Absorbed dose calorimetry

Metrology for advanced radiotherapy using particle beams with ultra-high dose rates

Dosimetry of ultra-high dose rate beams is one of the critical components which is required for safe implementation of FLASH radiotherapy (RT) into clinical practice. In the past years several national and international programmes have emerged with the aim to address some of the needs that are required for translation of this modality to clinics. These involve the establishment of dosimetry standards as well as the validation of protocols and dosimetry procedures. This review provides an overview of recent developments in the field of dosimetry for FLASH RT, with particular focus on primary and secondary standard instruments, and provides a brief outlook on the future work which is required to enable clinical implementation of FLASH RT.

Multi-point calorimeter using distributed fiber Bragg gratings for small field dosimetry in radiotherapy

BACKGROUND

The interest of using fiber Bragg gratings (FBGs) dosimeters in radiotherapy (RT) lies in their (i) microliter detection volume, (ii) customizable spatial resolution, (iii) multi-point dose measurement, (iv) real-time data acquisition and (v) insensitivity to Cherenkov light. These characteristics could prove very useful for characterizing dose distributions of small and nonstandard fields with high spatial resolution.

PURPOSE

We developed a multi-point FBGs dosimeter customized for small field RT dosimetry with a spatial resolution of ∼ $\sim$ 1 mm.

METHODS

The 3 cm-long multi-point dosimeter is made by embedding a 80μ m $\umu{\rm {m}}$ silica fiber containing an array of thirty (30) co-located ∼ $\sim$ 1 mm-long fs-written FBGs inside a plastic cylinder with an UV curing optical adhesive. With its higher thermal expansion coefficient, the plastic cylinder increases the sensitivity of the dosimeter by stretching the fiber containing the FBGs when the temperature rises slightly due to radiation energy deposition. Irradiations (2000 MU at 600 MU/min) were performed with a Varian TrueBeam linear accelerator.

RESULTS

The dose profile of a 2  × $ \times$ 2 cm2 $^{2}$ 6 MV beam was measured with a mean relative difference of 1.8% (excluding the penumbra region). The measured output factors for a 6 MV beam are in general agreement with the expected values within the experimental uncertainty (except for the 2 × $\,\times $ 2 cm2 $^{2}$ field). The detector response to different energy of photon and electron beams is within 5% of the mean response (0.068 ± 0.002 $0.068\pm 0.002$  pm/Gy). The calorimeter's post-irradiation thermal decay is in agreement with the theory.

CONCLUSIONS

An energy-independent small field calorimeter that allows dose profile and output factor measurements for RT using FBGs was developed, which, to our knowledge, has never been done before. This type of detector could prove really useful for small field dosimetry, but also potentially for MRI-LINAC since FBGs are insensitive to magnetic fields and for FLASH since FBGs have been used to measure doses up to 100 kGy.

Simulation-guided development of an optical calorimeter for high dose rate dosimetry

Optical Calorimetry (OC) is based on interferometry and provides a direct measurement of spatially resolved absorbed dose to water by measuring refractive index changes induced by radiation. The purpose of this work was to optimize and characterize in software an OC system tailored for ultra-high dose rate applications and to build and test a prototype in a clinical environment. A radiation dosimeter using the principles of OC was designed in optical modelling software. Traditional image quality instruments, fencepost and contrast phantoms, were utilized both in software and experimentally in a lab environment to investigate noise reduction techniques and to test the spatial and dose resolution of the system. Absolute dose uncertainty was assessed by measurements in a clinical 6 MV Flattening Filter Free (FFF) photon beam with dose rates in the range 0.2-6 Gy/s achieved via changing the distance from the source. Design improvements included: equalizing the pathlengths of the interferometer, isolating the system from external vibrations and controlling the system's internal temperature as well as application of mathematical noise reduction techniques. Simulations showed that these improvements should increase the spatial resolution from 22 to 35 lp/mm and achieve a minimum detectable dose of 0.2 Gy, which was confirmed experimentally. In the FFF beam, the absolute dose uncertainty was dose rate dependent and decreased from 2.5 ± 0.8 to 2.5 ± 0.2 Gy for dose rates of 0.2 and 6 Gy/s, respectively. A radiation dosimeter utilizing the principles of OC was developed and constructed. Optical modelling software and image quality phantoms allowed for iterative testing and refinement. The refined OC system proved capable of measuring absorbed dose to water in a linac generated photon beam. Reduced uncertainty at higher dose rates indicates the potential for OC as a dosimetry system for high dose rate techniques such as microbeam and ultra-high dose-rate radiotherapy.

