Dose verification in clinical imrt prostate incidents

Validation of complex radiotherapy techniques using polymer gel dosimetry

Modern radiotherapy delivers highly conformal dose distributions to irregularly shaped target volumes while sparing the surrounding normal tissue. Due to the complex planning and delivery techniques, dose verification and validation of the whole treatment workflow by end-to-end tests became much more important and polymer gel dosimeters are one of the few possibilities to capture the delivered dose distribution in 3D. The basic principles and formulations of gel dosimetry and its evaluation methods are described and the available studies validating device-specific geometrical parameters as well as the dose delivery by advanced radiotherapy techniques, such as 3D-CRT/IMRT and stereotactic radiosurgery treatments, the treatment of moving targets, online-adaptive magnetic resonance-guided radiotherapy as well as proton and ion beam treatments, are reviewed. The present status and limitations as well as future challenges of polymer gel dosimetry for the validation of complex radiotherapy techniques are discussed.

Engineering functional and anthropomorphic models for surgical training in interventional radiology: A state-of-the-art review

Training medical students in surgical procedures and evaluating their performance are both necessary steps to ensure the safety and efficacy of surgeries. Traditionally, trainees practiced on live patients, cadavers or animals under the supervision of skilled physicians, but realistic anatomical phantom models have provided a low-cost alternative because of the advance of material technology that mimics multi-layer tissue structures. This setup provides safer and more efficient training. Many research prototypes of phantom models allow rapid in-house prototyping for specific geometries and tissue properties. The gel-based method and 3D printing-based method are two major methods for developing phantom prototypes. This study excluded virtual reality based technologies and focused on physical phantoms, total 189 works published between 2015 and 2020 on anatomical phantom prototypes made for interventional radiology were reviewed in terms of their functions and applications. The phantom prototypes were first categorized based on fabrication methods and then subcategorized based on the organ or body part they simulated; the paper is organized accordingly. Engineering specifications and applications were analyzed and summarized for each study. Finally, current challenges in the development of phantom models and directions for future work were discussed.

Radiation Dosimetry by Use of Radiosensitive Hydrogels and Polymers: Mechanisms, State-of-the-Art and Perspective from 3D to 4D

Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise.

PAGAT gel dosimetry for everyone: gel production, measurement and evaluation

Polymer gel (PG) dosimetry is a valuable tool to measure complex dose distributions in 3D with a high spatial resolution. However, due to complex protocols that need to be followed for in-house produced PGs and the high costs of commercially available gels, PG gels are only rarely applied in quality assurance procedures worldwide. In this work, we provide an introduction to perform highly standardized dosimetric PG experiments using PAGAT (PolyAcrylamide Gelatine gel fabricated at ATmospheric conditions) dosimetry gel. PAGAT gel can be produced at atmospheric conditions, at low costs and is evaluated using magnetic resonance imaging (MRI). The conduction of PG experiments is described in great detail including the gel production, treatment planning, irradiation, MRI evaluation and post-processing procedure. Furthermore, a plugin in an open source image processing tool for post-processing is provided free of charge that allows a standardized and reproducible analysis of PG experiments.

Technical Note: On the feasibility of performing dosimetry in target and organ at risk using polymer dosimetry gel and thermoluminescence detectors in an anthropomorphic, deformable, and multimodal pelvis phantom

OBJECTIVE

To assess the feasibility of performing dose measurements in the target (prostate) and an adjacent organ at risk (rectum) using polymer dosimetry gel and thermoluminescence detectors (TLDs) in an anthropomorphic, deformable, and multimodal pelvis phantom (ADAM PETer).

METHODS

The 3D printed prostate organ surrogate of the ADAM PETer phantom was filled with polymer dosimetry gel. Nine TLD600 (LiF:Mg,Ti) were installed in 3 × 3 rows on a specifically designed 3D-printed TLD holder. The TLD holder was inserted into the rectum at the level of the prostate and fixed by a partially inflated endorectal balloon. Computed tomography (CT) images were taken and treatment planning was performed. A prescribed dose of 4.5 Gy was delivered to the planning target volume (PTV). The doses measured by the dosimetry gel in the prostate and the TLDs in the rectum ("measured dose") were compared to the doses calculated by the treatment planning system ("planned dose") on a voxel-by-voxel basis.

