p. 7179-7204Article in journal (Refereed)
    Abstract [en]

    Backscatter factors, B, and mass energy-absorption coefficient ratios, (mu(en)/rho)(omega,) (air), for the determination of the surface dose in diagnostic radiology were calculated using Monte Carlo simulations. The main purpose was to extend the range of available data to qualities used in modern x-ray techniques, particularly for interventional radiology. A comprehensive database for mono-energetic photons between 4 and 150 keV and different field sizes was created for a 15 cm thick water phantom. Backscattered spectra were calculated with the PENELOPE Monte Carlo system, scoring track-length fluence differential in energy with negligible statistical uncertainty; using the Monte Carlo computed spectra, B factors and (mu(en)/rho)(omega), air were then calculated numerically for each energy. Weighted averaging procedures were subsequently used to convolve incident clinical spectra with mono-energetic data. The method was benchmarked against full Monte Carlo calculations of incident clinical spectra obtaining differences within 0.3-0.6%. The technique used enables the calculation of B and (mu(en)/rho)(w), air for any incident spectrum without further time-consuming Monte Carlo simulations. The adequacy of the extended dosimetry data to a broader range of clinical qualities than those currently available, while keeping consistency with existing data, was confirmed through detailed comparisons. Mono-energetic and spectra-averaged values were compared with published data, including those in ICRU Report 74 and IAEA TRS-457, finding average differences of 0.6%. Results are provided in comprehensive tables appropriated for clinical use. Additional qualities can easily be calculated using a designed GUI interface in conjunction with software to generate incident photon spectra.

  • 8.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Fransson, Annette
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 2, p. 247-260Article in journal (Refereed)
    Abstract [en]

    Most of the existing backscatter factors for the dosimetry of clinical diagnostic x-ray beams have been calculated for 15 cm thick phantoms; these data are used for skin dose determinations which in general ignore the influence of phantom material and thickness. The former should strictly be required whenever dosimetry measurements are made on phantom materials different from those used for the backscatter factor calculations. The phantom or patient thickness is of special importance when skin dose determinations are made for infants or paediatric patients. In this work, the recently published formalism for reference dosimetry and comprehensive database of backscatter factors for clinical beams and water phantoms have been extended using two correction factors which account for phantom material and thickness. These were determined with simulations using the PENELOPE Monte Carlo system, for PMMA to analyse the influence of the phantom material relative to water, and for a broad range of thicknesses of water and PMMA to investigate the role of this parameter in patient dose estimates. The material correction factor was found to be in the range 3-10%, depending on the field size and the HVL. The thickness correction factor was in the range 2-12% for a 5 cm thick phantom and square field sizes between 5 and 35 cm, reaching a plateau of about ±1% for thicknesses beyond 13 cm. Expressions in the form of surface fits over the calculated data are provided which streamline the determination of backscatter factors for arbitrary thicknesses and phantom materials, as well as field sizes. Results demonstrate the inadequacy of using conventional backscatter factors (calculated for 15 cm thick phantoms) without correction factors that take into account the phantom material and its thickness.

  • 9.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
    Johansson, J.
    Paddick, I.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
    Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams2015In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 60, no 10, p. 3959-3973Article in journal (Refereed)
    Abstract [en]

    The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife (R) (LGK) radiotherapy units is a challenge for the physicist due to the under or over estimation of these factors by a vast majority of the detectors commercially available. Output correction factors, introduced in the international formalism published by Alfonso (2008 Med. Phys. 35 5179-86), standardize the determination of OFs for small photon beams by correcting detector-reading ratios to yield OFs in terms of absorbed-dose ratios. In this work output correction factors for a number of detectors have been determined for LGK Perfexion (TM) Co-60 gamma-ray beams by Monte Carlo (MC) calculations and measurements. The calculations were made with the MC system PENELOPE, scoring the energy deposited in the active volume of the detectors and in a small volume of water; the detectors simulated were two silicon diodes, one liquid ionization chamber (LIC), alanine and TLD. The calculated LIC output correction factors were within +/- 0.4%, and this was selected as the reference detector for experimental determinations where output correction factors for twelve detectors were measured, normalizing their readings to those of the LIC. The MC-calculated and measured output correction factors for silicon diodes yielded corrections of up to 5% for the smallest LGK collimator size of 4 mm diameter. The air ionization chamber measurements led to extremely large output correction factors, caused by the well-known effect of partial volume averaging. The corrections were up to 7% for the natural diamond detector in the 4 mm collimator, also due to partial volume averaging, and decreased to within about +/- 0.6% for the smaller synthetic diamond detector. The LIC, showing the smallest corrections, was used to investigate machine-to-machine output factor differences by performing measurements in four LGK units with different dose rates. These resulted in OFs within +/- 0.6% and +/- 0.2% for the 4 mm and 8 mm collimators, respectively, providing evidence for the use of generic OFs for these LGK beams. Using the experimentally derived output correction factors, OFs can be measured using a wide range of commercially available detectors.

