Gamma Agreement Research Articles

Effect of Different Dose Rates on Dosimetric Parameters and Accuracy of the Pretreatment Quality Assurance During Intensity-modulated Radiotherapy Planning Using Anthropomorphic Phantom

Background: This study investigates the impact of varying dose rates on dosimetric parameters and pretreatment quality assurance (QA) accuracy in intensity-modulated radiotherapy (IMRT) planning using anthropomorphic phantoms. The research explores the dosimetric effects of different dose rates ranging from 100 to 600 monitor units per minute (MU/min) on parameters such as Homogeneity Index (HI), Conformity Index (CI), and dose to organs at risk (OAR). Method: Anthropomorphic phantoms, mimicking human tissues, were employed for simulation. Treatment plans were generated using the Eclipse Treatment Planning System, and dosimetric evaluations were conducted using cumulative dose-volume histograms (DVH). Furthermore, pretreatment patient-specific QA was performed using EPID portal dosimetry and Delta4 dosimetry system. Result: Treatment plans adhere to institutional protocol, ensuring the target receives ≥95% of the prescription dose while meeting OAR constraints. All plans maintain target homogeneity and conformality, keeping maximum doses within the target below 107%. Low dose plans exhibit superior target coverage, conformality, and homogeneity compared to higher doses. However, increased dose rates elevate delivered Monitor Units and maximum doses to critical structures. Gamma passing rates vary with dose rates, with the lowest at 100 MU/min and the highest at 400 MU/min for 1% 1mm criteria. Dosimetric evaluations using Delta 4 and EPID QA methods confirm plan validity across different dose rates. Conclusion: Low dose rate dosimetry is superior to higher rates for target and organ-at-risk (OAR) delivery, albeit prolonging delivery time and affecting internal organ motion. Portal dosimetry offers a faster, more convenient tool for IMRT pretreatment quality assurance (QA). Optimal results are achieved at 400 MU/min, enhancing gamma agreement between calculated and measured portal doses for complex fields. This improvement in QA enhances IMRT treatment delivery quality. However, patient-specific QA using the DELTA4 dosimetry system and ICRU 83 recommendations are insufficient for discussing dosimetric differences relevant to patient treatment.

Dosimetric impact of CT metal artifact reduction for spinal implants in stereotactic body radiotherapy planning.

Metal artifacts due to spinal implants may affect the accuracy of dose calculation for radiotherapy. However, the dosimetric impact of metal artifact reduction (MAR) for spinal implants in stereotactic body radiotherapy (SBRT) plans has not been well studied. The objective of this study was to evaluate the dosimetric impact of MAR in spinal SBRT planning with three clinically common dose calculation algorithms. Gammex phantom and 10 patients' computed tomography (CT) images were studied to investigate the effects of titanium implants. A commercial orthopedic MAR algorithm was employed to reduce artifacts. Dose calculations for SBRT were conducted on both artifact-corrected and uncorrected images using three commercial algorithms [analytical anisotropic algorithm (AAA), Acuros XB (AXB), and Monte Carlo (MC)]. Dose discrepancies between artifact-corrected and uncorrected cases were appraised using a dose-volume histogram (DVH) and 3-dimensional (3D) gamma analysis with different distance to agreement (DTA) and dose difference criteria. The gamma agreement index (GAI) was denoted as G(∆D, DTA). Statistical analysis of t-test was utilized to evaluate the dose differences of different algorithms. The phantom study demonstrated that titanium metal artifacts can be effectively reduced. The patient cases study showed that dose differences between the artifact-corrected and uncorrected datasets were small evaluated by gamma index and DVH. Gamma analysis found that even the strict criterion local G(1,1) had average values ≥93.9% for the three algorithms. For all DVH metrics, average differences did not exceed 0.7% in planning target volume (PTV) and 2.1% in planning risk volume of spinal cord (PRV-SC). Statistical analysis showed that the observed dose differences of MC method were significantly larger than those of AAA (P<0.01 for D98% of PTV and P<0.001 for D0.1cc of spinal cord) and AXB methods (P<0.001 for D98% and P<0.0001 for D0.1cc). Dosimetric impact of artifacts caused by titanium implants is not significant in spinal SBRT planning, which indicates that dose calculation algorithms might not be very sensitive to CT number variation caused by titanium inserts.