Investigation of contrast mechanisms for MRI phase signal-based proton beam visualization in water phantoms

PURPOSE

The low sensitivity and limitation to water phantoms of convection-dependent MRI magnitude signal-based proton beam visualization hinder its in vivo applicability in MR-integrated proton beam therapy. The purpose of the present study was, therefore, to assess possible contrast mechanisms for MRI phase signal-based proton beam visualization that can potentially be exploited to enhance the sensitivity of the method and extend its applicability to tissue materials.

METHODS

To assess whether proton beam-induced magnetic field perturbations, changes in material susceptibility or convection result in detectable changes in the MRI phase signal, water phantom characteristics, experiment timing, and imaging parameters were varied in combined irradiation and imaging experiments using a time-of-flight angiography pulse sequence on a prototype in-beam MRI scanner. Velocity encoding was used to further probe and quantify beam-induced convection.

RESULTS

MRI phase signal-based proton beam visualization proved feasible. The observed phase difference contrast was evoked by beam-induced buoyant convection with flow velocities in the mm/s range. Proton beam-induced magnetic field perturbations or changes in magnetic susceptibility did not influence the MRI phase signal. Velocity encoding was identified as a means to enhance the detection sensitivity.

CONCLUSION

Because the MRI phase difference contrast observed during proton beam irradiation of water phantoms is caused by beam-induced convection, this method will unlikely be transferable to tightly compartmentalized tissue wherein flow effects are restricted. However, strong velocity encoded pulse sequences were identified as promising candidates for the future development of MRI-based methods for water phantom-based geometric quality assurance in MR-integrated proton beam therapy.

Monte Carlo modelling of a prototype small-body portable graphite calorimeter for ultra-high dose rate proton beams

Background and purpose

Accurate dosimetry in Ultra-High Dose Rate (UHDR) beams is challenging because high levels of ion recombination occur within ionisation chambers used as reference dosimeters. A Small-body Portable Graphite Calorimeter (SPGC) exhibiting a dose-rate independent response was built to offer reduced uncertainty on secondary standard dosimetry in UHDR regimes. The aim of this study was to quantify the effect of the geometry and material properties of the device on the dose measurement.

Materials and methods

A detailed model of the SPGC was built in the Monte Carlo code TOPAS (v3.6.1) to derive the impurity and gap correction factors, k i m p and k g a p . A dose conversion factor, D w MC / D g MC , was also calculated using FLUKA (v2021.2.0). These factors convert the average dose to its graphite core to the dose-to-water for a 249.7 MeV mono-energetic spot-scanned clinical proton beam. The effect of the surrounding Styrofoam on the dose measurement was examined in the simulations by substituting it for graphite.

Results

The k i m p and k g a p correction factors were 0.9993 ± 0.0002 and 1.0000 ± 0.0001, respectively when the Styrofoam was not substituted, and 1.0037 ± 0.0002 and 0.9999 ± 0.0001, respectively when substituted for graphite. The dose conversion factor was calculated to be 1.0806 ± 0.0001. All uncertainties are Type A.

Conclusions

Impurity and gap correction factors, and the dose conversion factor were calculated for the SPGC in a FLASH proton beam. Separating out the effect of scatter from Styrofoam insulation showed this as the dominating correction factor, amounting to 1.0043 ± 0.0002.

