Dose Delivery Errors Research Articles

Dose delivery error from respiratory motion in free breathing left breast irradiation

PurposeMany institutions worldwide currently deliver left breast radiotherapy in free breathing mode, mostly due to the unavailability of a Deep Inspiration Breath Hold technique (DIBH). This study aims at quantifying the error in dose delivery (compared to treatment plan) due to respiratory motion in free breathing irradiation of left breast or chest wall. Since subfields often consist in small, fine-tuned, highly targeted fields, slight intrafractional target motion may compromise their subtle benefit. Thus we analyzed the respiratory motion effect on target dose coverage, dose homogeneity and left lung dose. MethodsTreatment plans for twenty left breast or chest wall cancer patients previously treated at our center were retrieved and retrospectively planned with the introduction of an appropriate shift in isocenter location to simulate free breathing target motion. ResultsNo clinically significant dosimetric changes were found in all twenty cases when breathing motion was accounted for. Changes in target dose coverage (V95%), in target maximum dose (D2%) and in V20Gy lung dose were respectively less than 1.5%, 0.3% and 2.6%. ConclusionThe findings suggest that breast irradiation in free breathing mode does not undermine the dosimetric merits of the field-in-field technique and does not produce clinically significant dosimetric differences in dose delivery for target and lung compared to plan.

Evaluation of the deliverability of dynamic conformal arc therapy (DCAT) by gantry wobble and its influence on dose

We aimed to investigate the deliverability of dynamic conformal arc therapy (DCAT) by gantry wobble owing to the intrinsic inter-segment break of the Elekta linear accelerator (LINAC) and its adverse influence on the dose to the patient. The deliverability of DCAT was evaluated according to the plan parameters, which affect the gantry rotation speed and resultant positional inaccuracies; the deliverability according to the number of control points and dose rates was investigated by using treatment machine log files and dosimetry devices, respectively. A non-negligible degradation in DCAT deliverability due to gantry wobble was observed in both the treatment machine log files and dosimetry devices. The resulting dose-delivery error occurred below a certain number of control points or above a certain dose rate. Dose simulations in the patient domain showed a similar impact on deteriorated deliverability. For targets located primarily in the isocenter, the dose differences were negligible, whereas for organs at risk located mainly off-isocenter, the dose differences were significant up to − 8.77%. To ensure safe and accurate radiotherapy, optimal plan parameters should be selected, and gantry angle-specific validations should be conducted before treatment.

In Vivo Dosimetry in the Urethra During Prostate Carbon Ion Radiotherapy.

In vivo dosimetry can prevent dose delivery errors by directly measuring the dose of radiation administered to a patient. However, a method for in vivo dosimetry during carbon ion radiotherapy (CIRT) has not been established. Therefore, we investigated data from in vivo dosimetry of the urethra during CIRT for prostate cancer using small spherical diode dosimeters (SSDDs). This study included five patients enrolled in a clinical trial (jRCT identifier: jRCTs032190180) on which the use of four-fraction CIRT for prostate cancer was examined. The urethral dose during CIRT for prostate cancer was measured using the SSDDs inserted into the ureteral catheter. The relative error between the in vivo and calculated doses obtained using the Xio-N treatment planning system was determined. Additionally, a dose-response stability test for the in vivo dosimeter was performed under clinical conditions. The relative error between the in vivo and calculated urethral doses ranged from 6 to 12%. The dose-response stability under clinical conditions of the measured dose was ≤1%. Therefore, an error >1% would be due to an interfractional patient setup error in the large dose gradient in the urethra. The usefulness of in vivo dosimetry using SSDDs in CIRT and SSDDs' potential for detecting dose delivery errors during CIRT is herein highlighted.