  • 10.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
    Johansson, Jonas
    Paddick, Ian
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
    Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beamsManuscript (preprint) (Other academic)
    Abstract [en]

    The measurement of output factors (OF) for the small beams of the Leksell Gamma Knife® (LGK) is a challenge for the physicist due to the over- or underestimation of these factors by a vast majority of commercially available detectors. Output correction factors, introduced in the new international formalism published by Alfonso et al. (2008) standardizes the determination of OFs for small photon beams by correcting the detector reading ratios with output correction factors in order to yield the correct OF. Output correction factors have, in this work, been determined for LGK Perfexion™ 60Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The MC calculations were performed using the MC system PENELOPE scoring the doses to the active volumes of the detectors and to a small volume of water. Two silicon diodes, one liquid ionization chamber (LIC), one alanine and one TLD detector were included in the MC derivation of the output correction factors. The LIC resulted in correction factors within ±0.4% and was therefore selected as the reference detector for the measurements. Twelve detectors were used in the experimental determination of the output correction factors by normalizing their detector readings to those of the LIC. The MC-calculated and experimentally determined output correction factors for the silicon diodes resulted in up to a -4% correction for the smallest collimator size. The air ionization chamber measurements resulted in extremely large output correction factors, due to the well-known effect of partial volume averaging (PVA). The natural diamond detector resulted in 6% correction for the 4 mm collimator, also due to PVA, whereas the smaller synthetic diamond detector resulted in a correction within ±1%. The LIC, requiring the smallest correction, was used to explore machine-to-machine differences in the OFs by performing measurements in four LGK units with different dose rates. This resulted in OFs within ±0.6% and ±0.3% for the 4 mm and 8 mm collimators, respectively, favouring the use of generic OFs. Using these experimentally derived correction factors, OFs can now be measured using a wide range of commercially available detectors.

  • 11.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Sempau, Josep
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study2014In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 41, no 4, p. 041711-Article in journal (Refereed)
    Abstract [en]

    Purpose: To determine detector-specific output correction factors, k(Qclin,Qmsr)(fclin,fmsr) in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers. Methods: Field output factors, defined according to the international formalism published by Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 x 0.5 cm(2) and 10 x 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 x 10 cm(2). Results: The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about +/- 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or un-shielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates. Conclusions: The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model. (C) 2014 American Association of Physicists in Medicine.

  • 12. Goma, C.
    et al.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet.
    Sempau, J.
    Spencer-Attix water/medium stopping-power ratios for the dosimetry of proton pencil beams2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 8, p. 2509-2522Article in journal (Refereed)
    Abstract [en]

    This paper uses Monte Carlo simulations to calculate the Spencer-Attix water/medium stopping-power ratios (s(w, med)) for the dosimetry of scanned proton pencil beams. It includes proton energies from 30 to 350 MeV and typical detection materials such as air (ionization chambers), radiochromic film, gadolinium oxysulfide (scintillating screens), silicon and lithium fluoride. Track-ends and particles heavier than protons were found to have a negligible effect on the water/air stopping-power ratios (s(w, air)), whereas the mean excitation energy values were found to carry the largest source of uncertainty. The initial energy spread of the beam was found to have a minor influence on the s(w, air) values in depth. The water/medium stopping-power ratios as a function of depth in water were found to be quite constant for air and radiochromic film-within 2.5%. Also, the s(w, med) values were found to have no clinically relevant dependence on the radial distance-except for the case of gadolinium oxysulfide and proton radiography beams. In conclusion, the most suitable detection materials for depth-dose measurements in water were found to be air and radiochromic film active layer, although a small correction is still needed to compensate for the different s(w, med) values between the plateau and the Bragg peak region. Also, all the detection materials studied in this work-except for gadolinium oxysulfide-were found to be suitable for lateral dose profiles and field-specific dose distribution measurements in water.

