Klemens Zink

Efficiency improvements for ion chamber calculations in high energy photon beams

This article presents the implementation of several variance reduction techniques that dramatically improve the simulation efficiency of ion chamber dose and perturbation factor calculations. The cavity user code for the EGSnrc Monte Carlo code system is extended by photon cross-section enhancement (XCSE), an intermediate phase-space storage (IPSS) technique, and a correlated sampling (CS) scheme. XCSE increases the density of photon interaction sites inside and in the vicinity of the chamber and results-in combination with a Russian Roulette game for electrons that cannot reach the cavity volume-in an increased efficiency of up to a factor of 350 for calculating dose in a Farmer type chamber placed at 10 cm depth in a water phantom. In combination with the IPSS and CS techniques, the efficiency for the calculation of the central electrode perturbation factor Pcel can be increased by up to three orders of magnitude for a single chamber location and by nearly four orders of magnitude when considering the Pcel variation with depth or with distance from the central axis in a large field photon beam. The intermediate storage of the phase-space properties of particles entering a volume that contains many possible chamber locations leads to efficiency improvements by a factor larger than 500 when computing a profile of chamber doses in the field of a linear accelerator photon beam. All techniques are combined in a new EGSnrc user code egs_chamber. Optimum settings for the variance reduction parameters are investigated and are reported for a Farmer type ion chamber. A few example calculations illustrating the capabilities of the egs_chamber code are presented.

Monte-Carlo-based perturbation and beam quality correction factors for thimble ionization chambers in high-energy photon beams

This paper presents a detailed investigation into the calculation of perturbation and beam quality correction factors for ionization chambers in high-energy photon beams with the use of Monte Carlo simulations. For a model of the NE2571 Farmer-type chamber, all separate perturbation factors as found in the current dosimetry protocols were calculated in a fixed order and compared to the currently available data. Furthermore, the NE2571 Farmer-type and a model of the PTW31010 thimble chamber were used to calculate the beam quality correction factor kQ. The calculations of kQ showed good agreement with the published values in the current dosimetry protocols AAPM TG-51 and IAEA TRS-398 and a large set of published measurements. Still, some of the single calculated perturbation factors deviate from the commonly used ones; especially prepl deviates more than 0.5%. The influence of various sources of uncertainties in the simulations is investigated for the NE2571 model. The influence of constructive details of the chamber stem shows a negligible dependence on calculated values. A comparison between a full linear accelerator source and a simple collimated point source with linear accelerator photon spectra yields comparable results. As expected, the calculation of the overall beam quality correction factor is sensitive to the mean ionization energy of graphite used. The measurement setup (source-surface distance versus source-axis distance) had no influence on the calculated values.

Monte Carlo calculated correction factors for diodes and ion chambers in small photon fields

The application of small photon fields in modern radiotherapy requires the determination of total scatter factors Scp or field factors Ω(f(clin), f(msr))(Q(clin), Q(msr)) with high precision. Both quantities require the knowledge of the field-size-dependent and detector-dependent correction factor k(f(clin), f(msr))(Q(clin), Q(msr)). The aim of this study is the determination of the correction factor k(f(clin), f(msr))(Q(clin), Q(msr)) 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 k(f(clin), f(msr))(Q(clin), Q(msr)); this perturbation may contribute up to 50% to the correction factor. Field-dependent changes in stopping-power ratios are negligible. The magnitude of k(f(clin), f(msr))(Q(clin), Q(msr)) is of the order of 1.2 at a field size of 1 × 1 cm(2) 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 cm(2), but below this field size there is a steep decrease of k(f(clin), f(msr))(Q(clin), Q(msr)) 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 cm(2), 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 k(f(clin), f(msr))(Q(clin), Q(msr)) is closest to unity in small fields and mainly independent of the electron beam spot size.

Investigation of systematic uncertainties in Monte Carlo-calculated beam quality correction factors

Modern Monte Carlo codes allow for the calculation of ion chamber specific beam quality correction factors k(Q), which are needed for dosimetry in radiotherapy. While statistical (type A) uncertainties of the calculated data can be minimized sufficiently, the influence of systematic (type B) uncertainties is mostly unknown. This study presents an investigation of systematic uncertainties of Monte Carlo-based k(Q) values for a NE2571 thimble ion chamber, calculated with the EGSnrc system. Starting with some general investigation on transport parameter settings, the influence of geometry and source variations is studied. Furthermore, a systematic examination of uncertainties due to cross section is introduced by determining the sensitivity of k(Q) results to changes in cross section data. For this purpose, single components of the photon cross sections and the mean excitation energy I in the electron stopping powers are varied. The corresponding sensitivities are subsequently applied with information of standard uncertainties for the cross section data found in the literature. It turns out that the calculation of k(Q) factors with EGSnrc is mostly insensitive to transport settings within the statistical uncertainties of approximately 0.1%. Severe changes in the dimensions of the chamber lead to comparatively small, insignificant changes. Further, the inclusion of realistic beam models, delivering a complete phase space instead of simple photon spectra, does not significantly influence the result. However, the uncertainties in electron cross sections have an impact on the final uncertainty of k(Q) to a comparatively large degree. For the NE2571 chamber investigated in this work, this uncertainty amounts to 0.4% at 24 MV, decreasing to 0.2% at 6 MV.