Evaluation of plan complexity and dosimetric plan quality of total marrow and lymphoid irradiation using volumetric modulated arc therapy

To assess the impact of the planner's experience and optimization algorithm on the plan quality and complexity of total marrow and lymphoid irradiation (TMLI) delivered by means of volumetric modulated arc therapy (VMAT) over 2010-2022 at our institute. Eighty-two consecutive TMLI plans were considered. Three complexity indices were computed to characterize the plans in terms of leaf gap size, irregularity of beam apertures, and modulation complexity. Dosimetric points of the target volume (D2%) and organs at risk (OAR) (Dmean) were automatically extracted to combine them with plan complexity and obtain a global quality score (GQS). The analysis was stratified based on the different optimization algorithms used over the years, including a knowledge-based (KB) model. Patient-specific quality assurance (QA) using Portal Dosimetry was performed retrospectively, and the gamma agreement index (GAI) was investigated in conjunction with plan complexity. Plan complexity significantly reduced over the years (r=-0.50, p<0.01). Significant differences in plan complexity and plan dosimetric quality among the different algorithms were observed. Moreover, the KB model allowed to achieve significantly better dosimetric results to the OARs. The plan quality remained similar or even improved during the years and when moving to a newer algorithm, with GQS increasing from 0.019±0.002 to 0.025±0.003 (p<0.01). The significant correlation between GQS and time (r=0.33, p=0.01) indicated that the planner's experience was relevant to improve the plan quality of TMLI plans. Significant correlations between the GAI and the complexity metrics (r=-0.71, p<0.01) were also found. Both the planner's experience and algorithm version are crucial to achieve an optimal plan quality in TMLI plans. Thus, the impact of the optimization algorithm should be carefully evaluated when a new algorithm is introduced and in system upgrades. Knowledge-based strategies can be useful to increase standardization and improve plan quality of TMLI treatments.

Radiobiological Comparison of Acuros External Beam and Anisotropic Analytical Algorithm on Esophageal Carcinoma Radiotherapy Treatment Plans.

ObjectiveThe present study aimed to investigate the dose differences and radiobiological assessment between Anisotropic Analytical Algorithm (AAA) and Acuros External Beam (AXB) with its 2 calculation models, namely, dose-to-water (AXB-Dw) and dose-to-medium (AXB-Dm), on esophageal carcinoma radiotherapy treatment plans.Materials and methodsThe AXB-Dw and AXB-Dm plans were generated by recalculating the initial 66 AAA plans using the AXB algorithm with the same monitor units and beam parameters as those in the original plan. The dosimetric and radiobiological assessment parameters were calculated for the planning target volume (PTV) and organs at risk (OARs). The gamma agreement for the PTV and the correlation between it and the volume of the air cavity and bone among the different algorithms were compared simultaneously. The dose discrepancy between the theoretical calculation and treatment planning system (TPS) when switching from AXB-Dm to AXB-Dw was analyzed according to the composition of the structures.ResultsThe PTV dose of AXB-Dm plans was significantly smaller than that of the AAA and AXB-Dw plans (P < .05), except for D2. The difference values for AAA vs AXB-Dm (∆Dx,(AAA-AXB,Dm)) and AXB-Dw vs AXB-Dm (∆Dx,(AXB,Dw-AXB,Dm)) were 1.94% [1.27%, 2.64%] and 1.95% [1.56%, 2.27%], respectively. For the spinal cord and heart, there were obvious differences between the AAA vs AXB-Dm (spinal cord: 1.15%, heart: 2.89%) and AXB-Dw vs AXB-Dm (spinal cord: 1.88%, heart: 3.25%) plans. For the lung, the differences between AAA vs AXB-Dm and AAA vs AXB-Dw were significantly larger than those of AXB-Dm vs AXB-Dw. Compared to the case of AAA and AXB-Dw, the decrease in biologically effective dose (BED10, ) of AXB-Dm due to dose non-uniformity exceeded 6.5%, even for a small . The average values of equivalent uniform dose in the AAA, AXB-Dw, and AXB-Dm plans were 52.03±.39 Gy, 52.24 ± .81 Gy, and 51.13 ± .47 Gy, respectively. The tumor control probability (TCP) results for PTV in the AAA, AXB-Dw, and AXB-Dm plans were 62.29 ± 1.57%, 62.82 ± 1.69%, and 58.68±1.88%, respectively. With the 2%/2 mm and 3%/3 mm acceptance criteria, the mean values of , , and were 87.24, 63.3, and 64.81% vs 97.86, 91.77, and 89.25%, respectively. The dose discrepancy between the theoretical calculation and TPS when switching from AXB-Dm to AXB-Dw was approximately 1.63%.ConclusionsThe AAA and AXB-Dw algorithms overestimated the radiobiological parameters when the tumor particularly consisted of nonuniform tissues. A relatively small dose difference could cause a significant reduction in the corresponding TCP. Dose distribution algorithms should be carefully chosen by physicists and oncologists to improve tumor control, as well as to optimize OARs protection.

