Factors for converting dose measured in polystyrene phantoms to dose reported in water phantoms for incident proton beams.

PURPOSE: Previous dosimetry protocols allowed calibrations of proton beamline dose monitors to be performed in plastic phantoms. Nevertheless, dose determinations were referenced to absorbed dose-to-muscle or absorbed dose-to-water. The IAEA Code of Practice TRS 398 recommended that dose calibrations be performed with ionization chambers only in water phantoms because plastic-to-water dose conversion factors were not available with sufficient accuracy at the time of its writing. These factors are necessary, however, to evaluate the difference in doses delivered to patients if switching from calibration in plastic to a protocol that only allows calibration in water. METHODS: This work measured polystyrene-to-water dose conversion factors for this purpose. Uncertainties in the results due to temperature, geometry, and chamber effects were minimized by using special experimental set-up procedures. The measurements were validated by Monte Carlo simulations. RESULTS: At the peak of non-range-modulated beams, measured polystyrene-to-water factors ranged from 1.015 to 1.024 for beams with ranges from 36 to 315 mm. For beams with the same ranges and medium sized modulations, the factors ranged from 1.005 to 1.019. The measured results were used to generate tables of polystyrene-to-water dose conversion factors. CONCLUSIONS: The dose conversion factors can be used at clinical proton facilities to support beamline and patient specific dose per monitor unit calibrations performed in polystyrene phantoms.

Determination of consensus k Q values for megavoltage photon beams for the update of IAEA TRS-398.

The IAEA is currently coordinating a multi-year project to update the TRS-398 Code of Practice for the dosimetry of external beam radiotherapy based on standards of absorbed dose to water. One major aspect of the project is the determination of new beam quality correction factors, k Q , for megavoltage photon beams consistent with developments in radiotherapy dosimetry and technology since the publication of TRS-398 in 2000. Specifically, all values must be based on, or consistent with, the key data of ICRU Report 90. Data sets obtained from Monte Carlo (MC) calculations by advanced users and measurements at primary standards laboratories have been compiled for 23 cylindrical ionization chamber types, consisting of 725 MC-calculated and 179 experimental data points. These have been used to derive consensus k Q values as a function of the beam quality index TPR20,10 with a combined standard uncertainty of 0.6%. Mean values of MC-derived chamber-specific [Formula: see text] factors for cylindrical and plane-parallel chamber types in 60Co beams have also been obtained with an estimated uncertainty of 0.4%.

Commissioning of pencil beam and Monte Carlo dose engines for non-isocentric treatments in scanned proton beam therapy.

This work describes the dosimetric commissioning of the treatment planning system (TPS) RayStation v6.1 from RaySearch Laboratories (Stockholm, Sweden) for a synchrotron-based scanned proton beam delivery with isocentric and non-isocentric setups at MedAustron. Focus was on the comparison of the pencil beam (PBv4.1) and Monte Carlo (MCv4.0) calculation algorithms. Commissioning of dose calculations was done first for 1D/2D dose delivery where the performance of the beam model in reproducing dosimetric properties for the delivery of single static pencil beams and mono-energetic layers with multiple spots was evaluated. The commissioning for 3D beam delivery employed test cases with increasing complexity: from box-shaped fields in homogeneous phantoms to the introduction of oblique incidences and inhomogeneities. Dose calculations were compared to the measured data for different air gaps and using beams with and without range shifter (RaShi). Depth-dose curves and spot shape comparisons showed good agreement of the results obtained with PBv4.1 and MCv4.0 algorithms at isocentric setup for open beam configurations (without RaShi). Comparison of transverse dose profiles for lateral heterogeneities at different depths showed better performance of the MCv4.0 algorithm in comparison to the PBv4.1 algorithm. In the case of 3D delivery comparisons of measured and TPS-calculated dose with MCv4.0 algorithm in box-shaped fields in water showed an average agreement within 2%. The results for dose calculations with the PBv4.1 algorithm showed larger deviations for beams with RaShi at all evaluated air gaps (from 64.8 cm to 14.8 cm). Our results suggest that the MCv4.0 algorithm shall be used in clinics for final dose calculation when beams with RaShi are used especially in the presence of large air gaps, inclined patient surface and lateral inhomogeneities. The detailed stepwise methodology implemented for the RayStation commissioning in this work could serve as further guidance for other facilities introducing a new TPS for proton beam therapy.