RESULTS

In the prostate organ surrogate, the 3D-γ-index was 97.7% for the 3% dose difference and 3 mm distance to agreement criterium. In the center of the prostate organ surrogate, measured and planned doses showed only minor deviations (<0.1 Gy, corresponding to a percentage error of 2.22%). On the edges of the prostate, slight differences between planned and measured doses were detected with a maximum deviation of 0.24 Gy, corresponding to 5.3% of the prescribed dose. The difference between planned and measured doses in the TLDs was on average 0.08 Gy (range: 0.02-0.21 Gy), corresponding to 1.78% of the prescribed dose (range: 0.44%-4.67%).

CONCLUSIONS

The present study demonstrates the feasibility of using polymer dosimetry gel and TLDs for 3D and 1D dose measurements in the prostate and the rectum organ surrogates in an anthropomorphic, deformable and multimodal phantom. The described methodology might offer new perspectives for end-to-end tests in image-guided adaptive radiotherapy workflows.

Absolute dosimetry with polymer gels-a TLD reference system

BACKGROUND AND PURPOSE

Absolute dosimetry in 3D with polymer gels (PG) is generally complicated and usually requires a second independent measurement with conventional detectors. This is why, PG are often used only for relative dosimetry. To overcome this drawback, we combine PG with a 1D thermoluminescence (TL) detector within the same measurement. The TL detector information is then used as additional information for calibration of the gel.

MATERIALS AND METHODS

The PAGAT dosimetry gel was used in combination with TLD600 (LiF:Mg,Ti). TL detectors were attached on the surface of the PG container placed inside a cylindrical phantom. To test the usability of this setup, two irradiation geometries were carried out: (a) homogeneous target coverage and (b) small-field irradiation. PG was evaluated with magnetic resonance imaging (MRI) and the TL detectors with a Harshaw 5500 hot gas reader.

RESULTS

PG dosimetry alone showed deviations of up to 4% as compared to calculations. Including additionally the dose information of the TL detectors for PG calibration, this deviation was decreased to less than 1% for both irradiation geometries. This is also reflected by the very high [Formula: see text]-passing rates of  >  96% (3%/3 mm) and  >93% (2%/2 mm), respectively.

CONCLUSION

This study presents a novel method combining 3D PG and TL dose measurements for the purpose of absolute 3D dose measurements that can also be applied in complex anthropomorphic phantoms using only a single measurement. The method was validated for two different irradiation geometries including a homogeneous large field as well as a small field irradiation with sharp dose gradients.

Evaluation of three-dimensional polymer gel dosimetry using X-ray CT and R2 MRI

It is difficult to obtain images of thin slices from measurement of spin-spin relaxation (R2) with magnetic resonance imaging (MRI) using the traditional dose reading method of polymer gel dosimetry. In this study, the dose reading method was performed using X-ray computed tomography (CT) for proton beam measurements in order to enable collection of thin slices. In addition, three-dimensional (3D) images of polymer gels were constructed using volume rendering. As a result of acquisition of thin slices, more detailed 3D data consisting of smaller voxel sizes compared to R2 were acquired. However, it was found that with thin slice thicknesses and small voxels, the signal-to-noise ratio around the voxels deteriorated. In addition, the coefficient of variation of non-irradiated gels with CT was smaller than that with R2 MRI.