Validation of an MRI-only planning workflow for definitive pelvic radiotherapy

PurposePrevious work on Magnetic Resonance Imaging (MRI) only planning has been applied to limited treatment regions with a focus on male anatomy. This research aimed to validate the use of a hybrid multi-atlas synthetic computed tomography (sCT) generation technique from a MRI, using a female and male atlas, for MRI only radiation therapy treatment planning of rectum, anal canal, cervix and endometrial malignancies.Patients and methodsForty patients receiving radiation treatment for a range of pelvic malignancies, were separated into male (n = 20) and female (n = 20) cohorts for the creation of gender specific atlases. A multi-atlas local weighted voting method was used to generate a sCT from a T1-weighted VIBE DIXON MRI sequence. The original treatment plans were copied from the CT scan to the corresponding sCT for dosimetric validation.ResultsThe median percentage dose difference between the treatment plan on the CT and sCT at the ICRU reference point for the male cohort was − 0.4% (IQR of 0 to − 0.6), and − 0.3% (IQR of 0 to − 0.6) for the female cohort. The mean gamma agreement for both cohorts was > 99% for criteria of 3%/2 mm and 2%/2 mm. With dose criteria of 1%/1 mm, the pass rate was higher for the male cohort at 96.3% than the female cohort at 93.4%. MRI to sCT anatomical agreement for bone and body delineated contours was assessed, with a resulting Dice score of 0.91 ± 0.2 (mean ± 1 SD) and 0.97 ± 0.0 for the male cohort respectively; and 0.96 ± 0.0 and 0.98 ± 0.0 for the female cohort respectively. The mean absolute error in Hounsfield units (HUs) within the entire body for the male and female cohorts was 59.1 HU ± 7.2 HU and 53.3 HU ± 8.9 HU respectively.ConclusionsA multi-atlas based method for sCT generation can be applied to a standard T1-weighted MRI sequence for male and female pelvic patients. The implications of this study support MRI only planning being applied more broadly for both male and female pelvic sites.Trial registration This trial was registered in the Australian New Zealand Clinical Trials Registry (ANZCTR) (www.anzctr.org.au) on 04/10/2017. Trial identifier ACTRN12617001406392.

OD72 - Gamma agreement index test for dose distribution comparisons in selective internal radiation therapy (SIRT) procedures with 90Y

OD72 - Gamma agreement index test for dose distribution comparisons in selective internal radiation therapy (SIRT) procedures with 90Y

A Validation Method for EPID In Vivo Dosimetry Algorithms

The aim of this study was to develop and apply an evaluation method for assessing the accuracy of a novel 3D EPID back-projection algorithm for in vivo dosimetry. The novel algorithm of Dosimetry Check (DC) 5.8 was evaluated. A slab phantom homogeneously filled, or with air and bone inserts, was used for fluence reconstruction of different squared fields. VMAT plans in different anatomical sites were delivered on an anthropomorphic phantom. Dose distributions were measured with radiochromic films. The 2D Gamma Agreement Index (GAI) between the DC and the film dose distributions (3%, 3 mm) was computed for assessing the accuracy of the algorithm. GAIs between films and TPS and between DC and TPS were also computed. The fluence reconstruction accuracy was within 2% for all squared fields in the three slabs’ configurations. The GAI between the DC and the film was 92.7% in the prostate, 92.9% in the lung, 96.6% in the head and the neck, and 94.6% in the brain. An evaluation method for assessing the accuracy of a novel EPID algorithm was developed. The DC algorithm was shown to be able to accurately reconstruct doses in all anatomic sites, including the lung. The methodology described in the present study can be applied to any EPID back-projection in vivo algorithm.

Clinical implementation of 3D in vivo dosimetry for abdominal and pelvic stereotactic treatments

PurposeTo analyze results from three years of in vivo transit EPID dosimetry of abdominal and pelvic stereotactic radiotherapy and to establish tolerance levels for routine clinical use. Material80 stereotactic VMAT treatments (152 fractions) targeting the abdomen or pelvis were analyzed. In vivo 3D doses were reconstructed with an EPID commercial algorithm. Gamma Agreement Index (GAI) and DVH differences in Planning Target Volume (PTV) and Clinical Target Volume (CTV) were evaluated. Initial tolerance level was set to GAI > 85% in PTV. Fractions Over Tolerance Level (OTL) were deemed to be due to set-up errors, incorrect use of immobilization devices, 4D errors, transit EPID algorithm errors and unknown/unidentified errors. Statistical Process Control (SPC) was applied to determine local tolerance levels. ResultsAverage GAI were (82.7 ± 20.9) % in PTV and (72.9 ± 29.7) % in CTV. 37.8% of fractions resulted OTL and were classified as: set-up errors (3.3%), incorrect use of immobilization devices (2.1%), 4D errors (2.1%), EPID transit algorithm errors (17.1%). OTL causes for the remaining 13.2% of fractions were not identified. The differences between PTV and CTV measured in vivo and calculated mean dose (average difference ± standard deviation) were (−3.3% ± 3.2%) and (−2.3% ± 3.0%). When tolerance levels based on SPC to PTV mean dose differences were applied, the percentage of OTL decreased to 7% and no EPID algorithm error occurred. One error was not identified. ConclusionsThe application of local tolerance levels to EPID in vivo dosimetry proved to be useful for detecting extra-lung SBRT treatment errors.