A new formalism for reference dosimetry of small and nonstandard fields.

The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or nonuniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). This has been facilitated by the increased availability of standard and add-on multileaf collimators and a variety of new treatment units. For these fields, dosimetric errors have become considerably larger than in conventional beams mostly due to two reasons; (i) the reference conditions recommended by conventional Codes of Practice (CoPs) cannot be established in some machines and (ii) the measurement of absorbed dose to water in composite fields is not standardized. In order to develop standardized recommendations for dosimetry procedures and detectors, an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This paper outlines a new formalism for the dosimetry of small and composite fields with the intention to extend recommendations given in conventional CoPs for clinical reference dosimetry based on absorbed dose to water. This formalism introduces the concept of two new intermediate calibration fields: (i) a static machine-specific reference field for those modalities that cannot establish conventional reference conditions and (ii) a plan-class specific reference field closer to the patient-specific clinical fields thereby facilitating standardization of composite field dosimetry. Prior to progressing with developing a CoP or other form of recommendation, the members of this IAEA working group welcome comments from the international medical physics community on the formalism presented here.

Characterization of PTW-31015 PinPoint ionization chambers in photon and proton beams.

The increased use of complex forms of radiotherapy using small-field photon and proton beams has invoked a growing interest in the use of micro-ionization chambers. In this study, 48 PTW-TM31015 PinPoint-type micro-ionization chambers that are used in the commissioning and patient specific QA of a proton pencil beam scanning (PBS) delivery system have been characterized in proton and high-energy photon beams. In both beam modalities, the entire set of PinPoint chambers was characterized by imaging them, by evaluating their stability using check source measurements, by experimentally determining the ion recombination, polarity effect and by cross calibrating them in terms of absorbed dose to water against Farmer-type ionization chambers. Beam quality correction factors were theoretically derived for both beam modalities. None of the chambers' check source readings drifted by more than 0.5% over a one year period. Beam quality correction factors for the 6 MV photon with reference to 60Co were on average 1.0 ± 0.5% lower than the theoretical values calculated according to the data and procedures outlined in IAEA TRS-398. While this difference is within the overall dosimetric uncertainty, it is significant considering only uncorrelated uncertainties indicating inconsistencies in the theoretical data. Beam quality correction factors for the 179.2 MeV proton beam with reference to 60Co were in good agreement with the theoretical data. Ion recombination and polarity correction factors were very consistent for all chambers with standard deviations of 0.2% or below show that findings from more comprehensive investigations in the literature can be considered as representative for all the chambers of this type. The characterization of 48 PinPoint-type micro-ionization chambers performed in this study provided a unique opportunity to investigate chamber-to-chamber variations of calibration, beam quality correction factors, ion recombination and polarity correction factors for an unprecedented sample size of chambers for both high-energy photon and proton beams.

End-to-end tests using alanine dosimetry in scanned proton beams.