A probabilistic deep learning model of inter-fraction anatomical variations in radiotherapy

Objective. In radiotherapy, the internal movement of organs between treatment sessions causes errors in the final radiation dose delivery. To assess the need for adaptation, motion models can be used to simulate dominant motion patterns and assess anatomical robustness before delivery. Traditionally, such models are based on principal component analysis (PCA) and are either patient-specific (requiring several scans per patient) or population-based, applying the same set of deformations to all patients. We present a hybrid approach which, based on population data, allows to predict patient-specific inter-fraction variations for an individual patient. Approach. We propose a deep learning probabilistic framework that generates deformation vector fields warping a patient's planning computed tomography (CT) into possible patient-specific anatomies. This daily anatomy model (DAM) uses few random variables capturing groups of correlated movements. Given a new planning CT, DAM estimates the joint distribution over the variables, with each sample from the distribution corresponding to a different deformation. We train our model using dataset of 312 CT pairs with prostate, bladder, and rectum delineations from 38 prostate cancer patients. For 2 additional patients (22 CTs), we compute the contour overlap between real and generated images, and compare the sampled and ‘ground truth’ distributions of volume and center of mass changes. Results. With a DICE score of 0.86 ± 0.05 and a distance between prostate contours of 1.09 ± 0.93 mm, DAM matches and improves upon previously published PCA-based models, using as few as 8 latent variables. The overlap between distributions further indicates that DAM’s sampled movements match the range and frequency of clinically observed daily changes on repeat CTs. Significance. Conditioned only on planning CT values and organ contours of a new patient without any pre-processing, DAM can accurately deformations seen during following treatment sessions, enabling anatomically robust treatment planning and robustness evaluation against inter-fraction anatomical changes.

Radiotherapy delivery error detection with electronic portal imaging device (EPID)-based in vivo dosimetry.

Radiotherapy delivery error detection with electronic portal imaging device (EPID)-based in vivo dosimetry.

A phamtom study: In vivo rectal dosimetry of high dose rate brachytherapy in cervical cancer

The purpose of this study was to perform in-vivo dosimetry using a diode rectal dosimeter in phantom and compare the dose delivered to the rectum between the dose measured by the diode dosimeter and the dose calculated by the treatment planning system in cervical cancer. The PTW T9112 diode detector calibrations were performed to find the correction factor. Then the calibrated diode detector was used to measure the radiation dose received in the rectum area in the in-house pelvic phantom. An Iridium-192 source was loaded into the phantom with 7 Gy, the measurements were 3 times per treatment plan, with 15 total plans studied. The average cumulative charge (nC) of each plan was converted to the absorbed dose (mGy) for comparison with the treatment planning system. Finally, to test the hypothesis that an absorbed dose from the detector and the treatment planning system were not significantly different, dependent t–test statistical analysis was applied with p-value <0.05. For distance and direction correction factors, we found that the factors were approximately 1 at 5 cm and 180°. The percentage differences of radiation dose between the diode dosimeter and the treatment planning system were between −3.3 and 4.1%. Statistical analysis revealed that the doses from the detector and the treatment planning system were not statistically significant different. The comparison showed that the percent difference between diode dosimeter and treatment planning system was acceptable to perform the in vivo dosimetry in brachytherapy. Therefore, the diode detector may be a suitable candidate for a treatment verification system in cervical cancer brachytherapy to prevent the dose delivery errors that directly affect the prognosis and may cause complications for the patient.

Multi-Institutional Study of End-to-End Dose Delivery Quality Assurance Testing for Image-Guided Brachytherapy Using a Gel Dosimeter

To quantify dose delivery errors for high-dose-rate image-guided brachytherapy (HDR-IGBT) using an independent end-to-end dose delivery quality assurance test at multiple institutions. The novelty of our study is that this is the first multi-institutional end-to-end dose delivery study in the world. The postal audit used a polymer gel dosimeter in a cylindrical acrylic container for the afterloading system. Image acquisition using computed tomography, treatment planning, and irradiation were performed at each institution. Dose distribution comparison between the plan and gel measurement was performed. The percentage of pixels satisfying the absolute-dose gamma criterion was reviewed. Thirty-five institutions participated in this study. The dose uncertainty was 3.6% ± 2.3% (mean ± 1.96σ). The geometric uncertainty with a coverage factor of k = 2 was 3.5 mm. The tolerance level was set to the gamma passing rate of 95% with the agreement criterion of 5% (global)/3 mm, which was determined from the uncertainty estimation. The percentage of pixels satisfying the gamma criterion was 90.4% ± 32.2% (mean ± 1.96σ). Sixty-six percent (23/35) of the institutions passed the verification. Of the institutions that failed the verification, 75% (9/12) had incorrect inputs of the offset between the catheter tip and indexer length in treatment planning and 17% (2/12) had incorrect catheter reconstruction in treatment planning. The methodology should be useful for comprehensively checking the accuracy of HDR-IGBT dose delivery and credentialing clinical studies. The results of our study highlight the high risk of large source positional errors while delivering dose for HDR-IGBT in clinical practices.

