Transit Dosimetry Research Articles

Evaluation of Transit Dosimetry Software for Helical Tomotherapy

3D in vivo dosimetry is the only method capable of verifying the entire radiation therapy chain, including a potential machine malfunctioning, data corruption, or changes in patient anatomy. The purpose of this work was to evaluate limits and capabilities of a new transit dosimetry software for use with tomotherapy system by testing on heterogeneous phantom.

Commissioning and initial experience with a commercial software for in vivo volumetric dosimetry

Introduction and purposeDosimetry Check (DC) (Math Resolutions) is a commercial EPID-based dosimetry software, which allows performing pre-treatment and transit dosimetry. DC provides an independent verification of the treatment, being potentially of great interest due to the high benefits of the in vivo volumetric dosimetry, which guarantee the treatment delivery and anatomy constancy. The aim of this work is to study the differences in dose between DC and the Treatment Planning System (TPS) to establish an accuracy level of the system. Material and methodsDC v.3.8 was used along with Varian Clinac iX accelerator equipped with EPID aS1000 and Eclipse v.10.0 with AAA and Acuros XB calculation algorithms. The DC evaluated version is based on a pencil beam calculation algorithm. Various plans were generated over several homogeneous and heterogeneous phantoms. Isocentre point doses and gamma analysis were evaluated. ResultsTotal dose differences at the isocentre between DC and TPS for the studied plans are less than 2%, but single field contributions achieve greater values. In the presence of heterogeneities, the discrepancies can reach up to 15%. In transit mode, DC does not consider properly the couch attenuation, especially when there is an air gap between phantom and couch. ConclusionsThe possibility of this in vivo evaluation and the potentiality of this new system have a very positive impact on improving patient QA. But improvements are required in both calculation algorithm and integration with the record and verify system.

Transit dosimetry in dynamic IMRT with an a-Si EPID.

Using an amorphous silicon (a-Si) EPID for transit dosimetry requires detailed characterization of its dosimetric response in a variety of conditions. In this study, a measurement-based model was developed to calibrate an a-Si EPID response to dose for transit dosimetry by comparison with a reference ionization chamber. The ionization chamber reference depth and the required additional buildup thickness for electronic portal imaging devices (EPID) transit dosimetry were determined. The combined effects of changes in radiation field size, phantom thickness, and the off-axis distance on EPID transit dosimetry were characterized. The effect of scattered radiation on out-of-field response was investigated for different field sizes and phantom thicknesses by evaluation of the differences in image profiles and in-water measured profiles. An algorithm was developed to automatically apply these corrections to EPID images based on the user-specified field size and phantom thickness. The average phantom thickness and an effective field size were used for IMRT fields, and images were acquired in cine mode in the presence of an anthropomorphic phantom. The effective field size was defined as the percentage of the jaw-defined field that was involved during the delivery. Nine head and neck dynamic IMRT fields were tested by comparison with a MatriXX two-dimensional array dosimeter using the Gamma (3%, 3 mm) evaluation. A depth of 1.5 cm was selected as the ionization chamber reference depth. An additional 2.2 mm of copper buildup was added to the EPID. Comparison of EPID and MatriXX dose images for the tested fields showed that using a 10% threshold, the average number of points with Gamma index <1 was 96.5%. The agreement in the out-of field area was shown by selection of a 2% threshold which on average resulted in 94.8% of points with a Gamma index <1. The suggested method is less complicated than previously reported techniques and can be used for all a-Si EPIDs regardless of the manufacturer.