Dosimetry investigation of MOSFET for clinical IMRT dose verification

In IMRT, patient-specific dose verification is followed regularly at each centre. Simple and efficient dosimetry techniques play a very important role in routine clinical dosimetry QA. The MOSFET dosimeter offers several advantages over the conventional dosimeters such as its small detector size, immediate readout, immediate reuse, multiple point dose measurements. To use the MOSFET as routine clinical dosimetry system for pre-treatment dose verification in IMRT, a comprehensive set of experiments has been conducted, to investigate its linearity, reproducibility, dose rate effect and angular dependence for 6 MV x-ray beam. The MOSFETs shows a linear response with linearity coefficient of 0.992 for a dose range of 35 cGy to 427 cGy. The reproducibility of the MOSFET was measured by irradiating the MOSFET for ten consecutive irradiations in the dose range of 35 cGy to 427 cGy. The measured reproducibility of MOSFET was found to be within 4% up to 70 cGy and within 1.4% above 70 cGy. The dose rate effect on the MOSFET was investigated in the dose rate range 100 MU/min to 600 MU/min. The response of the MOSFET varies from -1.7% to 2.1%. The angular responses of the MOSFETs were measured at 10 degrees intervals from 90 to 270 degrees in an anticlockwise direction and normalized at gantry angle zero and it was found to be in the range of 0.98 ± 0.014 to 1.01 ± 0.014. The MOSFETs were calibrated in a phantom which was later used for IMRT verification. The measured calibration coefficients were found to be 1 mV/cGy and 2.995 mV/cGy in standard and high sensitivity mode respectively. The MOSFETs were used for pre-treatment dose verification in IMRT. Nine dosimeters were used for each patient to measure the dose in different plane. The average variation between calculated and measured dose at any location was within 3%. Dose verification using MOSFET and IMRT phantom was found to quick and efficient and well suited for a busy radiotherapy department.

A novel technique to enable experimental validation of deformable dose accumulation

PURPOSE

To propose a novel technique to experimentally validate deformable dose algorithms by measuring 3D dose distributions under the condition of deformation using deformable gel dosimeters produced by a novel gel fabrication method.

METHOD

Five gel dosimeters, two rigid control gels and three deformable gels, were manufactured and treated with the same conformal plan that prescribed 400 cGy to the isocenter. The control gels were treated statically; the deformable gels were treated while being compressed by an actuation device to simulate breathing motion (amplitude of compression  = 1, 1.5, and 2 cm, respectively; frequency  =  16 rpm). Comparison between the dose measured by the control gels and the corresponding static dose distribution calculated in the treatment planning system (TPS) has determined the intrinsic dose measurement uncertainty of the gel dosimeters. Doses accumulated using MORFEUS, a biomechanical model-based deformable registration and dose accumulation algorithm, were compared with the doses measured by the deformable gel dosimeters to verify the accuracy of MORFEUS using dose differences at each voxel as well as the gamma index test. Flexible plastic wraps were used to contain and protect the deformable gels from oxygen infiltration, which inhibits the gels' dose sensitizing ability. Since the wraps were imperfect oxygen barrier, dose comparison between MORFEUS and the deformable gels was performed only in the central region with a received dose of 200 cGy or above to exclude the peripheral region where oxygen penetration had likely affected dose measurements.

RESULTS

Dose measured with the control gels showed that the intrinsic dose measurement uncertainty of the gel dosimeters was 11.8 cGy or 4.7% compared to the TPS. The absolute mean voxel-by-voxel dose difference between the accumulated dose and the dose measured with the deformable gels was 4.7 cGy (SD = 36.0 cGy) or 1.5% (SD = 13.4%) for the three deformable gels. The absolute mean vector distance between the 250, 300, 350, and 400 cGy isodose surfaces on the accumulated and measured distributions was 1.2 mm (SD < 1.5 mm). The gamma index test that used the dose measurement precision of the control gels as the dose difference criterion and 2 mm as the distance criterion was performed, and the average pass rate of the accumulated dose distributions for all three deformable gels was 92.7%. When the distance criterion was relaxed to 3 mm, the average pass rate increased to 96.9%.

CONCLUSION

This study has proposed a novel technique to manufacture deformable volumetric gel dosimeters. By comparing the doses accumulated in MORFEUS and the doses measured with the dosimeters under the condition of deformation, the study has also demonstrated the potential of using deformable gel dosimetry to experimentally validate algorithms that include deformations into dose computation. Since dose less than 200 cGy was not evaluated in this study, future investigations will focus more on low dose regions by either using bigger gel dosimeters or prescribing a lower dose to provide a more complete experimental validation of MORFEUS across a wider dose range.