Commissioning of Clinac IX Trilogy Linear Accelerator for Stereotactic Radiosurgery

Comprehensive quality assurance (QA) and beam data acquisition has been performed for commissioning of Varian Trilogy linear accelerator for stereotactic radiosurgery (SRS). Mechanical, electrical and radiological tests were performed to check linac performance as per American Association of Physicists in Medicine (AAPM) Task Group (TG) 142. The geometric isocentre accuracy, congruence check and calibration of kV and MV imager were performed using the Isocal QA device. The off axis ratio profiles, depth dose data and beam output factors of 6 MV SRS beam for the field sizes ranging from 2 × 2 cm2 to 15 × 15 cm2 were measured using SFD diode detector and cc13 semiflex ion chamber. The multileaf collimator (MLC) characterization was done in the treatment planning system (TPS) using the parameters measured with the CC13 semiflex ionisation chamber in water phantom. Dose measured with 2D ion chamber array was compared to that calculated in TPS. The maximum change in MV and kV imager isocentre was within ±0.03 cm. The variation between SFD measured and ion chamber measured beam data is in positive correlation for fields sizes down up to 2 × 2 cm2. Variation in output factors is ±0.56 ± 0.05 %. The measured beam data is imported and calculated in the Eclipse TPS and is found within 1% gamma agreement index (GAI) with the Analytical Anisotropic Algorithm (AAA) model data. GAI for TPS verification QA is 97.8%, 98.5% and 98.3% respectively for fix jaw 2 × 2 cm2, 5 × 5 cm2 and rapid arc SRS plan. Successful modelling of the photon beam for high dose rate SRS mode is achieved for clinical treatment of patients.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

BackgroundPRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).Materials and methodsA 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm2), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.ResultsStatic output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.ConclusionsMC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

An investigation of a novel reusable radiochromic sheet for 2D dose measurement.

Radiochromic film remains a useful and versatile clinical dosimetry tool. Current film options are single use. Here, we introduce a novel prototype two-dimensional (2D) radiochromic sheet, which optically clears naturally at room temperature after irradiation and can be reused. We evaluate the sheets for potential as a 2D dosimeter and as a radiochromic bolus with capability for dose measurement. A novel derivative of reusable Presage® was manufactured into thin sheets of 5mm thickness. The sheets contained 2% cumin-leucomalachitegreen-diethylamine (LMG-DEA) and plasticizer (up to 25% by weight). Irradiation experiments were performed to characterize the response to megavoltage radiation, including dose sensitivity, temporal decay rate, consistency of repeat irradiations, intra and inter-sheet reproducibility, multi-modality response (electrons and photons), and temperature sensitivity (22°C to 36°C). The local change in optical-density (ΔOD), before and after radiation, was obtained with a flat-bed film scanner and extracting the red channel. Repeat scanning enabled investigation of the temporal decay of ΔOD. Additional studies investigated clinical utility of the sheets through application to IMRT treatment plans (prostate and a TG119 commissioning plan), and a chest wall electron boost treatment. In the latter test, the sheet performed as a radiochromic bolus. The radiation induced OD change in the sheets was found to be proportional to dose and to exponentially decay to baseline in ~24h (R2=0.9986). The sheet could be reused and had similar sensitivity (within 1% after the first irradiation) for at least eight irradiations. Importantly, no memory of previous irradiations was observed within measurement uncertainty. The consistency of dose response from photons (6 and 15MV) and electrons (6-20MeV) was found to be within calibration uncertainty (~1%). The dose sensitivity of the sheets had a temperature dependence of 0.0012 ΔOD/°C. For the short (1min) single field IMRT QA verification, good agreement was observed between the Presage sheet and EBT film (gamma pass rate 97% at 3% 3mm dose-difference and distance-to-agreement tolerance, with a 10% threshold). For the longer (~13min) TG-119 9-field IMRT verification the gamma agreement was lower at 93% pass rate at 5% 3mm, 10% threshold, when compared with Eclipse. The lower rate is attributed to uncertainty arising from signal decay during irradiation and indicates a current limitation. For the electron cutout treatment, both Presage and EBT agreed well (within 2% RMS difference) but differed from the Eclipse treatment plan (~7% RMS difference) indicating some limitations to the Eclipse modeling in this case. The worst case estimates of uncertainty introduced by the signal decay for deliveries of 2, 5, and 10min are 0.6%, 1.4%, and 2.8% respectively. Reusable Presage sheets show promise for 2D dose measurement and as a radiochromic bolus for in vivo dose measurement. The current prototype is suitable for deliveries of length up to 5min, where the uncertainty introduced by signal decay is anticipated to be ~1% (worst case 1.4%), or for longer deliveries where there is no temporal modulation (e.g. physical compensators, or open beams). Additionally, spatial resolution is limited by sheet thickness and scanner resolution, resulting in a practical resolution of 0.8mm.

