Benchmark data for small field dosimetry within a large multicentre study

4 Acquisition of small field dosimetry data: Establishment of acquisition protocols, comparison and characterization of different detectors

Purpose: An international IAEA/AAPM working group is preparing recommendations for reference dosimetry in small fields and composite fields. A proposed formalism for dosimetry in small and composite fields was published that introduces machine specific reference (msr) fields for static small fields and plan‐class specific reference (pcsr) fields for composite fields such as an IMRT sequence. The status of present activities, based on the proposed formalism, will be reviewed. Method and Materials: For msr fields a literature review is conducted and for some modalities experiments and Monte Carlo simulations are performed. For composite fields various routes are explored to arrive at suitable pcsr fields. Some of these start from clinical plans and try to distil most representative reference deliveries, while others start from treatment unit specific features to arrive at relevant modulated plans. Experiments and Monte Carlo simulations are performed as well to determine correction factors for ionisation chambers in pcsr fields and to evaluate their suitability to represent a class of clinical plans. One idea being explored is that for a composite field where full charged particle equilibrium is established, the overall correction factor equals the reciprocal of the gradient correction factor in the conventional reference field. Results: State‐of‐the‐art results on the correction factors for msr and pcsr fields will be reviewed. Experimental work performed on pcsr fields in TomoTherapy and step‐and‐shoot prostate plans indicates that correction factors for suitably sized ion chambers are not more than 1% different from unity. Monte Carlo simulations confirm this. Studies are underway to test the sensitivity of the correction factors to characteristics of the pcsr field. Conclusion: Results indicate that reference dosimetry in msr and pcsr fields is feasible with acceptably small corrections. A working document on static small field dosimetry is aimed for by the end of this year.

Background: Accurate small radiation field dosimetry is essential in modern radiotherapy techniques such as stereotactic radiosurgery (SRS) and intensity modulated radiotherapy (IMRT). Precise measurement of dosimetric parameters such as beam profile, percentage depth doses and output factor of these beams are complicated due to the electron disequilibrium and the steep dose gradients. In the present work the MAGIC polymer gel was used for dosimetry of small circular photon beams. The results of MAGIC were compared with EBT2 measurements and Monte Carlo (MC) calculations. Materials and Methods: Experimental measurements were made by mentioned dosimeters in four small field sizes 5, 10, 20 and 30 mm. The BEAMnrc code based on EGSnrc was used for simulation to calculate dosimetric parameters at these small fields. The phantoms were irradiated in a 6 MV photon beam Varian 2100C linear accelerator at SSD=100 cm. gel readout performed by 3 Tesla MRI scanner. Results: The results showed that the Percent depth dose (PDD) values measured and calculated by EBT2 film and MC had maximum local differences 4% and 5% with PDD values measured by MAGIC for field size of 5mm respectively. These differences decreased for larger field sizes. The measurements of output factor and penumbra (80%-20%) and (90%-10%) showed good agreement between the measurements and MC calculation. Conclusion: This study showed that the MAGIC polymer gel based on high resolution MRI images is useful detector for small field dosimetry but its agreement with MC is less than agreement of EBT2 film with MC.

BackgroundLack of an ideal detector for small field dosimetry has led to the development of many new types of detectors. Recent studies have shown that plastic scintillation detectors (PSDs) provide favorable dosimetric characteristics, such as minimal volume averaging and fluence perturbation effects, real time response rates, high signal to noise ratio (SNR), and independence to temperature, energy spectrum, dose rate, and irradiation direction, in the field of small field dosimetry, largely due to their small size and water‐equivalent composition materials, which eliminates the need for certain correction factors.PurposeThe goal of this study was to evaluate a new 0.5 × 0.5 mm2 PSD (called in this study Medscint0.5), with detailed uncertainty analysis, for its suitability in small field dosimetry and in comparison, to other commercially available PSDs.MethodsThis detector characterization was performed for small field output factors, pre‐ and post‐ irradiation leakage, short‐term repeatability, angular dependency, dose rate and energy dependency, dose response linearity, and temperate dependency. The small field output factors measured with this detector were also evaluated to determine if any correction factors were needed for extremely small fields.ResultsThe Medscint0.5 showed negligible leakage (< 0.1%) and short‐term repeatability (< 0.1% response deviations), similar to other commercially available PSDs. It also demonstrated comparable dose rate (< 0.47%) and dose response linearity (< 0.73%) to an ion chamber. Moreover, it also showed excellent angular (< 0.30%) and temperature independence (< 0.38%) relative to other commercially available PSDs. Stereotactic cone factor measurements showed good agreement (< 1.10%) with MC for all SRS cone sizes for both 6 and 10 MV FFF beams. The total uncertainty for absolute dosimetry using this detector was found to be ∼1.1% at one standard deviation from the mean. The uncertainty can be reduced to 0.8% at one standard deviation when this detector is used for relative small field dosimetry, that is, field output measurements.ConclusionThe latest small size Medscint0.5 detector is suitable for small field dosimetry with its unique advantage in measuring field output factors without need of any correction factor even in measuring small circular fields down to 4 mm diameter.