MRI magnitude signal-based proton beam visualisation in water phantoms reflects composite effects of beam-induced buoyant convection and radiation chemistry

Objective. Local magnetic resonance (MR) signal loss was previously observed during proton beam irradiation of free-floating water phantoms at ambient temperature using a research prototype in-beam magnetic resonance imaging (MRI) scanner. The emergence of this MR signal loss was hypothesised to be dependent on beam-induced convection. The aim of this study was therefore to unravel whether physical conditions allowing the development of convection must prevail for the beam-induced MRI signatures to emerge.Approach. The convection dependence of MRI magnitude signal-based proton beam visualisation was investigated in combined irradiation and imaging experiments using a gradient echo (GE)-based time-of-flight (ToF) angiography pulse sequence, which was first tested for its suitability for proton beam visualisation in free-floating water phantoms at ambient temperature. Subsequently, buoyant convection was selectively suppressed in water phantoms using either mechanical barriers or temperature control of water expansivity. The underlying contrast mechanism was further assessed using sagittal imaging and variation of T1 relaxation time-weighting.Main results. In the absence of convection-driven water flow, weak beam-induced MR signal changes occurred, whereas strong changes did occur when convection was not mechanically or thermally inhibited. Moreover, the degree of signal loss was found to change with the variation of T1-weighting. Consequently, beam-induced MR signal loss in free-floating water phantoms at ambient temperature does not exclusively originate from buoyant convection, but is caused by local composite effects of beam-induced motion and radiation chemistry resulting in a local change in the water T1 relaxation time.Significance. The identification of ToF angiography sequence-based proton beam visualisation in water phantoms to result from composite effects of beam-induced motion and radiation chemistry represents the starting point for the future elucidation of the currently unexplained motion-based MRI contrast mechanism and the identification of the proton beam-induced material change causing T1 relaxation time lengthening.

Absolute dosimetry for FLASH proton pencil beam scanning radiotherapy

A paradigm shift is occurring in clinical oncology exploiting the recent discovery that short pulses of ultra-high dose rate (UHDR) radiation-FLASH radiotherapy-can significantly spare healthy tissues whilst still being at least as effective in curing cancer as radiotherapy at conventional dose rates. These properties promise reduced post-treatment complications, whilst improving patient access to proton beam radiotherapy and reducing costs. However, accurate dosimetry at UHDR is extremely complicated. This work presents measurements performed with a primary-standard proton calorimeter and derivation of the required correction factors needed to determine absolute dose for FLASH proton beam radiotherapy with an uncertainty of 0.9% (1[Formula: see text]), in line with that of conventional treatments. The establishment of a primary standard for FLASH proton radiotherapy improves accuracy and consistency of the dose delivered and is crucial for the safe implementation of clinical trials, and beyond, for this new treatment modality.

Calorimeter measurements of absolute dose in aluminum, a surrogate of bone, to validate dose-to-medium in Acuros XB

Objective. While the accuracy of dose calculations in water with Acuros XB is well established, experimental validation of dose in bone is limited. Acuros XB reports both dose-to-medium and dose-to-water, and these values differ in bone, but there are no reports of measurements of validation in bone. This work compares Acuros XB calculations to measurements of absolute dose in aluminum (medium similar to bone). The validity of using selected relative dosimeters in aluminum is also investigated.Approach. A calorimeter with an aluminum core embedded in an aluminum phantom was selected as bone surrogate for the measurement of absolute dose. Matching the medium of the core to the medium of the phantom allowed eliminating the calculation of the conversion between media. The dose was measured at the fixed depth of 3.3 cm in aluminum (∼9 g·cm^-2) with 6X, 10X, 6FFF and 10FFF photon beams from a TrueBeam Varian linac. In addition, experimental cross-calibration between water and aluminum was performed for an IBA CC13 ionization chamber, a PTW microDiamond and EBT3 Gafchromic film.Main results. Calculations with Acuros XB dose-to-medium in aluminum differed from the calorimetry data by -2.8% to -3.5%, depending on the beam. Use of dose-to-water would have resulted in about 39% discrepancy. The cross calibration coefficient between water and aluminum yielded values of about 0.87 for the CC13 chamber, 0.91 for the microDiamond, and 0.88 for the film, and independent of the beam within about ±1%.Significance. It was demonstrated the value of the dose-to-medium in aluminum (surrogate of bone) computed with Acuros XB is close to the value of the absolute dose measured with a calorimeter, and there is a significant discrepancy when dose-to-water is used instead. The use of an ionization chamber, a microDiamond and Gafchromic film in aluminum required a considerable correction from calibration in water.