Monte Carlo calculations of beam quality correction factors kQ for electron dosimetry with a parallel-plate Roos chamber

Current dosimetry protocols (AAPM, IAEA, DIN) recommend the use of parallel-plate ionization chambers for the measurement of absorbed dose-to-water in clinical electron beams. For well-guarded plane-parallel chambers, it is assumed that the perturbation correction pQ is unity for all electron energies. In this study, we present detailed Monte Carlo simulations with the EGSnrc code for the widely used Roos parallel-plate chamber which is, besides other plane-parallel chamber types, recommended in all protocols. We have calculated the perturbation corrections pcav and pwall for a wide range of electron energies and for 60Co. While our results confirm the recommended value of unity for the cavity perturbation pcav, the wall-correction factor pwall depends on electron energy and decreases with increasing electron energy. For the lowest electron energies in this study (R50 approximately 2 cm), pwall deviates from unity by up to 1.5%. Using the perturbation factors for the different electron energies and those for the reference beam quality, 60Co, we have calculated the beam quality correction factor kQ. For electron energies E0>9 MeV (R50>4 cm), the calculated values are in good agreement with the data published in the IAEA protocol. Deviations in the range of 0.5-0.8% are found for R50<3 cm.

Physical and biological factors determining the effective proton range

PURPOSE Proton radiotherapy is rapidly becoming a standard treatment option for cancer. However, even though experimental data show an increase of the relative biological effectiveness (RBE) with depth, particularly at the distal end of the treatment field, a generic RBE of 1.1 is currently used in proton radiotherapy. This discrepancy might affect the effective penetration depth of the proton beam and thus the dose to the surrounding tissue and organs at risk. The purpose of this study was thus to analyze the impact of a tissue and dose dependent RBE of protons on the effective range of the proton beam in comparison to the range based on a generic RBE of 1.1. METHODS Factors influencing the biologically effective proton range were systematically analyzed by means of treatment planning studies using the Local Effect Model (LEM IV) and the treatment planning software TRiP98. Special emphasis was put on the comparison of passive and active range modulation techniques. RESULTS Beam energy, tissue type, and dose level significantly affected the biological extension of the treatment field at the distal edge. Up to 4 mm increased penetration depth as compared to the depth based on a constant RBE of 1.1. The extension of the biologically effective range strongly depends on the initial proton energy used for the most distal layer of the field and correlates with the width of the distal penumbra. Thus, the range extension, in general, was more pronounced for passive as compared to active range modulation systems, whereas the maximum RBE was higher for active systems. CONCLUSIONS The analysis showed that the physical characteristics of the proton beam in terms of the width of the distal penumbra have a great impact on the RBE gradient and thus also the biologically effective penetration depth of the beam.

Assessment of potential advantages of relevant ions for particle therapy: A model based study