Evaluation of perturbation effects for various size TLDs in small field dosimetry

Quantitative evaluation of dosimetric uncertainties associated with small electron fields

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad BorrasStereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT. Learning Objectives: To learn the physics of small fields in contrast to dosimetry of conventional fields To learn about detectors suitable for small fields To learn about the role of Monte Carlo simulations in determination of small field output factors To provide an overview of the IAEA small field dosimetry recommendations To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy. To learn about special technical considerations in delivering IMRT and SBRT treatments To appreciate specific challenges of IMRT implementationJ. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research

Purpose: To investigate the accuracy and efficiency of the microDiamond type 60019 detector compared to Gafchromic EBT3 film for relative dosimetry measurements in 1 cm × 1 cm small photon fields defined by a newly commissioned 160-MLC linear accelerator. Methods: Measurements were performed using an Elekta Synergy Agility linear accelerator equipped with 160 tungsten multi-leaf collimators at 6 MV energy. A 1 cm × 1 cm field was defined at 90 cm source-to-surface distance (SSD) and 1.5 cm depth. The microDiamond detector was oriented in radial direction and used in 2D scanning mode. A large area plane parallel transmission chamber (T-REF type 34091) served as the reference detector. Gafchromic EBT3 film was calibrated and irradiated under identical conditions. Beam profiles, dose distributions, and penumbra characteristics were analyzed and compared between detectors. Results: The microDiamond detector demonstrated superior spatial resolution and signal-to-noise ratio compared to Gafchromic film. Penumbra width measurements showed approximately 5% difference between detectors in the Y-profile, while X-profile measurements revealed narrower, more realistic dose distributions with the microDiamond. The diamond detector exhibited better sensitivity to low-dose regions and minimal inter-leaf and intra-leaf leakage detection capability. Film dosimetry showed wider penumbra, indicating potential overestimation of dose distribution. The microDiamond detector required significantly less processing time and eliminated calibration-related uncertainties associated with film dosimetry. Conclusions: The microDiamond type 60019 detector, when oriented in radial direction, provides a viable and potentially superior alternative to Gafchromic film for routine relative dosimetry in small fields ≤1 cm × 1 cm. Its advantages include faster workflow, elimination of film-specific uncertainties, excellent spatial resolution, and improved low-dose detection. The detector's near tissue-equivalence and energy independence make it particularly suitable for small field commissioning and quality assurance in modern radiotherapy techniques.

The purpose of this study was to investigate the effect of very small air gaps (less than 1 mm) on the dosimetry of small photon fields used for stereotactic treatments. Measurements were performed with optically stimulated luminescent dosimeters (OSLDs) for 6 MV photons on a Varian 21iX linear accelerator with a Brainlab µMLC attachment for square field sizes down to 6 mm × 6 mm. Monte Carlo simulations were performed using EGSnrc C++ user code cavity. It was found that the Monte Carlo model used in this study accurately simulated the OSLD measurements on the linear accelerator. For the 6 mm field size, the 0.5 mm air gap upstream to the active area of the OSLD caused a 5.3% dose reduction relative to a Monte Carlo simulation with no air gap. A hypothetical 0.2 mm air gap caused a dose reduction >2%, emphasizing the fact that even the tiniest air gaps can cause a large reduction in measured dose. The negligible effect on an 18 mm field size illustrated that the electronic disequilibrium caused by such small air gaps only affects the dosimetry of the very small fields. When performing small field dosimetry, care must be taken to avoid any air gaps, as can be often present when inserting detectors into solid phantoms. It is recommended that very small field dosimetry is performed in liquid water. When using small photon fields, sub-millimetre air gaps can also affect patient dosimetry if they cannot be spatially resolved on a CT scan. However the effect on the patient is debatable as the dose reduction caused by a 1 mm air gap, starting out at 19% in the first 0.1 mm behind the air gap, decreases to <5% after just 2 mm, and electronic equilibrium is fully re-established after just 5 mm.