Polymer gel dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

Polymer gel dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

Acrylonitrile Butadiene Styrene (ABS) plastic based low cost tissue equivalent phantom for verification dosimetry in IMRT

A novel IMRT phantom was designed and fabricated using Acrylonitrile Butadiene Styrene (ABS) plastic. Physical properties of ABS plastic related to radiation interaction and dosimetry were compared with commonly available phantom materials for dose measurements in radiotherapy. The ABS IMRT phantom has provisions to hold various types of detectors such as ion chambers, radiographic/radiochromic films, TLDs, MOSFETs, and gel dosimeters. The measurements related to pretreatment dose verification in IMRT of carcinoma prostate were carried out using ABS and Scanditronix‐Wellhofer RW3 IMRT phantoms for five different cases. Point dose data were acquired using ionization chamber and TLD discs, while Gafchromic EBT and radiographic EDR2 films were used for generating 2D dose distributions. Treatment planning system (TPS) calculated and measured doses in ABS plastic and RW3 IMRT phantom were in agreement within ± 2%. The dose values at a point in a given patient acquired using ABS and RW3 phantoms were found comparable within 1%. Fluence maps and dose distributions of these patients generated by TPS and measured in ABS IMRT phantom were also found comparable both numerically and spatially. This study indicates that ABS plastic IMRT phantom is a tissue‐equivalent phantom and, dosimetrically, it is similar to solid/plastic water IMRT phantoms. Although this material is demonstrated for IMRT dose verification, it can also be used as a tissue‐equivalent phantom material for other dosimetry purposes in radiotherapy.

PACS number: 87.53Kn, 87.55Qr, 87.53Bn and 87.55Km

Investigation of the feasibility of relative 3D dosimetry in the Radiologic Physics Center Head and Neck IMRT phantom using presage/optical-CT

This study presents the application of the Presage/optical-CT 3D dosimetry system for relative dosimetry in the Radiologic Physics Center (RPC) Head and Neck (H&N) IMRT phantom. Performance of the system was evaluated by comparison with the "gold-standard" RPC credentialing test. A modified Presage cylindrical insert was created that extended the capability of the RPC H&N phantom to 3D dosimetry. The RPC phantom was taken through the entire treatment planning procedure with both the standard RPC insert and the modified Presage insert. An IMRT plan was created to match the desired dose constraints of the credentialing test. This plan was delivered twice to the RPC phantom: first containing the standard insert, and then again containing the Presage insert. After irradiation, the standard insert was sent for routine credentialing analysis; including point dose measurements (TLD) and planar Gafchromic EBT film measurement. The 3D dose distribution from Presage was read out at Duke using the OCTOPUS 5X optical-CT scanner. The Presage distribution was compared with gold-standard EBT measurement (determined by the RPC) and the calculated Eclipse distribution. The agreement between the normalized EBT, Presage, and Eclipse distributions, in the central axial plane was evaluated using profiles and gamma-map comparisons (4% dose difference and 3 mm distance to agreement). Profiles showed good agreement between EBT, Presage, and Eclipse distributions. 2D gamma-map comparisons between all three modalities showed at least 98% pass rate. The excellent agreement between Presage and EBT in the central plane established Presage as a standard against which to evaluate the accuracy of the 3D calculated Eclipse distribution. A gamma comparison between normalized Presage and Eclipse 3D distributions gave an overall pass rate of approximately 94%. In conclusion, the Presage/optical-CT system was found to be feasible for relative 3D dosimetry in the RPC IMRT H&N phantom. The potential to extend the RPC IMRT credentialing procedure to 3D may be feasible provided accurate calibration to dose (Gy) and robustness to shipping stress are demonstrated.