SU-E-T-609: Evaluation of Transit Dosimetry Software Using Heterogeneous Phantom

Purpose: The purpose of this work was to evaluate limits and capabilities in the transit dosimetry software for use with the TomoTherapy system by irradiating a heterogeneous phantom. Methods: Helical TomoTherapy plan was created using CIRS phantom (model 062M) with nine various tissue equivalent inserts (lung inhale 0.2 g/cm3, lung exhale 0.5 g/cm3, adipose 0.96 g/cm3, breast 0.99 g/cm3, water 1.01 g/cm3, muscle 1.06 g/cm3, liver 1.07 g/cm3, trabecular bone 1.16 g/cm3, and dense bone 1.53 g/cm3). Targets were contoured within every insert. The phantom was scanned with a 50 cm field of view and 3 mm slice width. Images were imported into the TomoTherapy TPS. A plan was generated to deliver 20 Gy to every insert (2 Gy per fraction) with a jaw width of 2.5 cm, a pitch of 0.430 and an actual modulation factor of 2.621. After the radiation delivery the planning CT, the RT structure, the RT plan, and the RT dose (DICOM format) as well as the exit detector sinogram were imported into the Dosimetry Check software (Math Resolutions, LLC). The 3D delivered doses were reconstructed from the exit detector data by correcting for phantom and couch attenuation. The resulting dose distribution were compared with the TPS planned dose using gamma index. Results: Using the clinical gamma criteria, 3% and 3 mm, all tissue equivalent inserts had a passing percentage of 100% except for 0.2 g/cm3 and 0.5 g/cm3 density inserts (gamma value of 81.67% and 99.18% respectively). Conclusion: The evaluated transit dosimetry software provides an independent verification of helical TomoTherapy plans giving additional confidence in the treatment delivery, however, an overestimation of the reconstructed dose in low density materials has been revealed. Implementation of Monte Carlo algorithm for exit dose reconstruction should improve dosimetric accuracy in heterogeneous patient tissues. Agreement with Math Resolutions

TU‐C‐BRE‐10: A Streamlined Approach to EPID Transit Dosimetry

Purpose:To investigate the feasibility of a simple and efficient transit dosimetry method using the electronic portal imaging device (EPID) for dose delivery error detection and prevention.Methods:In the proposed method, 2D reference transit images are generated for comparison with online images acquired during treatment. Reference transit images are generated by convolving through‐air EPID measurements of each field with pixel‐specific kernels selected from a library of pre‐calculated Monte Carlo pencil kernels of varying radiological thickness. The kernel used for each pixel is selected based on the calculated radiological thickness of the patient along a line joining the pixel and the virtual source. The accuracy of the technique was evaluated in flat homogeneous and heterogeneous plastic water phantoms, a heterogeneous cylindrical phantom, and an anthropomorphic head phantom. Gamma criteria of 3%/3 mm was used to quantify the accuracy of the technique for the various cases.Results:An average of 99.9% and 99.7% of the points in the comparison between the measured and predicted images passed a 3%/3mm gamma for the homogeneous and heterogeneous plastic water phantoms, respectively. 97.1% of the points passed for the analysis of the heterogeneous cylindrical phantom. For the anthropomorphic head phantom, an average of 97.8% of points passed the 3%/3mm gamma criteria for all field sizes. Failures were observed primarily in areas of drastic thickness or material changes and at the edges of the fields.Conclusion:The data suggest that the proposed transit dosimetry method is a feasible approach to in vivo dose monitoring. Future research efforts could include implementation for more complex fields and sensitivity testing of the method to setup errors and changes in anatomy.Oncology Data Systems provided partial funding support but did not participate in the collection or analysis of data

Monte Carlo simulation of the transit dosimetric response of an a-Si electronic portal imaging device

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) are x-ray detectors frequently used in radiotherapy imaging and dosimetry applications. EPIDs employ a copper plate and gadolinium oxysulfide phosphor screen with an array of a-Si photodiodes to indirectly detect incident radiation. In this study, a previously developed Monte Carlo (MC) model of an a-Si EPID has been extended for transit dosimetry. The GEANT4 MC toolkit was used to integrate an a-Si EPID model with two phantoms and a 6 MV x-ray source. A solid water phantom was used to simulate EPID transmission factors, field size output factors and relative dose profiles and results were compared to experimental measurements. An anthropomorphic head phantom was used to qualitatively compare simulated and measured portal images of humanoid anatomy. Calculated transmission factors and field size output factors agreed to within 2.0% and 1.9% of experimental measurements, respectively. A comparison of calculated and measured relative dose profiles yielded >98% of points passing a gamma analysis with 3%/3 mm criterion for all field sizes. The simulated anthropomorphic head phantom image shows macroscopic anatomical features and qualitatively agrees with the measured image. Results validate the suitability of the MC model for predicting EPID response in transit dosimetry.