Validation of the RayStation Monte Carlo dose calculation algorithm using realistic animal tissue phantoms.

PurposeThe aim of this study is to validate the RayStation Monte Carlo (MC) dose algorithm using animal tissue neck phantoms and a water breast phantom.MethodsThree anthropomorphic phantoms were used in a clinical setting to test the RayStation MC dose algorithm. We used two real animal necks that were cut to a workable shape while frozen and then thawed before being CT scanned. Secondly, we made a patient breast phantom using a breast prosthesis filled with water and placed on a flat surface. Dose distributions in the animal and breast phantoms were measured using the MatriXX PT device.ResultsThe measured doses to the neck and breast phantoms compared exceptionally well with doses calculated by the analytical pencil beam (APB) and MC algorithms. The comparisons between APB and MC dose calculations and MatriXX PT measurements yielded an average depth difference for best gamma agreement of <1 mm for the neck phantoms. For the breast phantom better average gamma pass rates between measured and calculated dose distributions were observed for the MC than for the APB algorithms.ConclusionsThe MC dose calculations are more accurate than the APB calculations for the static phantoms conditions we evaluated, especially in areas where significant inhomogeneous interfaces are traversed by the beam.

Kv Intrafraction Verification for SBRT Amplitude-Gated Rapidarc Treatments: An Initial Experience

To report the workflow and the quality assurance program established for the first kV-IGRT intrafraction verification gated RapidArc (RA) treatment successfully delivered in our institution. One patient with stage I NSCLC was treated using the gating technique on a medical linear accelerator. Prescription dose was 48 Gy in four fractions and one fiducial marker was implanted near the tumor. During simulation a gated CT in a Stable Exhale (SE) phase (breath hold pause = 20 sec) was acquired. A 6 MV FFF plan was generated on the gated CT using four partial arcs; the fiducial and its 3 mm PRV were delineated in the planning CT. During each fraction a gating amplitude window of 2.5 mm was set, a gated CBCT on the SE was acquired and positioning was performed according to a tumor fusion. Two orthogonal kV images were then collected to verify the position of the marker. Triggered kV images were acquired immediately before beam-on to verify the intrafraction fiducial position during treatment and to perform an a posteriori evaluation of the marker displacements in the superior-inferior (SI) direction with respect to DRR images. Pretreatment quality assurance was performed using a Delta4 phantom (static delivery) and with EBT3 Gafchromic films coupled with a Quasar moving phantom (gated delivery) using three breathing pattern (BP): 1) FB: free BP of the patient, 2) EX1: SE obtained by modifying the FB pattern and 3) EX2: a random SE. A film irradiated in static delivery was taken as a reference to evaluate the gating deliveries and compared with TPS dose as additional static QA. Beam-on time was 1.8 ± 0.5 min per arc and 7.2 ± 1.4 min per fraction. The duty cycle averaged over all arcs was 42 ± 3.9 % and 56 ± 17 kV triggered images per fraction were acquired. The SI distance between the marker and its expected position was 1.7 ± 0.4 mm on average and 2.9 ± 0.7 mm at 95th percentile (range: 0 - 4.2 mm). During treatment the fiducial was always displayed inside its PRV, except during the delivery of arc 3 in fraction 3. This is in agreement with shifts evaluation, where we found an average displacement of 3.6 ± 0.3 mm. 2%/2mm gamma agreement for the static delivery was 97.8% and 100% for the Delta4 phantom and films, respectively. The gated deliveries were dosimetrically equivalent to the static procedure, with dose differences at isocenter below 1% and gamma pass-rates above 98.2%. Good agreement was also found with TPS (gamma scores > 98.9%). The system capability to superimpose marker PRV on real-time kV images made the clinicians confident during the delivery and the evaluation of the fiducial shifts has proven to be consistent to what was observed during the treatment. A 3 mm margin was required to account for 95% of the gating intrafraction uncertainties. Gated treatments were delivered to a moving phantom and no significant differences were found with respect to a static delivery. The use of a small gate coupled with a SE BP allows delivering acceptable dose distributions with reasonable duty cycles.