Due to multiple Coulomb scattering in complex geometries, small field dosimetry in proton therapy is challenging. Our goal was to define an indicator for the accuracy of dose delivery based on analytical dose calculations in treatment planning systems for small (e.g. radiosurgery) proton therapy fields. Seven patients whose treatment involved one or more small fields (below ∼3.6cm in diameter) were selected. We developed a fast methodology to quantify the inhomogeneity of the tissue traversed by a single beam using a heterogeneity index (HI). The implementation was based on the dose calculation approach taken by our pencil beam algorithm. Plans created with the treatment planning system were verified against Monte Carlo dose calculations on a field- by-field basis. DVHs were analyzed and differences in the dose to the GTV were assessed. The correlation between the HI-values and the discrepancies between planning system and Monte Carlo in terms of absolute dose to the target was studied. Our treatment planning system overestimates the dose within the GTV for very small fields by up to ∼8%, even if proper output factor normalization is done in water. The differences are strongly correlated to HI (Spearman's ρ=0.8, rho<0.0001). More complex heterogeneities within the beam path caused larger errors by the analytical algorithm. With the established correlation a threshold for the HI can be set by choosing a tolerance level. The HI as defined in this study appears to be a good indicator of the accuracy of proton field delivery in terms of GTV prescription dose when small fields are being delivered. Each HI-value was obtained in less than 2 minutes allowing implementation of the HI algorithm in clinical routine. For HI- values exceeding a certain threshold, either a change in beam incidence or a Monte Carlo dose calculation should be considered.

BackgroundPreclinical radiation biology has become increasingly sophisticated due to the implementation of advanced small animal image guided radiation platforms into laboratory investigation. These small animal radiotherapy devices enable state-of-the-art image guided therapy (IGRT) research to be performed by combining high-resolution cone beam computed tomography (CBCT) imaging with an isocentric irradiation system. Such platforms are capable of replicating modern clinical systems similar to those that integrate a linear accelerator with on-board CBCT image guidance.MethodsIn this study, we present a dosimetric evaluation of the small animal radiotherapy research platform (SARRP, Xstrahl Inc.) focusing on small field dosimetry. Physical dosimetry was assessed using ion chamber for calibration and radiochromic film, investigating the impact of beam focus size on the dose rate output as well as beam characteristics (beam shape and penumbra). Two film analysis tools) have been used to assess the dose output using the 0.5 mm diameter aperture.ResultsGood agreement (between 1.7–3%) was found between the measured physical doses and the data provided by Xstrahl for all apertures used. Furthermore, all small field dosimetry data are in good agreement for both film reading methods and with our Monte Carlo simulations for both focal spot sizes. Furthermore, the small focal spot has been shown to produce a more homogenous beam with more stable penumbra over time.ConclusionsFilmQA Pro is a suitable tool for small field dosimetry, with a sufficiently small sampling area (0.1 mm) to ensure an accurate measurement. The electron beam focus should be chosen with care as this can potentially impact on beam stability and reproducibility.

Due to their small collecting volume, diodes are commonly used in small field dosimetry. However, the relative sensitivity of a diode increases with decreasing small field size. Conversely, small air gaps have been shown to cause a significant decrease in the sensitivity of a detector as the field size is decreased. Therefore, this study uses Monte Carlo simulations to look at introducing air upstream to diodes such that they measure with a constant sensitivity across all field sizes in small field dosimetry. Varying thicknesses of air were introduced onto the upstream end of two commercial diodes (PTW 60016 photon diode and PTW 60017 electron diode), as well as a theoretical unenclosed silicon chip using field sizes as small as 5 mm × 5 mm. The metric used in this study represents the ratio of the dose to a point of water to the dose to the diode active volume, for a particular field size and location. The optimal thickness of air required to provide a constant sensitivity across all small field sizes was found by plotting as a function of introduced air gap size for various field sizes, and finding the intersection point of these plots. That is, the point at which was constant for all field sizes was found. The optimal thickness of air was calculated to be 3.3, 1.15 and 0.10 mm for the photon diode, electron diode and unenclosed silicon chip, respectively. The variation in these results was due to the different design of each detector. When calculated with the new diode design incorporating the upstream air gap, was equal to unity to within statistical uncertainty (0.5%) for all three diodes. Cross-axis profile measurements were also improved with the new detector design. The upstream air gap could be implanted on the commercial diodes via a cap consisting of the air cavity surrounded by water equivalent material. The results for the unclosed silicon chip show that an ideal small field dosimetry diode could be created by using a silicon chip with a small amount of air above it.