PO-0814: Clinical experience in the use of transit dosimetry for breast cancer treatment

PO-0814: Clinical experience in the use of transit dosimetry for breast cancer treatment

Initial Clinical Experience Performing Patient Treatment Verification With an Electronic Portal Imaging Device Transit Dosimeter

To prospectively evaluate a 2-dimensional transit dosimetry algorithm's performance on a patient population and to analyze the issues that would arise in a widespread clinical adoption of transit electronic portal imaging device (EPID) dosimetry. Eleven patients were enrolled on the protocol; 9 completed and were analyzed. Pretreatment intensity modulated radiation therapy (IMRT) patient-specific quality assurance was performed using a stringent local 3%, 3-mm γ criterion to verify that the planned fluence had been appropriately transferred to and delivered by the linear accelerator. Transit dosimetric EPID images were then acquired during treatment and compared offline with predicted transit images using a global 5%, 3-mm γ criterion. There were 288 transit images analyzed. The overall γ pass rate was 89.1%±9.8% (average±1 SD). For the subset of images for which the linear accelerator couch did not interfere with the measurement, the γ pass rate was 95.7%±2.4%. A case study is presented in which the transit dosimetry algorithm was able to identify that a lung patient's bilateral pleural effusion had resolved in the time between the planning CT scan and the treatment. The EPID transit dosimetry algorithm under consideration, previously described and verified in a phantom study, is feasible for use in treatment delivery verification for real patients. Two-dimensional EPID transit dosimetry can play an important role in indicating when a treatment delivery is inconsistent with the original plan.

A new approach for the pixel map sensitivity (PMS) evaluation of an electronic portal imaging device (EPID)

When using an electronic portal imaging device (EPID) for dosimetric verifications, the calibration of the sensitive area is of paramount importance. Two calibration methods are generally adopted: one, empirical, based on an external reference dosimeter or on multiple narrow beam irradiations, and one based on the EPID response simulation. In this paper we present an alternative approach based on an intercalibration procedure, independent from external dosimeters and from simulations, and is quick and easy to perform. Each element of a detector matrix is characterized by a different gain; the aim of the calibration procedure is to relate the gain of each element to a reference one. The method that we used to compute the relative gains is based on recursive acquisitions with the EPID placed in different positions, assuming a constant fluence of the beam for subsequent deliveries. By applying an established procedure and analysis algorithm, the EPID calibration was repeated in several working conditions. Data show that both the photons energy and the presence of a medium between the source and the detector affect the calibration coefficients less than 1%. The calibration coefficients were then applied to the acquired images, comparing the EPID dose images with films. Measurements were performed with open field, placing the film at the level of the EPID. The standard deviation of the distribution of the point‐to‐point difference is 0.6%. An approach of this type for the EPID calibration has many advantages with respect to the standard methods — it does not need an external dosimeter, it is not related to the irradiation techniques, and it is easy to implement in the clinical practice. Moreover, it can be applied in case of transit or nontransit dosimetry, solving the problem of the EPID calibration independently from the dose reconstruction method.PACS number: 87.56.‐v

Évaluation et mise en œuvre de la dosimétrie in vivo de transmission par imageurs portals

PurposeIn vivo dosimetry is now widely recommended to avoid major treatment error. Transit dosimetry using portal imagers allows fast and accurate in vivo dose verifications. Several teams have published clinical studies but no recommendation has been proposed to define tolerance levels and validation criteria. This study proposes a simple methodology to assess the overall standard deviation of transit dosimetry and was applied to our transit dosimetry method. Material and methodsIn a first step, the uncertainties due to the dose reconstruction method are evaluated. Their estimation is based on a set of geometries, representative of clinical situations for which 45 points of measurement have been defined. In a second step, we studied the variations of our method in clinical situations. During the treatment session of the patient, the dose was reconstructed and the differences between reconstructed dose and prescribed dose were used to define a realistic tolerance level, adapted to the clinical routine. Then, a methodology is proposed to determine if the transit dosimetry method, with the defined tolerance level allows detecting significant treatment errors (>5% of the prescribed dose). Results – conclusionApplying this methodology we concluded that a tolerance level of 6.5% (k=2) can be associated with our method. With this value, it is demonstrated that in many cases differences of 5% (or less) on the prescribed dose can be detected. This study demonstrates clearly that in vivo transit dosimetry is not able to detect all the treatment errors but remains an ultimate and efficient tool in many situations.