This paper describes end-to-end test procedures as the last fundamental step of medical commissioning before starting clinical operation of the MedAustron synchrotron-based pencil beam scanning (PBS) therapy facility with protons. One in-house homogeneous phantom and two anthropomorphic heterogeneous (head and pelvis) phantoms were used for end-to-end tests at MedAustron. The phantoms were equipped with alanine detectors, radiochromic films and ionization chambers. The correction for the 'quenching' effect of alanine pellets was implemented in the Monte Carlo platform of the evaluation version of RayStation TPS. During the end-to-end tests, the phantoms were transferred through the workflow like real patients to simulate the entire clinical workflow: immobilization, imaging, treatment planning and dose delivery. Different clinical scenarios of increasing complexity were simulated: delivery of a single beam, two oblique beams without and with range shifter. In addition to the dose comparison in the plastic phantoms the dose obtained from alanine pellet readings was compared with the dose determined with the Farmer ionization chamber in water. A consistent systematic deviation of about 2% was found between alanine dosimetry and the ionization chamber dosimetry in water and plastic materials. Acceptable agreement of planned and delivered doses was observed together with consistent and reproducible results of the end-to-end testing performed with different dosimetric techniques (alanine detectors, ionization chambers and EBT3 radiochromic films). The results confirmed the adequate implementation and integration of the new PBS technology at MedAustron. This work demonstrates that alanine pellets are suitable detectors for end-to-end tests in proton beam therapy and the developed procedures with customized anthropomorphic phantoms can be used to support implementation of PBS technology in clinical practice.

Evaluation of beam delivery and ripple filter design for non-isocentric proton and carbon ion therapy.

This study aims at selecting and evaluating a ripple filter design compatible with non-isocentric proton and carbon ion scanning beam treatment delivery for a compact nozzle. The use of non-isocentric treatments when the patient is shifted as close as possible towards the nozzle exit allows for a reduction in the air gap and thus an improvement in the quality of scanning proton beam treatment delivery. Reducing the air gap is less important for scanning carbon ions, but ripple filters are still necessary for scanning carbon ion beams to reduce the number of energy steps required to deliver homogeneous SOBP. The proper selection of ripple filters also allows a reduction in the possible transverse and depth-dose inhomogeneities that could appear in non-isocentric conditions in particular. A thorough review of existing ripple filter designs over the past 16 years is performed and a design for non-isocentric treatment delivery is presented. A unique ripple filter quality index (QIRiFi) independent of the particle type and energy and representative of the ratio between energy modulation and induced scattering is proposed. The Bragg peak width evaluated at the 80% dose level (BPW80) is proposed to relate the energy modulation of the delivered Bragg peaks and the energy layer step size allowing the production of homogeneous SOBP. Gate/Geant4 Monte Carlo simulations have been validated for carbon ion and ripple filter simulations based on measurements performed at CNAO and subsequently used for a detailed analysis of the proposed ripple filter design. A combination of two ripple filters in a series has been validated for non-isocentric delivery and did not show significant transverse and depth-dose inhomogeneities. Non-isocentric conditions allow a significant reduction in the spot size at the patient entrance (up to 350% and 200% for protons and carbon ions with range shifter, respectively), and therefore in the lateral penumbra in the patients.

Proton dosimetry intercomparison based on the ICRU report 59 protocol.

BACKGROUND AND PURPOSE: A new protocol for calibration of proton beams was established by the ICRU in report 59 on proton dosimetry. In this paper we report the results of an international proton dosimetry intercomparison, which was held at Loma Linda University Medical Center. The goals of the intercomparison were, first, to estimate the level of consistency in absorbed dose delivered to patients if proton beams at various clinics were calibrated with the new ICRU protocol, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration factors. MATERIALS AND METHODS: Eleven institutions participated in the intercomparison. Measurements were performed in a polystyrene phantom at a depth of 10.27 cm water equivalent thickness in a 6-cm modulated proton beam with an accelerator energy of 155 MeV and an incident energy of approximately 135 MeV. Most participants used ionization chambers calibrated in terms of exposure or air kerma. Four ionization chambers had 60Co-based calibration in terms of absorbed dose-to-water. Two chambers were calibrated in a 60Co beam at the NIST both in terms of air kerma and absorbed dose-to-water to provide a comparison of ionization chambers with different calibrations. RESULTS: The intercomparison showed that use of the ICRU report 59 protocol would result in absorbed doses being delivered to patients at their participating institutions to within +/-0.9% (one standard deviation). The maximum difference between doses determined by the participants was found to be 2.9%. Differences between proton doses derived from the measurements with ionization chambers with N(K)-, or N(W) - calibration type depended on chamber type. CONCLUSIONS: Using ionization chambers with 60Co calibration factors traceable to standard laboratories and the ICRU report 59 protocol, a distribution of stated proton absorbed dose is achieved with a difference less than 3%. The ICRU protocol should be adopted for clinical proton beam calibration. A comparison of proton doses derived from measurements with different chambers indicates that the difference in results cannot be explained only by differences in 60Co calibration factors.