Dose area product primary standards established by graphite calorimetry at the LNE-LNHB for small radiation fields in radiotherapy

PURPOSE

To present primary standards establishment in terms of Dose Area Product (DAP) for small field sizes.

METHODS

A large section graphite calorimeter and two plane-parallel ionization chambers were designed and built in-house. These chambers were calibrated in a 6MV FFF beam at the maximum dose rate of 1400 UM/min for fields defined by specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13 and 15 mm side length on a Varian TrueBeam linac.

RESULTS

The two chambers show the same behaviour regardless of field shape and size. From 5 to 15 mm, calibration coefficients slightly increase with the field size with a magnitude of 1.8% and 1.1% respectively for the two chambers, and are independent of the field shape. This tendency was confirmed by Monte Carlo calculations. The average associated uncertainty of the calibration coefficients is around 0.6% at k=1.

CONCLUSIONS

For the first time, primary standards in terms of DAP were established by graphite calorimetry for an extended range of small field sizes. These promising results open the door for an alternative approach in small fields dosimetry.

Experimental determination ofkQfactors for two types of ionization chambers in scanned proton beams

Objective.Experimental determination of beam qualityk_Qfactors for two types of Farmer ionization chambers, NE2571 and IBA FC65-G, in a scanned proton beam for three nominal energies (140 MeV, 180 MeV and 220 MeV) based on water calorimetry.Approach.Beam quality correction factors were determined comparing the results obtained with water calorimetry and ionometry. Water calorimetry was performed to determine the absorbed dose at a depth of measurement in water of 5 g cm^-2, limited by the extension of the calorimeter glass vessel used. For the ionometry, two chambers of each type were included in the study. The ionization chambers were calibrated in terms of absorbed dose to water in⁶⁰Co at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to the BIPM, and were used according to the IAEA TRS-398 Code of Practice.Main results. Thek_Qvalues determined in the present work have been compared with the values tabulated in TRS-398 and its forthcoming update and also with those obtained in previous water calorimetric measurements and Monte Carlo calculations. All results were found to agree within the combined uncertainties of the different data.Significance. It is expected that the present work will serve as an experimental contribution tok_Q-factors for the two chamber types and three scanned proton beam qualities used.

Feasibility of operating a millimeter-scale graphite calorimeter for absolute dosimetry of small-field photon beams in the clinic

PURPOSE

To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams.

METHODS

This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3 mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3 mK·Gy^-1 . The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite. The detector was irradiated with 6 MV ( % dd ( 10 ) x  = 63.5%) and 10 MV ( % dd ( 10 ) x  = 71.1%) flattening filter-free (FFF) photon beams for two field sizes, characterized by S clin dimensions of 2.16 and 11.0 cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at d max in water.

RESULTS

The field output correction factors for this dosimeter design were computed for field sizes ranging from S clin  = 0.54 to 11.0 cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, S clin  = 2.16 cm, which was irradiated with the 10 MV FFF beam, produced readings of 84.4 cGy (±1.3%) in the calorimeter and 84.5 cGy (±1.3%) in the ionization chamber.

CONCLUSION

The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.

Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice

With the rapid increase in clinical treatments with MRI-linacs, a consistent, harmonized and sustainable ground for reference dosimetry in MRI-linacs is needed. Specific for reference dosimetry in MRI-linacs is the presence of a strong magnetic field. Therefore, existing Code of Practices (CoPs) are inadequate. In recent years, a vast amount of papers have been published in relation to this topic. The purpose of this review paper is twofold: to give an overview and evaluate the existing literature for reference dosimetry in MRI-linacs and to discuss whether the literature and datasets are adequate and complete to serve as a basis for the development of a new or to extend existing CoPs. This review is prefaced with an overview of existing MRI-linac facilities. Then an introduction on the physics of radiation transport in magnetic fields is given. The main part of the review is devoted to the evaluation of the literature with respect to the following subjects: • beam characteristics of MRI-linac facilities; • formalisms for reference dosimetry in MRI-linacs; • characteristics of ionization chambers in the presence of magnetic fields; • ionization chamber beam quality correction factors; and • ionization chamber magnetic field correction factors. The review is completed with a discussion as to whether the existing literature is adequate to serve as basis for a CoP. In addition, it highlights subjects for future research on this topic.