Extended field radiotherapy measurements in a single shot using a BaFBr-based OSL-film

This work evaluated the use of a class solution specific calibration for an extra-large BaFBr-based optically stimulated luminescence film (OSL; 43 × 35 cm2; Zeff = 4.55). The clinical need for such large dosimeters follows from the increased use of extended-field radiation therapy (EFRT). E.g. for prostate cancer EFRT is currently used in the first prospective trial investigating the benefit of adding elective irradiation of the para-aortic lymph nodes in pN1 prostate cancer. The full extent of these EFRT dose distributions is not covered by the well-established standard sized radiochromic film or 2D detector arrays. Here we investigate an OSL calibration methodology, that tackles BaFBr-based OSL’s inherent energy dependence by a class solution specific calibration.10 EFRT treatment plans used in the PART trial were investigated. One plan was used to build a class solution specific bilinear calibration model, that distinguishes between in-field and penumbra dose contributions. The effect of this calibration was evaluated with respect to a standard linear calibration, using standard IMRT patterns, the nine remaining patient plans, and to smaller prostate treatment plans.A single OSL-dosimeter could be reused for all measurements. The dosimeter captured the full extent of the dose distributions (maximum EFRT field size = 33.5 cm). The bilinear correction reduced the residual dose differences from above 10% to an average of 0.7% (max 3.6%) in comparison with a Monte Carlo simulation. Consequently global gamma agreement scores (3%-3 mm) of 95.5% ± 2.7% were reached. A more strict local evaluation resulted in an average gamma-agreement score of 93.3% ± 3.2%.The BaFBr-based OSL film, with reduced Zeff requires a class-solution specific correction. The current work shows that such a correction can be as simple as a bilinear residual dose correction driven by the measured signal. As far as we know this is the first 2D dosimeter combining reusability, a sub-mm resolution, and a size covering the typical EFRT treatment plans.

Feasibility of MR-only radiation planning for hypofractionated stereotactic radiotherapy of schwannomas using non-coplanar volumetric modulated arc therapy.

To compare the dose calculation accuracy of plans done on a CT density-assigned MR image set for hypofractionated stereotactic radiotherapy (HSRT) using volumetric modulated radiation therapy containing non-coplanar beams. Eighteen patients diagnosed with schwannoma treated with HSRT were selected retrospectively. These patients underwent planning CT (pCT) for radiation therapy (RT) and contrast-enhanced three-dimensional fast-spoiled gradient-echo image (3D FSPGR) to assist tumor delineation. CTplan is plan done on pCT. The structures body, bone, and air are contoured exclusively on MR image and assigned Hounsfield units of 25, + 1000, and - 1000, respectively. This is termed as MRCT. After registration, original plans from pCT are pasted on the MRCT. Dose calculation is done in two ways: (1) with preset MU values (DDC) and (2) with optimization (OPT_DC). Conformity indices and Dmax and D0.5cc of brainstem, gamma agreement index and correlation coefficient are analyzed. ANOVA test is carried out to find the significance of difference between plans. The mean deviations of Dmax and D0.5cc of brainstem for CTplan versus DDC are 2.49% and 1.45% respectively. The mean deviations of Dmax and D0.5cc of brainstem for CTplan versus OPT_DC are - 1.56% and - 1.97%, respectively. Mean deviations of conformity index for DDC and OPT_DC are 0.84% and 0.89%, respectively. No significant difference was found with ANOVA test. Results show that there is no difference between plans generated with actual CT data and MRCT data. Thus MR scans could be employed for radiation planning provided the verification image is available. This gives us confidence to reduce treatment margins where image registration process is avoided.

Validation and IMRT/VMAT delivery quality of a preconfigured fast-rotating O-ring linac system.