Proton dosimetry intercomparison.

BACKGROUND AND PURPOSE: Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols. MATERIALS AND METHODS: Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV). RESULTS: The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%. CONCLUSION: The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Application of solid state detectors for dosimetry of therapeutic proton beams.

A PTW Riga diamond detector and LiF TLDs have been evaluated for use in proton beam dosimetry by comparing results of proton beam calibration with those obtained using thimble ionization chambers. The thimble ionization chambers were calibrated in terms of exposure while the TLDs and diamond detector were calibrated in terms of absorbed dose in a 60Co beam. Absorbed doses to muscle in proton beams for ionization chambers were derived using the TG 20 charged particle protocol. Absorbed doses to muscle for solid state detectors were derived using absorbed dose proton beam quality correction factors. Differences between the derived doses for ionization chambers and solid state detectors were found to be within the uncertainties of measurements: 4.5% for ionization chambers and 5% for solid state detectors.

Calorimetric determination of the absorbed dose-to-water beam quality correction factor kQ for high-energy photon beams.

A method has been developed for measuring photon beam quality correction factors kQ using direct transfer with a water calorimeter. kQ values were measured for beam qualities varying between 6 and 23 MV for a Capintec PR-06 and a PTW W30001 cylindrical ionization chamber. Measured values were intercompared with published sets of computed kQ values and agreement was found to be within measurement uncertainty (1%, one standard deviation).

kQ factors for ionization chamber dosimetry in clinical proton beams.

We discuss a formalism for clinical proton beam dosimetry based on the use of ionization chamber absorbed dose-to-water calibration and beam quality correction factors. A quantity kQ, the beam quality correction factor, is defined which corrects the absorbed dose-to-water calibration factor ND,w in a reference beam of quality Q0 to that in a user's beam of quality Q1. This study of proton beam quality correction factors used 60Co (kQ gamma) and proton (kQp) reference beams. The kQ gamma factors were measured using combined water calorimetry and ionometry for PTW and Capintec-Farmer-type ionization chambers, and were computed from standard dosimetry protocols. Agreement between measured and calculated kQ gamma values for both chambers was found within 1.2% in the plateau region for a monoenergetic 250-MeV beam and within 1.8% at the spread-out Bragg peak for a 155-MeV range-modulated beam. Comparison of absorbed doses to water determined in the range-modulated 155-MeV beam was performed with the PTW chamber using three calibration methods: Ngas calibration (AAPM Report 16), ND,w,gamma calibration in a 60Co beam in conjunction with a kQ gamma factor, and ND,w,p calibration in a proton beam in conjunction with a kQp factor. Absorbed doses to water obtained with the three methods agreed within 2% when ionization chamber dosimetry data were analyzed using the proton W-value for air from the AAPM Report 16 and the ICRU 49 proton stopping powers. The use of the proton-calibrated reference ionization chamber, in conjunction with the beam quality correction factor kQp, significantly reduced the systematic uncertainty of the absorbed dose determination.

Determination of the air w-value in proton beams using ionization chambers with gas flow capability.

The purpose of this work was to determine the w-value of air for protons using the paired gas method. Several plastic- and magnesium-walled chambers were used with air, synthetic air, nitrogen, and argon flowing gases. Using argon as a reference gas, the w-value of air was measured and ranged from 32.7 to 34.5 J/C for protons with energies encountered in radiotherapy. Using nitrogen as a reference gas, the w-value of air ranged from 35.2 to 35.4 J/C over the same range of proton energies. The w-value was found, at a given energy, to be independent of the ion chamber used. The uncertainty in these measurements was estimated at 5.2% at the 2sigma level. This uncertainty was dominated by the 4.4% uncertainty in the w-value of the reference gas.