Image guided IMRT dosimetry using anatomy specific MOSFET configurations

We have investigated the feasibility of using a set of multiple MOSFETs in conjunction with the mobileMOSFET wireless dosimetry system, to perform a comprehensive and efficient quality assurance (QA) of IMRT plans. Anatomy specific MOSFET configurations incorporating 5 MOSFETs have been developed for a specially designed IMRT dosimetry phantom. Kilovoltage cone beam computed tomography (kV CBCT) imaging was used to increase the positional precision and accuracy of the detectors and phantom, and so minimize dosimetric uncertainties in high dose gradient regions. The effectiveness of the MOSFET based dose measurements was evaluated by comparing the corresponding doses measured by an ion chamber. For 20 head and neck IMRT plans the agreement between the MOSFET and ionization chamber dose measurements was found to be within −0.26±0.88% and 0.06±1.94% (1σ) for measurement points in the high dose and low dose respectively. A precision of 1 mm in detector positioning was achieved by using the X‐Ray Volume Imaging (XVI) kV CBCT system available with the Elekta Synergy Linear Accelerator. Using the anatomy specific MOSFET configurations, simultaneous measurements were made at five strategically located points covering high dose and low dose regions. The agreement between measurements and calculated doses by the treatment planning system for head and neck and prostate IMRT plans was found to be within 0.47±2.45%. The results indicate that a cylindrical phantom incorporating multiple MOSFET detectors arranged in an anatomy specific configuration, in conjunction with image guidance, can be utilized to perform a comprehensive and efficient quality assurance of IMRT plans.

PACS number: 87.55.Qr

Introducing gel dosimetry in a clinical environment: customization of polymer gel composition and magnetic resonance imaging parameters used for 3D dose verifications in radiosurgery and intensity modulated radiotherapy

Radiation sensitive gels have been used as dosimeters for clinical dose verification of different radiation therapy modalities. However, the use of gels is not widespread, because careful techniques are required to achieve the dose precision and accuracy aimed for in clinical dose verification. Here, the introduction of gel dosimetry in a clinical environment is described, including the whole chain of customizations and preparations required to introduce magnetic resonance (MR) based gel dosimetry into clinical routine. In order to standardize gel dosimetry in dose verifications for radiosurgery and intensity modulated radiotherapy (IMRT), we focused on both the customization of the gel composition and of the MR imaging parameters to increase its precision. The relative amount of the components of the normoxic, methacrylic acid based gel (MAGIC) was changed to obtain linear and steep dose response relationships. MR imaging parameters were customized for the different dose ranges used in order to lower the relative standard deviation of the measured transversal relaxation rate (R2). An optimization parameter was introduced to quantify the change in the relative standard deviation of R2 (sigma(R2,rel)) taking the increase in MR time into account. A 9% methacrylic acid gel customized for radiosurgery was found to give a linear dose response up to 40 Gy with a slope of 0.94 Gy(-1) s(-1), while a 6% methacrylic acid gel customized for IMRT had a linear range up to 3 Gy with a slope of 1.86 Gy(-1) s(-1). With the help of an introduced optimization parameter, the mean sigma(R2,rel) was improved by 13% for high doses and by 55% for low doses, without increasing MR time to unacceptable values. A mean dose resolution of less than 0.13 Gy has been achieved with the gel and imaging parameters customized for IMRT and a dose resolution from 0.97 Gy (at 5 Gy) to 2.15 Gy (at 40 Gy) for the radiosurgery dose range. The comparisons of calculated and measured relative 3D dose distributions performed for radiosurgery and IMRT showed an acceptable overall correlation. The gamma criterion for the radiosurgery verification with a voxel size of 1.5 x 1.5 x 1.5 mm3 was passed by 96.8% of the voxels (1.5 mm distance, 8% in dose). For the IMRT verification using a voxel size of 1.25 x 1.25 x 5 mm3 the gamma criterion was passed by 50.3% of the voxels (3 mm distance, 3% dose uncertainty). Using dedicated data analysis and visualization software, MR based normoxic gel dosimetry was found to be a valuable tool for clinically based dose verification, provided that customized gel compositions and MR imaging parameters are used. While high dose precision was achieved, further work is required to achieve clinically acceptable dose accuracy.

Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy

The prototype of a stereotactic collimator set developed in our department is evaluated for clinical use. This set consists of three cylindrical blocks mounted on a tray which slides in the wedge insert of a Siemens Primus accelerator. Each block has a collimating hole along its long axis to produce radiation fields of circular cross-section at the isocentre plane with diameters of 15 mm, 20 mm and 25 mm. Different geometric and dosimetric quality assurance tests were performed and results are found within the limits set for stereotactic radiotherapy. Dosimetry results measured using Kodak EDR-2 radiographic film and a pinpoint ion chamber also show good agreement with corresponding results calculated by Monte Carlo simulation of the linear accelerator head and the collimators. Measured dosimetry data were used to adapt a conventional PLATO treatment planning system for stereotactic radiotherapy using the prototype collimator set. Treatment planning system calculations and film measurements for treatment of an intracranial lesion in an anthropomorphic head phantom using coplanar 180 degrees arcs are compared and found to agree within 2 mm. This supports the accuracy of dose delivery using the prototype stereotactic collimators. Despite their increased penumbra (2.5-3.5 mm relative to 2-2.5 mm for commercially available collimators) the ease of construction makes the proposed stereotactic collimators an interesting alternative for accomplishing cost effective stereotactic treatments.

Mechanical and dose delivery accuracy evaluation in radiosurgery using polymer gels

The polymer gel–magnetic resonance imaging (MRI) dosimetry technique was used to evaluate the mechanical and dose delivery accuracy in Leksell gamma‐knife stereotactic radiosurgery for the treatment of multiple targets. Two different polymer gel dosimeter formulations reported in the literature were prepared in‐house. A plan for the treatment of four brain metastases (targets) was generated. It involved the delivery of four 8‐mm collimator shots using different prescription isodose lines and different prescription doses for each target, keeping the maximum dose constant for all targets. A sample of each gel formulation was irradiated using a custom‐made phantom with an experimental procedure capable of testing the increased nominal mechanical accuracy of stereotactic radiosurgery. The irradiated dosimeters were evaluated using a clinical 1.5 T MR imager. Result manipulation in 3D allowed for the determination of the mechanical accuracy in the delivery of each shot through the comparison of measured versus planned shot center coordinates. Dose delivery accuracy was also evaluated by comparison of maximum dose values measured at the center of each shot as well as dose distribution measurements, with corresponding treatment‐planning calculations. Polymer gel dosimetry was found capable of verifying the complete chain of radiosurgery treatment in gamma‐knife applications involving the irradiation of multiple targets.

PACS numbers: 87.53.Dq, 87.53.Ly, 87.53.Xd

Sensitivity of linear CCD array based film scanners used for film dosimetry

Film dosimetry is commonly performed by using linear CCD array transmission optical densitometers. However, these devices suffer from a variation in response along the detector array. If not properly corrected for, this nonuniformity may lead to significant overestimations of the measured dose as one approaches regions close to the edges of the scanning region. In this note, we present measurements of the spatial response of an AGFA Arcus II document scanner used for radiochromic film dosimetry. Results and methods presented in this work can be generalized to other CCD based transmission scanners used for film dosimetry employing either radiochromic or radiographic films.

Experimental determination of the effect of detector size on profile measurements in narrow photon beams

The aim of this work is to investigate experimentally the detector size effect on narrow beam profile measurements. Polymer gel and magnetic resonance imaging dosimetry was used for this purpose. Profile measurements (Pm(s)) of a 5 mm diameter 6 MV stereotactic beam were performed using polymer gels. Eight measurements of the profile of this narrow beam were performed using correspondingly eight different detector sizes. This was achieved using high spatial resolution (0.25 mm) two-dimensional measurements and eight different signal integration volumes A X A X slice thickness, simulating detectors of different size. "A" ranged from 0.25 to 7.5 mm, representing the detector size. The gel-derived profiles exhibited increased penumbra width with increasing detector size, for sizes >0.5 mm. By extrapolating the gel-derived profiles to zero detector size, the true profile (Pt) of the studied beam was derived. The same polymer gel data were also used to simulate a small-volume ion chamber profile measurement of the same beam, in terms of volume averaging. The comparison between these results and actual corresponding small-volume chamber profile measurements performed in this study, reveal that the penumbra broadening caused by both volume averaging and electron transport alterations (present in actual ion chamber profile measurements) is a lot more intense than that resulted by volume averaging effects alone (present in gel-derived profiles simulating ion chamber profile measurements). Therefore, not only the detector size, but also its composition and tissue equivalency is proved to be an important factor for correct narrow beam profile measurements. Additionally, the convolution kernels related to each detector size and to the air ion chamber were calculated using the corresponding profile measurements (Pm(s)), the gel-derived true profile (Pt), and convolution theory. The response kernels of any desired detector can be derived, allowing the elimination of the errors associated with narrow beam profile measurements.