Isodoses-aset theory-based patient-specific QA measure to compare planned and delivered isodose distributions in photon radiotherapy.

The gamma index and dose-volume histogram (DVH)-based patient-specific quality assurance (QA) measures commonly applied in radiotherapy planning are unable to simultaneously deliver detailed locations and magnitudes of discrepancy between isodoses of planned and delivered dose distributions. By exploiting statistical classification performance measures such as sensitivity or specificity, compliance between aplanned and delivered isodose may be evaluated locally, both for organs-at-risk (OAR) and the planning target volume (PTV), at any specified isodose level. Thus, apatient-specific QA tool may be developed to supplement those presently available in clinical radiotherapy. Amethod was developed to locally establish and report dose delivery errors in three-dimensional (3D) isodoses of planned (reference) and delivered (evaluated) dose distributions simultaneously as afunction the dose level and of spatial location. At any given isodose level, the total volume of delivered dose containing the reference and the evaluated isodoses is locally decomposed into four subregions: true positive-subregions within both reference and evaluated isodoses, true negative-outside of both of these isodoses, false positive-inside the evaluated isodose but not the reference isodose, and false negatives-inside the reference isodose but not the evaluated isodose. Such subregions may be established over the whole volume of delivered dose. This decomposition allows the construction of aconfusion matrix and calculation of various indices to quantify the discrepancies between the selected planned and delivered isodose distributions, over the complete range of values of dose delivered. The 3D projection and visualization of the spatial distribution of these discrepancies facilitates the application of the developed method in clinical practice. Several clinical photon radiotherapy plans were analyzed using the developed method. In some plans at certain isodose levels, dose delivery errors were found at anatomically significant locations. These errors were not otherwise highlighted-neither by gamma analysis nor by DVH-based QA measures. Aspecially developed 3D projection tool to visualize the spatial distribution of such errors against anatomical features of the patient aids in the proposed analysis of therapy plans. The proposed method is able to spatially locate delivery errors at selected isodose levels and may supplement the presently applied gamma analysis and DVH-based QA measures in patient-specific radiotherapy planning.

Are gamma passing rate and dose-volume histogram QA metrics correlated?

The quality of a measured distribution of dose delivered against its corresponding radiotherapy plan is routinely assessed by gamma index (GI) and dose-volume histogram (DVH) metrics. Any correlation between error detection rates, as based on either of these approaches, while argued, has never been convincingly demonstrated. The dependence of the strength of correlation between the GI passing rate ( ) and DVH quality assurance (QA) metrics on various elements of the therapy plan has not been systematically investigated. A formal analysis of the relation between and DVH metrics has been undertaken, leading to a relationship which may partly approximate with respect to the DVH. This relationship was further validated by studying examples of simulated clinical radiotherapy plans and by studying the correlation between and the derived relationship using a simple two-dimensional representations of the planning target volume (PTV) and organs at risk (OAR), where penumbra regions, distance-to-agreement tolerances and dose delivery errors were systematically varied. It is shown formally that there cannot be any correlation between and other commonly applied DVH-derived QA measures. However, may be partly approximated given the planned and measured DVH. The derived approximation (the " -slope indicator") may be clinically useful in some practical cases of radiotherapy plan QA. In formal terms, there cannot be any correlation between and any common DVH-calculated patient-specific measures, with respect to PTV or OAR. However, as demonstrated analytically and further confirmed in our simulation studies, the approximation derived in this study (the " -slope indicator") may in some cases offer a degree of correlation between and the PTV and OAR DVH QA metrics in measured and planned patient-specific dose distributions-which may be potentially useful in clinical practice.