A fast-rotating O-ring dedicated intensity modulated radiotherapy (IMRT)/volumetric modulated arc therapy (VMAT) delivery system, the Halcyon, is delivered by default with a fully preconfigured photon beam model in the treatment planning system (TPS). This work reports on the validation and achieved IMRT/VMAT delivery quality on the system. Acceptance testing followed the vendor's installation product acceptance and was supplemented with mechanical QA. The dosimetric calibration was performed according to the IAEA TRS-398 code-of-practice, delivering 600 cGy/min at 10 cm depth, a 90 cm source-surface distance, and a 10 × 10 cm² field size. The output factors, multileaf collimator (MLC) transmission and dosimetric leaf gap (DLG) were validated by comparing measurements with the modeled values in the TPS. Validation of IMRT/VMAT was conducted following AAPM reports (MPPG 5.a, TG-119). Next, dose measurements were performed for end-to-end (E2E) checks in heterogeneous anthropomorphic phantoms using radiochromic film in multiple planes and using ionization chambers (IC) point measurements. E2E checks were performed for VMAT (cranial, rectum, spine, and head and neck) and IMRT (lung). Additionally, IROC Houston mailed dosimetry audits were performed for the beam calibration and E2E measurements using a thorax phantom (IMRT) and a head and neck phantom (VMAT). Lastly, extensive patient-specific QA was performed for the first patients of each new indication, 26 in total (nrectum = 2, nspine = 5, nlung = 5, nesophagus = 2, nhead and neck = 7, ncranial = 5), treated on the fast-rotating O-ring linac. The patient-specific QA followed the AAPM TG-218 guidelines and comprised of portal dosimetry, ArcCHECK diode array, radiochromic film dosimetry in a MultiCube phantom, and IC point measurements. The measured output factors showed an agreement <1% for fields ≥3 × 3 cm². Field sizes ≤2 × 2 cm² had a difference of <2%. The measured single-layer MLC transmission was 0.42 ± 0.01% and the measured DLG was 0.27 ± 0.22 mm. The AAPM MPPG 5.a measurements were fully compliant with the guideline criteria. Dose differences larger than 2% were found for the PDD at large depths (>25 cm). TG-119's confidence limits were achieved for the VMAT point dose measurements and for both the IMRT and VMAT radiochromic film measurements. The TG-119 confidence limits were not achieved for IMRT point dose measurements in both the target (5.9%) and the avoidance structure (6.4%). All E2E tests had point differences below 2.3% and gamma agreement scores above 90.6%. The IROC beam calibration audit showed agreement of <1%. The IROC lung IMRT audit and head and neck VMAT audit had results compliant with the IROC Houston's credentialing criteria. All IMRT and VMAT plans selected for patient-specific QA were within the action limits suggested by TG-218. The fast-rotating O-ring linac and its preconfigured TPS are compliant with the international commissioning criteria of AAPM MPPG 5.a and AAPM TG-119. E2E measurements on heterogeneous anthropomorphic phantoms were within clinically acceptable tolerances. IROC Houston's audits satisfied the credentialing criteria. This work comprises the first extensive dataset reporting on the preconfigured fast-rotating O-ring linac.

Characterization of EPID software for VMAT transit dosimetry.

Dosimetry check (DC) is a commercial software that allows reconstruction of 3D dose distributions using transit electronic portal imaging device (EPID) images. In this work, we evaluated the suitability of DC software for volumetric modulated arc therapy (VMAT) transit dosimetry. The volumetric gamma agreement index 3%/3mm between twenty VMAT dose distributions reconstructed by DC and calculated with treatment planning system (TPS) were compared to those obtained using PTW OCTAVIUS®4D to assess DC accuracy in VMAT quality assurance (QA). The sensitivity of DC in detecting VMAT delivery and set-up errors and anatomical variations has been investigated by measuring the variation of the gamma agreement index before and after the introduction of specific errors in four VMAT plans related to different anatomical sites. The influence of dose computation algorithm in presence of density inhomogeneity was also assessed. The assessment of VMAT QA shows agreements with TPS maps comparable to OCTAVIUS® 4D (OCT) in homogeneous phantom (p < 0.001). DC mean gamma agreement index was 94.2%±3.4, versus 95.6%±2.5 of OCT, lower dose threshold was set to 10%. Introduction of deliberate errors resulted in lower gamma agreement index and in 38/56 cases the gamma agreement index was over the detection threshold. The dose computation algorithm of DC is accurate in all anatomical sites except lung. However in lung cases, the aqua vivo approach used in this work reduced the algorithm dependence of DC results. DC accurately reproduced VMAT 3D dose distributions in phantom and is sensitive to detect errors caused by delivery inaccuracy and anatomical variations of patients.

Impact of the Use of Homogeneous and Heterogeneous Phantoms in Pretreatment Verification for Volumetric Modulated Arc Radiotherapy