A calibration procedure for beam monitors in a scanned beam of heavy charged particles.

An international code of practice (CoP) for dosimetry based on standards of absorbed dose to water has recently been published by the IAEA [Technical Report Series No. 398, 2000] (TRS-398). This new CoP includes procedures for proton and heavy ion beams as well as all other beam qualities. In particular it defines reference conditions to which dose measurements should refer to. For proton and ion beams these conditions include dose measurements in the center of all possible modulated Bragg peaks. The recommended reference conditions in general are used also for the calibration of beam monitors. For a dynamic beam delivery system using beam scanning in combination with energy variation, like, e.g., at the German carbon ion radiotherapy facility, this calibration procedure is not appropriate. We have independently developed a different calibration procedure. Similar to the IAEA CoP this procedure is based on the measurement of absorbed dose to water. This is translated in terms of fluence which finally results in an energy-dependent calibration of the beam monitor in units of particle number per monitor unit, which is unique for all treatment fields. In contrast to the IAEA CoP, the reference depth is chosen to be very small. The procedure enables an accurate and reliable determination of calibration factors. In a second step, the calibration is verified by measurements of absorbed dose in various modulated Bragg peaks by comparing measured against calculated doses. The agreement between measured and calculated doses is usually better than 1% for homogeneous fields and the mean deviation for more inhomogeneous treatment fields, as they are used for patient treatments, is within 3%. It is proposed that the CoP in general, and in particular the IAEA TRS-398 should include explicit recommendations for the beam monitor calibration. These recommendations should then distinguish between systems using static and dynamic beams.

Application of a natural diamond detector for the measurement of relative dose distributions in radiotherapy.

The suitability of a natural diamond detector with a special contact system for the measurement of relative dose distributions in selected radiotherapy applications was studied. The performance of the diamond detector was analysed by comparison with measurements using an ionization chamber and a silicon diode detector. The good stability, high response and good spatial resolution of the diamond detector were demonstrated by relative measurements in high-energy photon and electron beams and around the 137Cs source of an afterloading device. The application of the diamond detectors for relative as well as absolute dose measurements of beta ray ophthalmic applicators was also highlighted. It is concluded that the diamond with the special contact system is a suitable detector for relative dose measurements in a wide variety of applications, especially when high response and high spatial resolution are required.

Radiochromic film dosimetry for verification of dose distributions delivered with proton-beam radiosurgery.

In this work we studied the feasibility of radiochromic film for dosimetry verification of proton Bragg peak stereotactic radiosurgery with multiple beams. High-sensitivity MD-55 radiochromic film was calibrated for proton beam irradiation and the RIT 113 system was employed for film evaluation. Simulated stereotactic radiosurgery with a special phantom arrangement for film dosimetry was performed, following the same procedure as for a patient undergoing treatment. Five-beam irradiation was developed using a 3D treatment planning system. This plan was then delivered to the phantom in a one-day experiment. Planned and measured composite dose distributions were compared. Spatial accuracy of dose delivery to a region containing a simulated critical structure was evaluated for a single portal. Radiochromic film dosimetry validated the prescribed dose delivery within +/- 5%, one standard deviation, by comparing calculated doses with measured values. The alignment of apertures and boluses, as well as the alignment of the phantom with respect to the isocentre, was confirmed. Spatial accuracy of the method would have been able to detect possible misalignments greater than +/- 2 mm. We have demonstrated how radiochromic film dosimetry can be used to measure complex dose distributions in an irradiated phantom, thus enabling us to verify planned dose delivery of proton Bragg peak stereotactic radiosurgery with multiple beams. We assume that the dosimetric agreement between planned and measured dose distributions for the reported simulations will apply to patient treatments.