Dynamic conformal arcs-based single-isocenter VMAT planning technique for radiosurgery of multiple brain metastases

Multiple small beamlets in the delivery of highly modulated single-isocenter HyperArc VMAT plan can lead to dose delivery errors associated with small-field dosimetry, which can be a major concern for stereotactic radiosurgery for multiple brain lesions. Herein, we describe and compare a clinically valuable dynamic conformal arc (DCA)-based VMAT (DCA-VMAT) approach for stereotactic radiosurgery of multiple brain lesions using flattening filter free beams to minimize this effect. Original single-isocenter HyperArc style VMAT and DCA-VMAT plans were created on 7 patients with 2 to 8 brain lesions (total 35 lesions) for 10 MV- flattening filter free beam. 20 Gy was prescribed to each lesion. For identical planning criteria, DCA-VMAT utilizes user-controlled field aperture shaper before VMAT optimization. Plans were evaluated for conformity and target coverage, low- and intermediate dose spillages to brain volume that received more than 30% (V30%) and 50% (V50%) of prescription dose. Additionally, mean brain dose, V8, V12 and maximal dose to adjacent organs-at-risk (OAR) including hippocampi were reported. Total monitor units, beam modulation factor, treatment delivery efficiency, and accuracy were recorded. Comparing with original VMAT, DCA-VMAT plans provided similar tumor dose, target coverage and conformity, yet tighter radio-surgical dose distribution with lower dose to normal brain V30% (p = 0.009), V50% (p = 0.05) and other OAR including lower dose to hippocampi. Lower total number of monitor units and smaller beam modulation factor reduced beam on time by 2.82 min (p < 0.001), on average (maximum up to 3.8 min). Beam delivery accuracy was improved by 8%, on average (p < 0.001) and maximum up to 13% in some cases for DCA-VMAT plans. This novel DCA-VMAT approach provided excellent plan quality, reduced dose to normal brain, and other OAR while significantly reducing beam-on time for radiosurgery of multiple brain lesions–improving patient compliance and clinic workflow. It also provided less MLC modulation through the targets–potentially minimizing small field dosimetry errors as demonstrated by quality assurance results. Incorporating DCA-based VMAT optimization in HyperArc module for radiosurgery of multiple brain lesions merits future investigation.

Dosimetric and mechanical equivalency of Varian TrueBeam linear accelerators.

PurposeTo investigate and improve the level of equivalency of Varian TrueBeam linear accelerators (linacs) in energy‐, dosimetric leaf gap‐ (DLG) and jaw calibration.MethodsEight linacs with four photon energies: 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV, and three electron energies (on two linacs): 6, 9, and 12 MeV were commisioned and beam‐matched. Initially, symmetry of lateral profiles was calibrated for maximum field size. Energy‐matching was then performed for photons by adjusting diagonal profiles at maximum field size and depth of maximum dose to coincide with the reference linac, and for electrons by matching the range at percentage depth of ionization of 90%, 80%, and 50%. Calibration of DLG was performed for 6 MV and evaluated among the linacs. The relationship between DLG and the Gap value was investigated. A method using electronic portal imaging device (EPID) was developed and implemented for jaw calibration.ResultsSymmetry calibration for photons (electrons) was within 1% (0.7%), further improving the vendor's acceptance criteria. Photon and electron energy‐matching was within 0.5% and 0.1 mm, respectively. Calibration of DLG was within 0.032 mm among the linacs and utilizing the relationship between DLG and the Gap value resulted in an empirical calibration method which was implemented to simplify DLG adjustment. Using EPID‐based method of calibration, evaluation of the jaw‐positioning among the linacs for 30 cm × 30 cm field size was within 0.4 mm and in the junction area within 0.2 mm. Dose delivery error of VMAT‐plans were at least 99.2% gamma pass rate (1%, 1 mm).ConclusionsHigh level of equivalency, beyond clinically accepted criteria, of TrueBeam linacs could be achieved which reduced dose delivery systematic errors and increased confidence in interchanging patients among linacs.