Dosimeters often consist of several components whose mass densities differ substantially from water. These components cause small-field correction factors to vary significantly as lateral electronic equilibrium breaks down. Even amongst instruments designed for small-field dosimetry, inter-detector variation in the correction factors associated with very small (∼0.5 cm) fields can amount to tens of per cent. For a given dosimeter, small-field correction factors vary not only with field size but also with detector azimuthal angle and position within the field. Furthermore the accurate determination of these factors typically requires time-intensive Monte Carlo simulations. Thus, if achievable, ‘correction factor free’ small-field dosimetry would be highly desirable. This study demonstrates that a new generation of mass-density compensated detectors could take us towards this goal. Using a 6 MV beam model, it shows that ‘mass-density compensation’ can be utilized to improve the performance of a range of different detectors under small-field conditions. Non-sensitive material of appropriate mass-density is incorporated into detector designs in order to make the instruments behave as if consisting only of water. The dosimeter perturbative effects are then reduced to those associated with volume averaging. An even better solution—which modifies detectors to obtain profiles that look like those measured by a point-like water structure—is also considered. Provided that adequate sensitivity can be achieved for a small measurement volume, this study shows that it may be possible to use mass-density compensation (and Monte Carlo-driven design) to produce a solid-state dosimeter/ionization chamber with a near-perfect non-equilibrium response.

Investigation of 2D radiophotoluminescence films in radiotherapy: Multileaf collimator Quality Assurance and small field dosimetry

The use of MR-guided radiation therapy (MRgRT) is increasing with the introduction of commercially-available parallel MR-linac systems. Treatment fields used by these machines can be very small and highly modulated. Literature on the small field dosimetry of parallel MR-linacs and the impact of the magnetic field on dose perturbations is scarce. To investigate the impact of magnetic fields on small field dosimetry of parallel MR-linacs using Monte Carlo (MC) methods. A TOPAS MC model of a 6 MV FFF beam was developed and validated against a commercial 0.5 T parallel MR-linac. The impact of varying magnetic field of up to 3.0 T on water phantom dosimetry such as percent depth dose (PDD), beam profiles, and output factors (OFs) was studied. A lung phantom with embedded spherical tumors of diameters 1-3cm was employed to investigate the impact of parallel magnetic fields on lung lesions using an arc delivery with a 4ο gantry spacing providing a surrogate for rotational delivery techniques. Dose distributions were compared with the 0.0 T scenario. Negligible differences were noted in the PDD, except for the build-up region, between the investigated parallel magnetic fields. Besides the tail region of the 1.5 and 3.0 T profiles, minimal differences were observed in the lateral beam profiles. Similarly, parallel magnetic fields of up to 1 T had a negligible impact on small field OFs. Compared to the 0 T magnetic field, the OF for field sizes of <2×2 cm2 was found to increase significantly, by up to 9%, for the 1.5 and 3.0 T fields. The increase in magnetic field strength led to a more uniform dose distribution across the tumor inside the lung phantom with a slight reduction in penumbra due to the electron focusing effect. Normalized to the 0 T lung phantom dose distribution, differences ranging from 3% to 8% were found when the magnetic field strength was varied from 0.5 to 3.0 T. These differences were proportional to the field strength and were more significant for the smaller tumor diameters employing smaller fields. The impact of parallel magnetic fields of up to 1.0 T strength is minimal for in-water small field dosimetry. However, the increase in OF, compared to the 0 T case, was found to be significant for the 3.0 T magnetic field. For all field strengths compared to 0 T, significant dose differences were found around the periphery of the tumors inside a lung phantom that must be accounted for during treatment planning and dosimetry.