Polymer gel dosimeters with reduced toxicity: a preliminary investigation of the NMR and optical dose-response using different monomers

In this work, three new polymer gel dosimeter recipes were investigated that may be more suitable for widespread applications than polyacrylamide gel dosimeters, since the extremely toxic acrylamide has been replaced with the less harmful monomers N-isopropylacrylamide (NIPAM), diacetone acrylamide and N-vinylformamide. The new gel dosimeters studied contained gelatin (5 wt%), monomer (3 wt%), N,N'-methylene-bis-acrylamide crosslinker (3 wt%) and tetrakis (hydroxymethyl) phosphonium chloride antioxidant (10 mM). The NMR response (R2) of the dosimeters was analysed for conditions of varying dose, dose rate, time post-irradiation, and temperature during irradiation and scanning. It was shown that the dose-response behaviour of the NIPAM/Bis gel dosimeter is comparable to that of normoxic polyacrylamide gel (PAGAT) in terms of high dose-sensitivity and low dependence on dose rate and irradiation temperature, within the ranges considered. The dose-response (R2) of NIPAM/Bis appears to be linear over a greater dose range than the PAGAT gel dosimeter. The effects of time post-irradiation (temporal instability) and temperature during NMR scanning on the R2 response were more significant for NIPAM/Bis dosimeters. Diacetone acrylamide and N-vinylformamide gel dosimeters possessed considerably lower dose-sensitivities. The optical dose-response, measured in terms of the attenuation coefficient for each polymer gel dosimeter, showed potential for the use of optical imaging techniques in future studies.

Polymer gel dosimetry for the TG-43 dosimetric characterization of a new 125I interstitial brachytherapy seed

In this work, a polymer gel-magnetic resonance (MR) imaging method is employed for the dosimetric characterization of a new 125I low dose rate seed (IsoSeed model I25.S17). Two vials filled with PABIG gel were prepared in-house and one new seed as well as one commercially available 125I seed of similar dose rate and well-known dosimetric parameters (IsoSeed model I25.S06) were positioned in each vial. Both seeds in each vial were MR scanned simultaneously on days 11 and 26 after implantation. The data obtained from the known seed in each vial are used to calibrate the gel dose response which, for the prolonged irradiation duration necessitated by the investigated dose rates, depends on the overall irradiation time. Data for this study are presented according to the AAPM TG-43 dosimetric formalism. Polymer gel results concerning the new seed are compared to corresponding, published dosimetric results obtained, for the purpose of the new seed clinical implementation, by our group using the established methods of Monte Carlo (MC) simulation and thermo-luminescence dosimetry (TLD). Polymer gel dosimetry yields an average dose rate constant value of lambda = (0.921 +/- 0.031) cGy h(-1) U(-1) relative to (MC)lambda = (0.929 +/- 0.014) cGy h(-1) U(-1), (TLD)lambda = (0.951 +/- 0.044) cGy h(-1) U(-1) and the average value of Lambda = (0.940 +/- 0.051) cGy h(-1) U(-1) proposed for the clinical implementation of the new seed. Results for radial dose function, g(L)(r), and anisotropy function, F(r, theta), also agree with corresponding MC calculations within experimental uncertainties which are smaller for the polymer gel method compared to TLD. It is concluded that the proposed polymer gel-magnetic resonance imaging methodology could be used at least as a supplement to the established techniques for the dosimetric characterization of new low energy and low dose rate interstitial brachytherapy seeds.