Comparison of ICRU 38 rectal reference point dose estimates with measured dose in vivo in cobalt-60 HDR brachytherapy for cervical cancer

The objective of this study was to compare the ICRU 38 calculated rectal dose with in vivo dosimetry measured doses in cobalt-60 HDR brachytherapy for cervical cancer. A total of 48 brachytherapy insertions done on 15 patients treated from January to March 2017 at our institution were included in this prospective cross-sectional study. The results demonstrated no significant difference between the computed ICRU rectal point dose and in vivo maximum measured rectal dose ((r) 0.6208, p < 0.0001 [S]; t test p = 0.1578 [NS] 95% CI − 0.78 to 0.46), but a significant difference between ICRU rectal point and in vivo mean measured rectal dose ((r) 0.6033, p < 0.0001 [S]; t test p < 0.0001 [S] 95% CI − 0.81 to 0.35). These findings were seen even when sub-analyzed for the two used fraction sizes of 7 Gy and 8 Gy. The results also showed no significant differences in the maximum ((r) 0.9029, p < 0.0001 [S]; t test p = 0.2576 [NS], 95% CI − 0.21 to 0.06) and mean ((r) 0.9766, p < 0.0001 [S]; t test p = 0.2786 [NS], 95% CI − 0.93 to 0.03) doses taken from treatment planning system assigned dose points coinciding with the imaged probes of the in vivo dosimeter. Overall, this study was able to provide additional evidence that in vivo dosimetry can be validly used in the clinical setting to estimate the dose to the rectum during Co-60 HDR brachytherapy. Use of this technique allows for an additional quality assurance method that can contribute to reductions of errors in dose delivery.

Use of statistical approaches to improve the quality control of the dose delivery in radiotherapy

The gamma index is a measure used routinely for the quality control of dose delivery in radiotherapy, implemented in commercial systems for the verification of treatment plans. It involves comparison of the difference between planned and delivered doses to a single reference. The same reference value is selected for all points in the plan that can potentially hide dose delivery errors, especially in medium and low dose areas. In this study, a receiver operating characteristic analysis is used to demonstrate the limits of the performance of the global gamma index as a method for detecting dose delivery errors. The performance of a global gamma index is compared with two approaches based on statistical tests for outlier detection. Two statistical approaches are considered: according to the first, the distribution of the delivered doses is estimated based on an appropriate calibration procedure. According to the second, the distribution of the delivered doses is estimated based on the detection of relatively homogeneous regions of a plan and analyzing the distributions of planned doses within these regions. The performance of the three approaches is compared based on analytical considerations and in simulations in which errors are intentionally introduced to the plan delivery and noise related to dose delivery is modeled. We have shown that a statistics-based approach to gamma analysis generally leads to better detection of true delivery errors. The results of analytical consideration coincide with the simulations. In simulations, we observe that both statistical approaches are better detectors of true delivery errors than the global method for the gamma-index passing rate in the range from 0.9–1.0. It is shown that the global gamma index is a weak detector of dose delivery errors, which in some circumstances behaves only slightly better than a purely random classifier.

Interplay effect in lung cancer proton therapy

Pencil beam scanning proton therapy is used increasingly for cancer treatment. Although this technique allows superior dose avoidance to critical organs in comparison to passive scattering and X-rays photon-type radiation therapy, it is more susceptible to changes in the patient’s inherent anatomical motion, especially in the thoracic region. The uncertainties in dose distribution due to motion interplay in this region are significant and cannot be ignored. This paper focuses on the errors in dose delivery when motion interplay is involved, and how we can quantify them practically. With the proper tools for evaluating the magnitude of such errors, the best method to mitigate motion effects can be decided and designed for each individual patient.

Error detection during VMAT delivery using EPID-based 3D transit dosimetry

PurposeTo investigate the effectiveness of an EPID-based 3D transit dosimetry system in detecting deliberately introduced errors during VMAT delivery. MethodsAn Alderson phantom was irradiated using four VMAT treatment plans (one prostate, two head-and-neck and one lung case) in which delivery, thickness and setup errors were introduced. EPID measurements were performed to reconstruct 3D dose distributions of “error” plans, which were compared with “no-error” plans using the mean gamma (γmean), near-maximum gamma (γ1%) and the difference in isocenter dose (ΔDisoc) as metrics. ResultsOut of a total of 42 serious errors, the number of errors detected was 33 (79%), and 27 out of 30 (90%) if setup errors are not included. The system was able to pick up errors of 5 mm movement of a leaf bank, a wrong collimator rotation angle and a wrong photon beam energy. A change in phantom thickness of 1 cm was detected for all cases, while only for the head-and-neck plans a 2 cm horizontal and vertical shift of the phantom were alerted. A single leaf error of 5 mm could be detected for the lung plan only. ConclusionAlthough performed for a limited number of cases and error types, this study shows that EPID-based 3D transit dosimetry is able to detect a number of serious errors in dose delivery, leaf bank position and patient thickness during VMAT delivery. Errors in patient setup and single leaf position can only be detected in specific cases.