PURPOSE Different ion types offer different physical and biological advantages for therapeutic applications. The purpose of this work is to assess the advantages of the most commonly used ions in particle therapy, i.e., carbon ((12)C), helium ((4)He), and protons ((1)H) for different treatment scenarios. METHODS A treatment planning analysis based on idealized target geometries was performed using the treatment planning software TRiP98. For the prediction of the relative biological effectiveness (RBE) that is required for biological optimization in treatment planning the local effect model (LEM IV) was used. To compare the three ion types, the peak-to-entrance ratio (PER) was determined for the physical dose (PERPHY S), the RBE (PERRBE), and the RBE-weighted dose (PERBIO) resulting for different dose-levels, field configurations, and tissue types. Further, the dose contribution to artificial organs at risk (OAR) was assessed and a comparison of the dose distribution for the different ion types was performed for a patient with chordoma of the skull base. RESULTS The study showed that the advantages of the ions depend on the physical and biological properties and the interplay of both. In the case of protons, the consideration of a variable RBE instead of the clinically applied generic RBE of 1.1 indicates an advantage in terms of an increased PERRBE for the analyzed configurations. Due to the fact that protons show a somewhat better PERPHY S compared to helium and carbon ions whereas helium shows a higher PERRBE compared to protons, both protons and helium ions show a similar RBE-weighted dose distribution. Carbon ions show the largest variation of the PERRBE with tissue type and a benefit for radioresistant tumor types due to their higher LET. Furthermore, in the case of a two-field irradiation, an additional gain in terms of PERBIO is observed when using an orthogonal field configuration for carbon ions as compared to opposing fields. In contrast, for protons, the PERBIO is almost independent on the field configuration. Concerning the artificial lateral OAR, the volume receiving 20% of the prescribed RBE-weighted dose (V20) was reduced by over 35% using helium ions and by over 40% using carbon ions compared to protons. The analysis of the patient plan showed that protons, helium, and carbon ions are similar in terms of target coverage whereas the dose to the surrounding tissue is increasing from carbon ions toward protons. The mean dose to the brain stem can be reduced by more than 55% when using helium ions and by further 25% when using carbon ions instead of protons. CONCLUSIONS The comparison of the PERRBE and PERPHY S of the three ion types suggests a strong dependence of the advantages of the three ions on the dose-level, tissue type, and field configuration. In terms of conformity, i.e., dose to the normal tissue, a clear gain is expected using carbon or helium ions compared to protons.

Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry

Current dosimetry protocols (AAPM, IAEA, IPEM, DIN) recommend parallel-plate ionization chambers for dose measurements in clinical electron beams. This study presents detailed Monte Carlo simulations of beam quality correction factors for four different types of parallel-plate chambers: NACP-02, Markus, Advanced Markus and Roos. These chambers differ in constructive details which should have notable impact on the resulting perturbation corrections, hence on the beam quality corrections. The results reveal deviations to the recommended beam quality corrections given in the IAEA TRS-398 protocol in the range of 0%-2% depending on energy and chamber type. For well-guarded chambers, these deviations could be traced back to a non-unity and energy-dependent wall perturbation correction. In the case of the guardless Markus chamber, a nearly energy-independent beam quality correction is resulting as the effects of wall and cavity perturbation compensate each other. For this chamber, the deviations to the recommended values are the largest and may exceed 2%. From calculations of type-B uncertainties including effects due to uncertainties of the underlying cross-sectional data as well as uncertainties due to the chamber material composition and chamber geometry, the overall uncertainty of calculated beam quality correction factors was estimated to be <0.7%. Due to different chamber positioning recommendations given in the national and international dosimetry protocols, an additional uncertainty in the range of 0.2%-0.6% is present. According to the IAEA TRS-398 protocol, the uncertainty in clinical electron dosimetry using parallel-plate ion chambers is 1.7%. This study may help to reduce this uncertainty significantly.

Thimble ionization chambers in medium-energy x-ray beams and the role of constructive details of the central electrode : Monte Carlo simulations and measurements

This paper presents investigations of thimble ionization chamber response in medium-energy kilovoltage x-ray beams (70-280 kVp, 0.09-3.40 mm Cu HVL). Two thimble ionization chambers (PTW30015 and PTW30016) were investigated, regarding the influence of the central electrode dimensions made of aluminum. Measurements were carried out in photon fields of different beam quality. Corresponding Monte Carlo simulations employing the EGSnrc Monte Carlo code system were performed. The simulations included the modelling of the x-ray tube and measurement setup for generation of x-ray spectra. These spectra were subsequently used to calculate the absorbed energy in the air cavity of the two thimble ionization chamber models and the air kerma at the reference point of the chambers. Measurements and simulations revealed an optimal diameter of the central electrode, concerning an almost energy-independent response of the ionizaton chamber. The Monte Carlo simulations are in good agreement with the measured values, expressed in beam quality correction factors k(Q). The agreement was generally within 0.6% but could only be achieved with an accurate model of the central electrode including its exact shape. Otherwise, deviations up to 8.5% resulted, decreasing with higher photon energies, which can be addressed to the high yield of the photoelectric effect in the electrode material aluminum at low photon energies.