SU‐E‐J‐44: Imaging and Radiation Therapy: GATE Monte Carlo Simulation of a MV‐CBCT Flat Panel with Specific Application in Head and Neck Cancer

Purpose: In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the “electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods: The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. As a second step, 2D and 3D images simulations were performed according to clinical protocols: projections over a 200° arc, 1° increments, and 41cm square field of view. IMRT QA and, RANDO anthropomorphic phantom and head‐and‐neck patient CT were used as input objects for simulations. Results: From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion: We have developed within the GATE platform a complete and detailed model of a 160MLC LINAC with its MVCBCT flat panel. Future work will consist in simulating epid‐based transit dosimetry.

SU‐E‐T‐194: Radiochromic Film Based Transit Dosimetry for Verification of Dose Delivery with Intensity Modulated Radiotherapy

Purpose: To evaluate the transit dose based patient specific quality assurance (QA) of intensity modulated radiation therapy (IMRT) for verification of the accuracy of dose delivered to the patient. Methods: Five IMRT plans were selected and utilized to irradiate a homogeneous plastic water phantom and an inhomogeneous anthropomorphic phantom. The transit dose distribution was measured with radiochromic film and was compared with the computed dose map on the same plane using a gamma index with a 3 % dose and a 3 mm distance‐to‐dose agreement tolerance limit. Results: While the average gamma index for comparisons of dose distributions was less than one for 98.9 % of all pixels from the transit dose with the homogeneous phantom, the passing rate was reduced to 95.0 % for the transit dose with the inhomogeneous phantom. Transit doses due to a 5 mm setup error may cause up to a 50 % failure rate of the gamma index. Conclusion: Transit dose based IMRT QA may be superior to the traditional QA method since the former can show whether the inhomogeneity correction algorithm from TPS is accurate. In addition, transit dose based IMRT QA can be used to verify the accuracy of the dose delivered to the patient during treatment by revealing significant increases in the failure rate of the gamma index resulting from errors in patient positioning during treatment.

Imaging and radiation therapy: Gate Monte Carlo simulation of a MV-CBCT flat panel with specific application in head and neck cancer

Purpose In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the ‘‘electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. Assessments of portal imaging were performed by performing dose calculations in a CQ IMRT phantom with GATE for two measurement points with protocols of 8, 15 and 60 UM and comparing the doses with clinical measurements. And with performing 2D CT projections over a 200° arc, 1° increments, and 41 cm × 41 cm field of view for homogeneous and heterogeneous structures (introduction of a patient head and neck CT patient). Results For the calculation of the deposit dose at the isocentre of the CQ IMRT phantom, respectively GATE gave doses of 5.734 ± 0.169, 11.008 ± 0.777 and 46.006 ± 0.711 cGy compared with measurements are: 6.821, 12,469 and 49.600 cGy protocols for 8, 15 and 60 UM. From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion We have successfully developed within the GATE platform a complete and detailed model of a 160 MLC LINAC with its MV-CBCT flat panel. Future work will consist in simulating epidbased transit dosimetry.

Two-dimensional dosimetric calibration of an electronic portal imaging device (EPID) for transit in vivo dose verification in external radiotherapy