Treatment plan complexity does not predict IROC Houston anthropomorphic head and neck phantom performance

Previous works indicate that intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans that are highly complex may produce more errors in dose calculation and treatment delivery. Multiple complexity metrics have been proposed and associated with IMRT QA results, but their relationships with plan performance using in situ dose measurements have not been thoroughly investigated. This study aimed to evaluate the relationships between IMRT treatment plan complexity and anthropomorphic phantom performance in order to assess the extent to which plan complexity is related to dosimetric performance in the IROC phantom credentialing program. Sixteen complexity metrics, including the modulation complexity score (MCS), several modulation indices, and total monitor units (MU) delivered, were evaluated for 343 head and neck phantom irradiations, comprising both IMRT (step-and-shoot and sliding window techniques) and VMAT. Spearman’s correlations were used to explore the relationship between complexity and plan performance, as measured by the dosimetric differences between the treatment planning system (TPS) and thermoluminescent dosimeter (TLD) measurement, as well as film gamma analysis. Relationships were likewise determined for several combinations of subpopulations, based on the linear accelerator model, TPS used, and delivery modality. Evaluation of the complexity metrics presented here yielded no significant relationships (p > 0.01, Bonferroni-corrected) and all correlations were weak (less than ±0.30). These results indicate that complexity metrics have limited predictive utility in assessing plan performance in multi-institutional comparisons of IMRT plans. Other factors affecting plan accuracy, such as dosimetric modeling or multileaf collimator (MLC) performance, should be investigated to determine a more probable cause for dose delivery errors.

Trajectory log file sensitivity: A critical analysis using DVH and EPID

The aim of this study was to investigate the sensitivity of the trajectory log file based quality assurance to detect potential errors such as MLC positioning and gantry positioning by comparing it with EPID measurement using the most commonly used criteria of 3%/3 mm. An in-house program was used to modified plans using information from log files, which can then be used to recalculate a new dose distribution. The recalculated dose volume histograms (DVH) were compared with the originals to assess differences in target and critical organ dose. The dose according to the differences in DVH was also compared with dosimetry from an electronic portal imaging device. In all organs at risk (OARs) and planning target volumes (PTVs), there was a strong positive linear relationship between MLC positioning and dose error, in both IMRT and VMAT plans. However, gantry positioning errors exhibited little impact in VMAT delivery. For the ten clinical cases, no significant correlations were found between gamma passing rates under the criteria of 3%/3 mm for the composite dose and the mean dose error in DVH (r < 0.3, P > 0.05); however, a significant positive correlation was found between the gamma passing rate of 3%/3 mm (%) averaged over all fields and the mean dose error in the DVH of the VMAT plans (r = 0.59, P < 0.001). This study has successfully shown the sensitivity of the trajectory log file to detect the impact of systematic MLC errors and random errors in dose delivery and analyzed the correlation of gamma passing rates with DVH.

P183] Replacement of EPID-based patient-specific QA by treatment analysis software

Purpose It is standard practice to perform QA of IMRT and VMAT plans by delivering the plans to the EPID (Electronic Portal Imaging Device) and comparing the measured dose to the calculated dose by an independent dose calculation engine prior to treatment. However, this procedure is both time consuming and reduces the patient throughput because the whole treatment must be delivered and measured using a clinical accelerator. In addition, noise and drift of the EPID require considerable extra resources to investigate false dose deviations and to perform repetitive EPID measurements. Accordingly, there is a need for a more streamlined QA approach with the same level of patient safety. Methods The Treatment Delivery Tool is an in house-developed C# application that can analyze both Varian Clinac and Varian TrueBeam treatment delivery log files. For each treatment the deviation between the delivered and planned fluence is calculated using the planned/actual MLC positions and the planned/actual MU for each time step in the log file. In addition, the application calculates all key technical parameters of the treatment delivery (e.g. max. MLC deviation, max. MU deviation, and max. MLC speed). Results The Treatment Delivery Tool automatically analyzes all daily treatments of an accelerator within one minute using a standard PC. Over a test period of two months 14 treatments with a significant deviation of the delivered fluence (more than 2% for more than 2% of the fluence area) were found. The deviations are related to a high MLC modulation (small MLC gap) and a large MU deviation. The test period also demonstrates that the Treatment Delivery Tool detects treatment errors with a much higher accuracy than possible with EPID-based QA. Conclusions The Treatment Delivery Tool is an effective and accurate tool for detecting dose delivery errors. In our department the application is complemented by an independent dose calculation by MobiusCalc (Mobius, USA) and a comprehensive accelerator QA program detecting any significant deviation between treatment delivery and data in the log file. This combined approach is very efficient and provides the same level of patient safety as for EPID-based patient-specific QA.