The purpose of this study was to investigate the impact of quality assurance (QA) phantoms designed with different material to verify the accuracy of dose distribution for volumetric modulated arc radiotherapy (VMAT). Homogeneous and heterogeneous QA phantoms are developed to compare the pretreatment verification according to the different materials made in the phantom for routine VMAT of lung cancer. These phantoms have a quadrature rectangular shape with horizontal and vertical lengths of 300 mm and 315 mm, respectively, and with inserts of various slabs. The homogeneous QA phantom was constructed using only polymethyl methacrylate (ρ = 1.19 g/cm3). The heterogeneous QA phantom was constructed using a ceramic fiber board with a density of 0.3 g/cm3, which is similar to that of the lung in human thorax. Overall, the average gamma agreement values for the homogeneous and the heterogeneous QA phantoms were observed to be higher than 94.9% and 95.4% for two energies and the usual criteria. In addition, this study showed that the gamma agreement values obtained for both the homogeneous and the heterogeneous QA phantom agreed to within 2% for both the 2%/2 mm and the 3%/3 mm criteria. Furthermore, no statistically significant difference was observed in the gamma agreement values obtained using the homogeneous and the heterogeneous QA phantoms for both energies and the usual criteria (p > 0.05). Consequentially, this study found that patient specific QA results were not affected by phantoms made of the different materials.

Validation of the Acuros XB dose calculation algorithm versus Monte Carlo for clinical treatment plans.

The two distinct dose computation paradigms of Boltzmann equation solvers and Monte Carlo simulation both promise in principle maximum accuracy. In practice, clinically acceptable calculation times demand approximations and numerical short-cuts on one hand, and modeling the beam characteristics of a real linear accelerator to the required accuracy on the other. A thorough benchmark of both algorithm types therefore needs to start with beam modeling, and needs to include a number of clinically challenging treatment plans. The Acuros XB (v 13.7, Varian Medical Systems) and SciMoCa (v 1.0, Scientific RT) algorithms were commissioned for the same Varian Clinac accelerator for beam qualities 6 and 15 MV. Beam models were established with water phantom measurements and MLC calibration protocols. In total, 25 patients of five case classes (lung/three-dimensional (3D) conformal, lung/IMRT, head and neck/VMAT, cervix/IMRT, and rectum/VMAT) were randomly selected from the clinical database and computed with both algorithms. Statistics of 3D gamma analysis for various dose/distance-to-agreement (DTA) criteria and differences in selected DVH parameters were analyzed. The percentage of points fulfilling a gamma evaluation was scored as the gamma agreement index (GAI), denoted as G(ΔD, DTA). G(3,3), G(2,2), and G(1,1) were evaluated for the full body, PTV, and selected organs at risk (OARs). For all patients, G(3,3) ≥ 99.9% and G(2,2) > 97% for the body. G(1,1) varied among the patients. However, for all patients, G(1,1) > 70% and G(1,1) > 80% for 68% of the patients. For each patient, the mean dose deviation was ΔD < 1% for the body, PTV, and all evaluated OARs, respectively. In dense bone and at off-axis distance > 10 cm, the Acuros algorithm yielded slightly higher doses. In the first layer of voxels of the patient surface, the calculated doses deviated between the algorithms. However, at the second voxel, good agreement was observed. The differences in D(98%PTV) were <1.9% between the two algorithms and for 76% of the patients, deviations were below 1%. Overall, an outstanding agreement was found between the Boltzmann equation solver and Monte Carlo. High-accuracy dose computation algorithms have matured to a level that their differences are below common experimental detection thresholds for clinical treatment plans. Aside from residual differences which could be traced back to implementation details and fundamental cross-section data, both algorithms arrive at identical dose distributions.

A New Anthropomorphic Pediatric Spine Phantom for Proton Therapy Clinical Trial Credentialing.

To design and evaluate an anthropomorphic spine phantom for use in credentialing proton therapy facilities for clinical trial participation by the Imaging and Radiation Oncology Core Houston QA Center. A phantom was designed to perform an end-to-end audit of the proton spine treatment process, including simulation, dose calculation, and proton treatment delivery. Because plastics that simulate bone in proton beams are unknown, 11 potential materials were tested to identify suitable phantom materials. Once built, preliminary testing using passive scattering and spot scanning treatment plans (including a field junction) were created in-house and delivered 3 times to test reproducibility. The following measured attributes were compared with the calculated values: absolute dose agreement using thermoluminescent dosimeters, planar gamma agreement, distal range, junction match, and right and left profile alignment using radiochromic film. Finally, credentialing results from 10 institutions were also assessed. A suitable bone substitute was identified (Techtron HPV Bearing Grade), which had a measured relative stopping power that agreed within 1.1% of its value calculated by Eclipse. In-house passive scatter testing of the phantom demonstrated that the phantom was suitable for assessing craniospinal irradiation dose delivery. However, the in-house scanning beam results were more mixed, highlighting challenges in treatment delivery. Seven of ten institutions passed the proposed criteria for this phantom, a pass rate consistent with other Imaging and Radiation Oncology phantoms. An anthropomorphic proton spine phantom was developed to evaluate proton therapy delivery. This phantom provides a realistic challenge for centers wishing to participate in proton clinical trials and highlights the need for caution in applying advanced treatments.