Introduction An algorithm to convert EPID signal (SEPID) into dose in water (DWATER) in transit conditions (i.e. with patient) is proposed and evaluated. This work is a necessary first step to perform patient dose reconstruction during his treatment. Materials and methods Conversion of EPID signal into dose is performed according the 4 following steps: 1.Conversion of (SEPID) into relative dose (DREL) thanks to a relative dose response function (fRD) defined for a reference field size (cREF). 2.Deconvolution of DREL with a field size dependance kernel (Kc). 3.Convolution of the array with a dose to water redistribution kernel (Kw). 4.Use of a transmission function to take into account EPID response variation induced by the modification of the beam spectrum energy in case of transit dosimetry. Development and evaluation of our algorithm were perform using in-house MatlabTMsoftware. On a first part, the dose determined from SEPID was compared to ionization chamber measurements on beam axis for several fields. Then, dose profiles were reconstructed from SEPID and compared with those calculated by our treatment planning system for field size ranging from 3 × 3 cm2 to 20 × 20 cm2. Results The relative dose response function established for cREF = 10 × 10 cm2 was found to be linear. Indeed, fRD can be written as SEPID = a * DWATER + b. Kernels were both modeled as 2D functions dependent on pixel coordinates (x,y). ØKc (x,y) is an exponential function. ØKw (x,y) is a Gaussian function. For all fields, the deviation between the calculated dose and the one measured with the ionization chamber was within ±0.5%. Concerning dose profiles, more than 90% of studied points have a gamma index below than 1 (3%-3 mm). Conclusion All results obtained from our model are in line with experimental measurements. It also confirms that the proposed method will allow a 2D and 3D in vivo dose reconstruction from EPID signal. An algorithm to convert EPID signal (SEPID) into dose in water (DWATER) in transit conditions (i.e. with patient) is proposed and evaluated. This work is a necessary first step to perform patient dose reconstruction during his treatment. Conversion of EPID signal into dose is performed according the 4 following steps: 1.Conversion of (SEPID) into relative dose (DREL) thanks to a relative dose response function (fRD) defined for a reference field size (cREF). 2.Deconvolution of DREL with a field size dependance kernel (Kc). 3.Convolution of the array with a dose to water redistribution kernel (Kw). 4.Use of a transmission function to take into account EPID response variation induced by the modification of the beam spectrum energy in case of transit dosimetry. Development and evaluation of our algorithm were perform using in-house MatlabTMsoftware. On a first part, the dose determined from SEPID was compared to ionization chamber measurements on beam axis for several fields. Then, dose profiles were reconstructed from SEPID and compared with those calculated by our treatment planning system for field size ranging from 3 × 3 cm2 to 20 × 20 cm2. The relative dose response function established for cREF = 10 × 10 cm2 was found to be linear. Indeed, fRD can be written as SEPID = a * DWATER + b. Kernels were both modeled as 2D functions dependent on pixel coordinates (x,y). ØKc (x,y) is an exponential function. ØKw (x,y) is a Gaussian function. For all fields, the deviation between the calculated dose and the one measured with the ionization chamber was within ±0.5%. Concerning dose profiles, more than 90% of studied points have a gamma index below than 1 (3%-3 mm). All results obtained from our model are in line with experimental measurements. It also confirms that the proposed method will allow a 2D and 3D in vivo dose reconstruction from EPID signal.

Evaluation of a new 3D in vivo dosimetry software using portal imaging for VMAT treatments of prostate: Preliminary results

Introduction/Objectives The control of VMAT treatments is a key issue in the Quality Assurance process. Current solutions are often cumbersome to implement and unsuitable for routine use. An innovative solution is proposed by the Dosimetry Check software (Math- Resolutions) allowing both to control pre-treatment as well as socalled transit dosimetry using the portal imager. Here we present the implementation and the evaluation of the software to control VMAT treatments of prostate. Material and methods Acquisition where made on a Clinac 2100C equipped with an IaS500. Images, acquired in continuous mode (@60 frames/rotation), CT scanner and associated structures and 3D dose matrix are imported in Dosimetry Check. Fluence matrix in air is obtained by filtered backprojection. The dose volume in the patient is calculated with Pencil Beam algorithm. Analysis is based on the comparison of point dose, dose profiles, and dose distributions in 2D and 3D. The calculation of Dose Volume Histogram and Gamma Index is also available. Results and conclusion The study focuses on 10 cases of prostate cancer treated in VMAT according to simultaneous integrated boost. The deviation of measured dose at the isocenter in pre-treatment and in transit mode is of the order of −4.0%(±1.3%) and 4.9%(±4.1%) respectively. These results improve significantly when corrected by the acquisition of an integrated image: +1.4%(±0.7%) in pre-treatment and −1.1%(±0.7%) in transit mode. Dosimetry Check software is a suitable tool to do pre-treatment control and in vivo dosimetry. The first results are satisfactory and we wish to test it on other sites, but weaknesses of Pencil Beam algorithm have to be taken into account.