Methodology to reduce 6D patient positional shifts into a 3D linear shift and its verification in frameless stereotactic radiotherapy

The aim of this article is to derive and verify a mathematical formulation for the reduction of the six-dimensional (6D) positional inaccuracies of patients (lateral, longitudinal, vertical, pitch, roll and yaw) to three-dimensional (3D) linear shifts.The formulation was mathematically and experimentally tested and verified for 169 stereotactic radiotherapy patients. The mathematical verification involves the comparison of any (one) of the calculated rotational coordinates with the corresponding value from the 6D shifts obtained by cone beam computed tomography (CBCT). The experimental verification involves three sets of measurements using an ArcCHECK phantom, when (i) the phantom was not moved (neutral position: 0MES), (ii) the position of the phantom shifted by 6D shifts obtained from CBCT (6DMES) from neutral position and (iii) the phantom shifted from its neutral position by 3D shifts reduced from 6D shifts (3DMES). Dose volume histogram and statistical comparisons were made between and .The mathematical verification was performed by a comparison of the calculated and measured yaw (γ°) rotation values, which gave a straight line, Y = 1X with a goodness of fit as R2 = 0.9982. The verification, based on measurements, gave a planning target volume receiving 100% of the dose (V100%) as 99.1 ± 1.9%, 96.3 ± 1.8%, 74.3 ± 1.9% and 72.6 ± 2.8% for the calculated treatment planning system values TPSCAL, 0MES, 3DMES and 6DMES, respectively. The statistical significance (p-values: paired sample t-test) of V100% were found to be 0.03 for the paired sample and 0.01 for .In this paper, a mathematical method to reduce 6D shifts to 3D shifts is presented. The mathematical method is verified by using well-matched values between the measured and calculated γ°. Measurements done on the ArcCHECK phantom also proved that the proposed methodology is correct. The post-correction of the table position condition introduces a minimal spatial dose delivery error in the frameless stereotactic system, using a 6D motion enabled robotic couch.This formulation enables the reduction of 6D positional inaccuracies to 3D linear shifts, and hence allows the treatment of patients with frameless stereotactic radiosurgery by using only a 3D linear motion enabled couch.

A simple method to back-project isocenter dose of radiotherapy treatments using EPID transit dosimetry

The aim of this work was to implement a simple algorithm to evaluate isocenter dose in a phantom using the back-projected transmitted dose acquired using an Electronic Portal Imaging Device (EPID) available in a Varian Trilogy accelerator with two nominal 6 and 10 MV photon beams. This algorithm was developed in MATLAB language, to calibrate EPID measured dose in absolute dose, using a deconvolution process, and to incorporate all scattering and attenuation contributions due to photon interactions with phantom. Modeling process was simplified by using empirical curve adjustments to describe the contribution of scattering and attenuation effects. The implemented algorithm and method were validated employing 19 patient treatment plans with 104 clinical irradiation fields projected on the phantom used. Results for EPID absolute dose calibration by deconvolution have showed percent deviations lower than 1%. Final method validation presented average percent deviations between isocenter doses calculated by back-projection and isocenter doses determined with ionization chamber of 1,86% (SD of 1,00%) and -0,94% (SD of 0,61%) for 6 and 10 MV, respectively. Normalized field by field analysis showed deviations smaller than 2% for 89% of all data for 6 MV beams and 94% for 10 MV beams. It was concluded that the proposed algorithm possesses sufficient accuracy to be used for in vivo dosimetry, being sensitive to detect dose delivery errors bigger than 3-4% for conformal and intensity modulated radiation therapy techniques.