  • 13. Omar, Artur
    et al.
    Benmakhlouf, Hamza
    Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
    Marteinsdottir, Maria
    Bujila, Robert
    Nowik, Patrik
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics.
    Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures2014In: Medical Imaging 2014: Physics of Medical Imaging. Proceedings vol. 9033 / [ed] Bruce R. Whiting; Christoph Hoeschen, SPIE - International Society for Optical Engineering, 2014, , p. 8Conference paper (Refereed)
    Abstract [en]

    Complex interventional and diagnostic x-ray angiographic (XA) procedures may yield patient skin doses exceeding thethreshold for radiation induced skin injuries. Skin dose is conventionally determined by converting the incident air kermafree-in-air into entrance surface air kerma, a process that requires the use of backscatter factors. Subsequently, theentrance surface air kerma is converted into skin kerma using mass energy-absorption coefficient ratios tissue-to-air,which for the photon energies used in XA is identical to the skin dose. The purpose of this work was to investigate howthe cranial bone affects backscatter factors for the dosimetry of interventional neuroradiology procedures.The PENELOPE Monte Carlo system was used to calculate backscatter factors at the entrance surface of a spherical anda cubic water phantom that includes a cranial bone layer. The simulations were performed for different clinical x-rayspectra, field sizes, and thicknesses of the bone layer.The results show a reduction of up to 15% when a cranial bone layer is included in the simulations, compared withconventional backscatter factors calculated for a homogeneous water phantom. The reduction increases for thicker bonelayers, softer incident beam qualities, and larger field sizes, indicating that, due to the increased photoelectric crosssectionof cranial bone compared to water, the bone layer acts primarily as an absorber of low-energy photons.For neurointerventional radiology procedures, backscatter factors calculated at the entrance surface of a water phantomcontaining a cranial bone layer increase the accuracy of the skin dose determination.

  • 14. Palmans, H.
    et al.
    Al-Sulaiti, L.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics.
    Shipley, D.
    Luehr, A.
    Bassler, N.
    Martinkovic, J.
    Dobrovodsky, J.
    Rossomme, S.
    Thomas, R. A. S.
    Kacperek, A.
    Fluence correction factors for graphite calorimetry in a low-energy clinical proton beam: I. Analytical and Monte Carlo simulations2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 10, p. 3481-3499Article in journal (Refereed)
    Abstract [en]

    The conversion of absorbed dose-to-graphite in a graphite phantom to absorbed dose-to-water in a water phantom is performed by water to graphite stopping power ratios. If, however, the charged particle fluence is not equal at equivalent depths in graphite and water, a fluence correction factor, k(fl), is required as well. This is particularly relevant to the derivation of absorbed dose-to-water, the quantity of interest in radiotherapy, from a measurement of absorbed dose-to-graphite obtained with a graphite calorimeter. In this work, fluence correction factors for the conversion from dose-to-graphite in a graphite phantom to dose-to-water in a water phantom for 60 MeV mono-energetic protons were calculated using an analytical model and five different Monte Carlo codes (Geant4, FLUKA, MCNPX, SHIELD-HIT and McPTRAN.MEDIA). In general the fluence correction factors are found to be close to unity and the analytical and Monte Carlo codes give consistent values when considering the differences in secondary particle transport. When considering only protons the fluence correction factors are unity at the surface and increase with depth by 0.5% to 1.5% depending on the code. When the fluence of all charged particles is considered, the fluence correction factor is about 0.5% lower than unity at shallow depths predominantly due to the contributions from alpha particles and increases to values above unity near the Bragg peak. Fluence correction factors directly derived from the fluence distributions differential in energy at equivalent depths in water and graphite can be described by k(fl) = 0.9964 + 0.0024 . z(w-eq) with a relative standard uncertainty of 0.2%. Fluence correction factors derived from a ratio of calculated doses at equivalent depths in water and graphite can be described by k(fl) = 0.9947 + 0.0024 . z(w-eq) with a relative standard uncertainty of 0.3%. These results are of direct relevance to graphite calorimetry in low-energy protons but given that the fluence correction factor is almost solely influenced by non-elastic nuclear interactions the results are also relevant for plastic phantoms that consist of carbon, oxygen and hydrogen atoms as well as for soft tissues.

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