Operating a graphite calorimeter in quasi-isothermal mode under high-energy x-ray beams

In this study, we developed a semi-active method to run a graphite calorimeter in the quasi-isothermal mode under high-energy x-ray beams. The rate of energy imparted by the beam during irradiation was compensated mainly by removing the electrical heating power based on the pre-calculation and in part by an active automated algorithm, as well, while the temperature of the calorimeter core was kept constant. Irradiations were performed under the linear electron accelerator x-ray beams at 6, 8, 10, 15, and 18 MV. A simple model was applied to analyze the results. The energy imparted to the core was determined with an uncertainty level of 0.2%-0.3%, and the results were reaffirmed by comparing it with that obtained by the quasi-adiabatic mode. The normalized root-mean-square deviation to the mean from the quasi-adiabatic mode was 0.11%, and the associated uncertainty was 0.16% taking into account the correlation of the uncertainty components. This level of agreement showed that the present method is practical for the high-energy x-ray dosimetry.

First-stage validation of a portable imageable MR-compatible water calorimeter

PURPOSE

The purpose of this study is to design a water calorimeter with three goals in mind: (a) To be fully magnetic resonance (MR)-compatible; (b) To be imaged using kV cone beam computed tomography (CBCT), MV portal imaging or MRI for accurate positioning; (c) To accommodate both vertical and horizontal beam incidence, as well as volumetric deliveries or Gamma Knife®. Following this, the calorimeter performance will be measured using an accelerator-based high-energy photon beam.

METHODS

A portable 4°C cooled stagnant water calorimeter was built using MR-compatible materials. The walls consist of layers of acrylic plastic, aerogel-based material acting as thermal insulation, as well as tubing for coolant to flow to keep the calorimeter temperature stable at 4°C. The lid contains additional pathways for coolant to flow through as well as two hydraulically driven stirrers. The water calorimeter was positioned in an Elekta Versa using kV CBCT imaging as well as orthogonal MV image pairs. Absolute absorbed dose to water was then determined under a 6 MV flattening filter-free (FFF) beam. This was compared against reference dosimetry results that were measured under identical conditions with an Exradin A1SL ionization chamber with a calibration coefficient directly traceable to the National Research Council Canada.

RESULTS

The dose to water determined with the calorimeter (n = 30) agreed with the A1SL ionization chamber reference dose measurements (n = 15) to within 0.25%. The uncertainty associated with the water calorimeter absorbed dose measurement was estimated to be 0.54% (k = 1).

CONCLUSIONS

An MR-compatible water calorimeter was successfully built and absolute absorbed dose to water under a conventional 6 MV FFF beam was determined successfully as a first-stage validation of the system.

Absolute dosimetry of a 1.5 T MR-guided accelerator-based high-energy photon beam in water and solid phantoms using Aerrow

PURPOSE

In this work, the fabrication, operation, and evaluation of a probe-format graphite calorimeter - herein referred to as Aerrow - as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)-guided radiotherapy.

METHODS

Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in-house. Graphite-to-water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI-linac and were directly compared to the results obtained using two calibrated reference-class IC types. The feasibility of performing solid phantom-based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference-type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water.

RESULTS

In the absence of the B-field, as well as in the parallel orientation while in the presence of the B-field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG-51 using calibrated reference-class ICs. Statistically significant differences on the order of (2-4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B-field. Aerrow had a peak-to-peak response of about 0.5% when rotated within the solid phantom regardless of whether the B-field was present or not.

CONCLUSIONS

This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high-energy photon fields in the presence of a 1.5 T B-field to a greater accuracy than currently achievable with ICs. The detector-phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.