Einsatz normoxischer Polymergele in der 3D-Dosimetrie für die Radiochirurgie

The aim of this study was to describe the manufacture of normoxic polymer gels, to characterize their dose response relationship, to optimize MR imaging parameters in order to minimize the standard deviation in the measured dose and to use the gel in a dose verification experiment in radiosurgery. The normoxic polymer gel used is simple to manufacture under normal atmospheric conditions and is characterized by a linear dose relationship up to 40 Gy. MR imaging was performed using 2-dimensional (20) single spin echo pulse sequences with two different echo times. The imaging parameters were optimized in order to minimize the standard deviation of the measured transversal relaxation rate R2 and to achieve a geometrical resolution of 1.5 mm. Comparisons of calculated and measured relative 3D dose distributions using a multi isocentric irradiation with Gamma Knife B showed a good overall agreement of both the isodose levels and the differential and cumulative dose volume histograms. The standard deviation in the measured dose was approximately 9% at 30 Gy. The evaluation according to the gamma criterion showed that 96% of the dose voxels remained within a spatial uncertainty of 1.5 mm and a dose uncertainty of 8%.

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.

Fricke gel as a tool for dose distribution verification: optimization and characterization

With the introduction of conformal techniques in radiation therapy, gel dosimetry plays an important role as a 3D dose verification system. There are two main types of gels in use for dosimetry: Fricke gels and polymer gels. The advantages of polymer gels are improved dose response and stability with no diffusion problems. However, the more complicated fabrication procedure and the greater cost compared to Fricke gels makes polymer gels less attractive in routine clinical use. Dose resolution has recently been introduced as a concept for comparing and optimizing the performance of different types of gel dosimeters. This parameter has not yet been investigated for Fricke gels. In this study, the effect on the dose resolution and the diffusion from different gelatine- and Fe2+-concentrations and different pH was evaluated. Increasing the concentration of gelatine from 6 wt% to 10 wt% influenced the diffusion coefficient the most, while reducing the pH from 2.0 to 1.5 had the largest effect on the dose resolution. For a gel consisting of 10 wt% gelatine, 1.0 mM Fe2+ and pH 1.5 the diffusion coefficient was found to be 1.5 mm2 h-1 and the dose resolution was about 4.1% (at 95% confidence level), for a dose of 40 Gy. By evaluating different dose gradients by the gamma-method, the diffusion was shown to have no clinically relevant impact on the dose distribution and plan acceptance within 3 h of irradiation. The results indicate a potential use of Fricke gels for IMRT verification.

An evaluation of the TSE MR sequence for time efficient data acquisition in polymer gel dosimetry of applications involving high doses and steep dose gradients

The use of magnetic resonance imaging as a readout method for polymer gel dosimetry commonly involves long imaging sessions, particularly when high spatial resolution is required in all three dimensions, for the investigation of dose distributions with steep dose gradients and stringent dose delivery specifications. In this work, a volume selective turbo spin echo (TSE) pulse sequence is compared to the established Carr-Purcell-Meiboom-Gill (CPMG) multiecho acquisition with regard to providing accurate dosimetric results in significantly reduced imaging times. Polyethylene glycol diacrylate based (PABIG) gels were irradiated and subsequently scanned to obtain R2 relaxation rate measurements, using a CPMG multiecho sequence and a dual echo TSE utilizing an acceleration (turbo) factor of 64. R2 values, plotted against corresponding Monte Carlo dose calculations, provided calibration data of PABIG gels dose response over a wide dose range. A linear R2 versus dose relationship was demonstrated for both sequences with TSE results presenting reduced dose sensitivity. Although TSE data were found to deviate from linearity at lower doses compared to CPMG data, a relatively wide dynamic dose range of response extending up to approximately 100 Gy was observed for both sequences. The TSE and CPMG sequences were evaluated with a brachytherapy irradiation using a high dose rate 192Ir source and a gamma knife stereotactic radiosurgery irradiation with a single 4 mm collimator helmet shot. Dosimetric results obtained with the TSE and CPMG are shown to compare equally well with the expected dose distributions for these irradiations. The 60-fold scan time reduction achieved with TSE implies that this sequence could prove to be a useful tool for the introduction of polymer gel dosimetry in clinical radiation therapy applications involving high doses and steep dose gradients.