Radiochromic film based transit dosimetry for verification of dose delivery with intensity modulated radiotherapy

To evaluate the transit dose based patient specific quality assurance (QA) of intensity modulated radiation therapy (IMRT) for verification of the accuracy of dose delivered to the patient. Five IMRT plans were selected and utilized to irradiate a homogeneous plastic water phantom and an inhomogeneous anthropomorphic phantom. The transit dose distribution was measured with radiochromic film and was compared with the computed dose map on the same plane using a gamma index with a 3% dose and a 3 mm distance-to-dose agreement tolerance limit. While the average gamma index for comparisons of dose distributions was less than one for 98.9% of all pixels from the transit dose with the homogeneous phantom, the passing rate was reduced to 95.0% for the transit dose with the inhomogeneous phantom. Transit doses due to a 5 mm setup error may cause up to a 50% failure rate of the gamma index. Transit dose based IMRT QA may be superior to the traditional QA method since the former can show whether the inhomogeneity correction algorithm from TPS is accurate. In addition, transit dose based IMRT QA can be used to verify the accuracy of the dose delivered to the patient during treatment by revealing significant increases in the failure rate of the gamma index resulting from errors in patient positioning during treatment.

Fast transit portal dosimetry using density‐scaled layer modeling of aSi‐based electronic portal imaging device and Monte Carlo method

Fast and accurate transit portal dosimetry was investigated by developing a density-scaled layer model of electronic portal imaging device (EPID) and applying it to a clinical environment. The model was developed for fast Monte Carlo dose calculation. The model was validated through comparison with measurements of dose on EPID using first open beams of varying field sizes under a 20-cm-thick flat phantom. After this basic validation, the model was further tested by applying it to transit dosimetry and dose reconstruction that employed our predetermined dose-response-based algorithm developed earlier. The application employed clinical intensity-modulated beams irradiated on a Rando phantom. The clinical beams were obtained through planning on pelvic regions of the Rando phantom simulating prostate and large pelvis intensity modulated radiation therapy. To enhance agreement between calculations and measurements of dose near penumbral regions, convolution conversion of acquired EPID images was alternatively used. In addition, thickness-dependent image-to-dose calibration factors were generated through measurements of image and calculations of dose in EPID through flat phantoms of various thicknesses. The factors were used to convert acquired images in EPID into dose. For open beam measurements, the model showed agreement with measurements in dose difference better than 2% across open fields. For tests with a Rando phantom, the transit dosimetry measurements were compared with forwardly calculated doses in EPID showing gamma pass rates between 90.8% and 98.8% given 4.5 mm distance-to-agreement (DTA) and 3% dose difference (DD) for all individual beams tried in this study. The reconstructed dose in the phantom was compared with forwardly calculated doses showing pass rates between 93.3% and 100% in isocentric perpendicular planes to the beam direction given 3 mm DTA and 3% DD for all beams. On isocentric axial planes, the pass rates varied between 95.8% and 99.9% for all individual beams and they were 98.2% and 99.9% for the composite beams of the small and large pelvis cases, respectively. Three-dimensional gamma pass rates were 99.0% and 96.4% for the small and large pelvis cases, respectively. The layer model of EPID built for Monte Carlo calculations offered fast (less than 1 min) and accurate calculation for transit dosimety and dose reconstruction.