Axilladissektion oder Axillabestrahlung bei postmenopausalen Patientinnen mit Mammakarzinom? Langzeitergebnisse und Langzeitfolgen bei 655 Patientinnen

Hintergrund: Vor 1993 erhielten postmenopausale Patientinnen mit Mammakarzinom auch bei axillärem Lymphknotenbefall (pN+) in unserer Klinik keine adjuvante Chemotherapie. Für diese Patientinnen hatt die diagnostische Funktion der Axilladissektion keinen therapieentscheidenden Wert. Deshalb wurde untersucht, ob bei ihnen die therapeutische Funktion der Dissektion durch eine Radiatio ersetzt werden kann oder im Hinblick auf die Langzeitfolgen ersetzt werden sollte. Patientinnen und Methoden: Von 1986 bis 1993 wurden 655 Patientinnen mit Mammakarzinom nach brusterhaltender Operation (BET) bestrahlt. Davon waren präoperativ 144 cN1− und 511 cN0-Patientinnen. Bei allen 144 cN1− und allen 29 prämenopausalen cN0-Patientinnen war eine Axilladissektion vorgesehen. Von 302 postmenopausalen cN0-Patientinnen erhielten alle 129 Patientinnen in unserer Klinik operierten Fälle keine Dissektion, sondern eine alleinige Axillabestrahlung (AxRT-Gruppe). Vergleichskollektiv waren alle 173 postmenopausalen cN0-Patientinnen, die in auswärtigen Kliniken eine Axilladissektion erhalten hatten (AxOP-Gruppe) und in unserer Klinik postoperativ nach BET bestrahlt wurden. Die Prognoseparameter waren in beiden Gruppen gleich bzw. sogar etwas ungünstiger in der AxRT-Gruppe. Bei massivem Befall oder weniger als acht entfernten Lymphknoten wurde die Axilla zusätzlich bestrahlt. Spätfolgen nach Dissektion und/oder Radiatio der Axilla wurden bei 502 lokoregionär rezidivfreien Patientinnen des Gesamtkollektivs ausgewertet, die mindestens 3 Jahre nachkontrolliert werden konnten (median 9,5 Jahre). Ergebnisse: Nach 5, 10 und 15 Jahren beträgt das tumorfreie Überleben bei 173 Patientinnen mit Dissektion 90%, 82% und 79% bzw. bei 129 Patientinnen mit Radiatio der Axilla 91%, 82% und 80% (p = 0,95), Gesamtüberleben (p = 0,98), lokale (p = 0,47) und axilläre Rezidivfreiheit (p = 0,12) waren ebenfalls gleich. Gravierende Spätfolgen traten bei 26% der axilladissezierten Patientinnen auf und bei nur 1% der axillabestrahlten. Die zusätzliche Radiatio hatten keinen Einfluss auf die Morbidität durch die Dissektion (26 vs. 27%). Schlussfolgerung: Die Axillaradiatio bietet bei geringerer Morbidität gleich gute Heilungschance wie die Dissektion und sollte zumindest bei postmenopausalen cN0-Patientinnen eingesetzt werden.Background: Until 1993 postmenopausal women with breast cancer did not receive adjuvant chemotherapy in our institution even if axillary nodes were involved. So in these patients axillary dissection had no diagnostic value for further treatment. Therefore we started a prospective study in which dissection of axillary nodes was replaced by irradiation in postmenopausal cN0 patients. Patients and Methods: From 1986 to 1993 we irradiated 655 patients with breast cancer after breast conserving surgery (BET). In all 144 cN1− and all 209 premenopausal cN0-patients axillary dissection was recommended. Of 302 postmenopausal cN0 patients 129 had breast surgery in our institution. In a total of 129 patients axillary dissection was replaced by irradiation (AxRT-group). They were compared with all 173 patients referred from other hospitals for irradiation after both breast conserving surgery and axillary dissection (AxOP-group). Dissected patients with gross tumor involvement of the axilla or less than eight nodes removed had additional axillary irradiation. Patients age, tumor size, vessel-, muscle- or skin invasion and grading were similar in both groups (Table 1). However, in the AxRT-group there were more patients with negative hormon receptors, multifocal and medial sited tumors. Late complications after dissection and/or irradiation of the axilla were evaluated in 502 patients free of locoregional relapse and with a minimal follow up of 3 years (median 9.5 years). Results: After 5, 10 and 15 years tumor free survival rates were 90%, 82% and 79% in the AxOP-group vs 91%, 82% and 80% in the AxRT-group, respectively (p = 0,95) (Figure 1). Overall survival (p = 0.98) (Figure 2), local (p = 0.47) and axillary control (p = 0.12) were equal in both groups (Figures 3 and 4). However, serious problems like lymphedema of the arm, pain mobility impairment occurred in 26% patients following axillary dissection but only in 1% after axillary irradiation. No difference in late squelae after axillary dissection with or without irradiation could be detected (26 vs 27%) (Table 2). Conclusion: In postmenopausal cN0-patients axillary dissection should be replaced by axillary irradiation, since it offers the same chance for cure with much lower morbidity.