Deduction of the air w value in a therapeutic proton beam.

Utilization of air-filled ionization chambers with 60Co-based reference calibrations in proton dosimetry requires application of water to air stopping power ratios and the mean energy required to produce an ion pair (W or w). Accepted uncertainties in current w values for protons leads to a dosimetric uncertainty of 4 per cent when ionization chambers are employed to measure absorbed dose. For this reason, proton dosimetry protocols recommend the use of calorimetry as the absorbed dose standard. We used calorimetry in conjunction with an ionization chamber with 60Co reference calibrations to deduce the proton w value in the entrance region of a 250 MeV proton beam: 34.2 +/- 0.5 eV. Application of this w value, with its 1.5 per cent uncertainty, allows determination of dose in therapeutic proton beams, with uncertainties comparable to photon and electron values.

Dosimetry techniques for narrow proton beam radiosurgery.

Characterization of narrow beams used in proton stereotactic radiosurgery (PSRS) requires special efforts, since the use of finite size detectors can lead to distortion of the measured dose distributions. Central axis depth doses, lateral profiles and field size dependence factors are the most important beam characteristics to be determined prior to dosimetry calculations and beam modelling for PSRS. In this paper we report recommendations for practical dosimetry techniques which were developed from a comparison of beam characteristics determined with a variety of radiation detectors for 126 and 155 MeV narrow proton beams shaped with 2-30 mm circular brass collimators. These detectors included small-volume ionization chambers, a diamond detector, an Hi-p Si diode, TLD cubes, radiographic and radiochromic films. We found that both types of film are suitable for profile measurements in narrow beams. Good agreement between depth dose distributions measured with ionization chambers, diamond and diode detectors was demonstrated in beams with diameters of 20-30 mm. The diode detector can be used in smaller beams, down to 5 mm diameter. For beams with diameters less than 5 mm, reliable depth dose data may be obtained only with radiochromic film. The tested ionization chambers are appropriate for calibration of beams with diameters of 20-30 mm. TLD cubes and diamond detectors are useful to determine relative dose in beams with diameters of 10-20 mm. Field size factors for smaller beams should be obtained with diode and radiochromic film. We conclude that dosimetry characterization of proton beams down to several millimetres in diameter can be performed using the described procedures.

IAEA/WHO postal dose audits for radiotherapy hospitals in Eastern and South-Eastern Europe.

The IAEA/WHO TLD programme has been in operation for 34 years. In this period the calibration of approximately 5200 high-energy photon beams in over 1300 radiotherapy hospitals in 115 countries worldwide was checked. Of these, 18% of the audits were performed in Eastern and South-Eastern Europe. There are large contrasts in the region; while the results are very good for most countries, a few countries struggle with basic problems in dosimetry. The hospitals operating radiotherapy services without qualified medical physicists or dosimetry equipment have poorer results than those properly equipped and staffed. Only about 2/3 of TLD audit participants in Eastern Europe have the appropriate dosimetry equipment. To achieve consistency of the audit results within Eastern and South-Eastern Europe, strengthening of radiotherapy infrastructure in a few countries would be necessary.

Proton beam output measurement with an extrapolation chamber.

A variable air-volume, parallel-plate extrapolation chamber forming an integral part of a polystyrene phantom was used in measurement of dose rate in a 250 MeV clinical proton beam. The sensitive air-volume of the extrapolation chamber is controlled through the movement of the chamber piston by means of a micrometer mounted on the phantom body. The relative displacement of the piston is monitored by a calibrated mechanical distance travel indicator. The proton beam dose rate determined with the uncalibrated extrapolation chamber was 5% lower than the dose rate determined with a calibrated Farmer-type thimble chamber at the same depth in the polystyrene phantom. Despite the current 5% discrepancy, uncalibrated extrapolation chambers may offer a simple and practical alternative to current techniques used in output measurements of proton beam machines.