Interfractional trend analysis of dose differences based on 2D transit portal dosimetry

Dose delivery of a radiotherapy treatment can be influenced by a number of factors. It has been demonstrated that the electronic portal imaging device (EPID) is valuable for transit portal dosimetry verification. Patient related dose differences can emerge at any time during treatment and can be categorized in two types: (1) systematic—appearing repeatedly, (2) random—appearing sporadically during treatment. The aim of this study is to investigate how systematic and random information appears in 2D transit dose distributions measured in the EPID plane over the entire course of a treatment and how this information can be used to examine interfractional trends, building toward a methodology to support adaptive radiotherapy. To create a trend overview of the interfractional changes in transit dose, the predicted portal dose for the different beams is compared to a measured portal dose using a γ evaluation. For each beam of the delivered fraction, information is extracted from the γ images to differentiate systematic from random dose delivery errors. From the systematic differences of a fraction for a projected anatomical structures, several metrics are extracted like percentage pixels with |γ| > 1. We demonstrate for four example cases the trends and dose difference causes which can be detected with this method. Two sample prostate cases show the occurrence of a random and systematic difference and identify the organ that causes the difference. In a lung cancer case a trend is shown of a rapidly diminishing atelectasis (lung fluid) during the course of treatment, which was detected with this trend analysis method. The final example is a breast cancer case where we show the influence of set-up differences on the 2D transit dose. A method is presented based on 2D portal transit dosimetry to record dose changes throughout the course of treatment, and to allow trend analysis of dose discrepancies. We show in example cases that this method can identify the causes of dose delivery differences and that treatment adaptation can be triggered as a result. It provides an important element toward informed decision-making for adaptive radiotherapy.

Transit dosimetry in IMRT with an a-Si EPID in direct detection configuration

In this study an amorphous silicon electronic portal imaging device (a-Si EPID) converted to direct detection configuration was investigated as a transit dosimeter for intensity modulated radiation therapy (IMRT). After calibration to dose and correction for a background offset signal, the EPID-measured absolute IMRT transit doses for 29 fields were compared to a MatriXX two-dimensional array of ionization chambers (as reference) using Gamma evaluation (3%, 3 mm). The MatriXX was first evaluated as reference for transit dosimetry. The accuracy of EPID measurements was also investigated by comparison of point dose measurements by an ionization chamber on the central axis with slab and anthropomorphic phantoms in a range of simple to complex fields. The uncertainty in ionization chamber measurements in IMRT fields was also investigated by its displacement from the central axis and comparison with the central axis measurements. Comparison of the absolute doses measured by the EPID and MatriXX with slab phantoms in IMRT fields showed that on average 96.4% and 97.5% of points had a Gamma index<1 in head and neck and prostate fields, respectively. For absolute dose comparisons with anthropomorphic phantoms, the values changed to an average of 93.6%, 93.7% and 94.4% of points with Gamma index<1 in head and neck, brain and prostate fields, respectively. Point doses measured by the EPID and ionization chamber were within 3% difference for all conditions. The deviations introduced in the response of the ionization chamber in IMRT fields were<1%. The direct EPID performance for transit dosimetry showed that it has the potential to perform accurate, efficient and comprehensive in vivo dosimetry for IMRT.

Exit fluence analysis using portal dosimetry in volumetric modulated arc therapy

AimIn measuring exit fluences, there are several sources of deviations which include the changes in the entrance fluence, changes in the detector response and patient orientation or geometry. The purpose of this work is to quantify these sources of errors. BackgroundThe use of the volumetric modulated arc therapy treatment with the help of image guidance in radiotherapy results in high accuracy of delivering complex dose distributions while sparing critical organs. The transit dosimetry has the potential of Verifying dose delivery by the linac, Multileaf collimator positional accuracy and the calculation of dose to a patient or phantom. Materials and methodsThe quantification of errors caused by a machine delivery is done by comparing static and arc picket fence test for 30 days. A RapidArc plan, created for the pelvis site was delivered without and with Rando phantom and exit portal images were acquired. The day to day dose variation were analysed by comparing the daily exit dose images during the course of treatment. The gamma criterion used for analysis is 3% dose difference and 3mm distance to agreement with a threshold of 10% of maximum dose. ResultsThe maximum standard deviation for the static and arc picket fence test fields were 0.19CU and 1.3CU, respectively. The delivery of the RapidArc plans without a phantom shows the maximum standard deviation of 1.85CU and the maximum gamma value of 0.59. The maximum gamma value for the RapidArc plan delivered with the phantom was found to be 1.2. The largest observed fluence deviation during the delivery to patient was 5.7% and the maximum standard deviation was 4.1CU. ConclusionIt is found from this study that the variation due to patient anatomy and interfraction organ motion is significant.