Unflattened Beams Research Articles

Performance of cone-beam computed tomography imaging during megavoltage beam irradiation under phase-gated conditions

PurposeTarget positions should be acquired during beam delivery for accurate lung stereotactic body radiotherapy. We aimed to perform kilovoltage (kV) imaging during beam irradiation (intra-irradiation imaging) under phase-gated conditions and evaluate its performance. MethodsCatphan 504 and QUASAR respiratory motion phantoms were used to evaluate image quality and target detectability, respectively. TrueBeam STx linac and the Developer Mode was used. The imaging parameters were 125 kVp and 1.2 mAs/projection. Flattened megavoltage (MV) X-ray beam energies 6, 10 and 15 MV and un-flattened beam energies 6 and 10 MV were used with field sizes of 5 × 5 and 15 × 15 cm2 and various frame rates for intra-irradiation imaging. In addition, using a QUASAR phantom, intra-irradiation imaging was performed during intensity-modulated plan delivery. The root-mean-square error (RMSE) of the CT-number for the inserted rods, image noise, visual assessment, and contrast-to-noise ratio (CNR) were evaluated. ResultsThe RMSEs of intra-irradiation cone-beam computed tomography (CBCT) images under gated conditions were 50–230 Hounsfield Unit (HU) (static < 30 HU). The noise of the intra-irradiation CBCT images under gated conditions was 15–35 HU, whereas that of the standard CBCT images was 8.8–27.2 HU. Lower frame rates exhibited large RMSEs and noise; however, the iterative reconstruction algorithm (IR) was effective at improving these values. Approximately 7 fps with the IR showed an equivalent CNR of 15 fps without the IR. The target was visible on all the gated intra-irradiation CBCT images. ConclusionSeveral image quality improvements are required; however, intra-irradiated CBCT images showed good visual target detection.

Impact of flattening filter-free beams on remaining volume at risk in lung cancer treatment.

Modern radiotherapy machines offer a new modality, like flattening filter-free beam (FFF), which is used especially in stereotactic body radiation therapy (SBRT) to reduce treatment time. The remaining volume at risk (RVR) is known as undefined normal tissue, and assists in evaluating late effects such as carcinogenesis. This study aimed to compare the effects of flattening and un-flattened beams on RVR in lung cancer treated by conventional doses using volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT). Twenty-three lung cancer patients with a prescribed dose of 60Gy delivered in 30 fractions were selected retrospectively. Four treatment plans were generated for each case (VMAT FF, VMAT FFF, IMRT FF and IMRT FFF). Mean doses to RVR and volumes that received low doses (V15Gy, V10Gy and V5Gy) were introduced as RVR evaluation parameters. Variance percentage comparison between flattening filter (FF) and FFF for the RVR evaluation parameters gave 2.38, 1.10, 1.80 and 2.22 for VMAT, and 1.73, 1.18, 1.62 and 1.81 for IMRT. In contrast, VMAT and IMRT RVR evaluation parameters resulted in variance percentage differences of 10.29, 5.02, -8.84 and -4.82 for FF, and 11.18, 4.96, -8.59 and -4.48for FFF. It is concluded that in terms of RVR evaluation parameters, FFF is clinically beneficial compared to FF for RVR, due to the decrease in mean RVR dose and low-dose irradiated RVR volume. Furthermore, VMAT is preferred in the mean RVR dose and V15Gy, while IMRT is better in V10Gy and V5Gy for RVR.

Optimization of a commercial portal dose image prediction algorithm for pre-treatment verifications of plans using unflattened photon beams.

The aim was to improve the portal dosimetry-based quality assurance results of conventional treatment plans by adjusting the multileaf collimator (MLC) dosimetric leaf gap (DLG) and transmission (T) values of the anisotropic analytic algorithm (AAA) used for portal dose image prediction (PDIP). The AAA-based PDIP v. 16.1 algorithm (PDIP-AAA) of the Eclipse TPS was configured for 6 MV FFF energy. Optimal DLG and T values were achieved for this algorithm by comparing predicted versus measured portal images of the Chair pattern. Twenty clinical plans using 6 MV FFF beams were verified using the optimal PDIP-AAA algorithm and the standard PDIP v. 16 algorithm (PDIP-vE), configured using the van Esch package. The 3% global/2 mm gamma passing rates (GPRs) and average gamma indexes (AGIs) were computed for each acquired image. For each plan, the mean GPR (GPRmean) and mean GAI (GAImean) were compared for both algorithms. A 2-tailed Student t-test (α = 0.05) was used to evaluate whether there was a statistically significant difference. Optimal values of DLG = 0.1 mm and T = 0.01 were found for the PDIP-AAA algorithm, providing significantly better values of GPRmean and AGImean than PDIP-vE (p < 0.001). All plans verified with PIDP-AAA showed GPRmean ≥ 95%. In contrast, only 45% of the plans reported GPRmean ≥ 95% with the PDIP-vE algorithm. The MLC parameters available in the PDIP-AAA model must be tuned to improve the accuracy of the predicted dose image. This work-around is not possible using the standard PDIP algorithm. The adjusted PDIP-AAA resulted in significantly better results than PDIP-vE.

Development of Inclined Applicators for Flattened Beam Intraoperative Radiotherapy

A 3D-printed applicator for intraoperative radiation therapy (IORT) using the INTRABEAM system has been introduced, designed to protect normal tissue, expand indications, and assess its effectiveness in various intraoperative settings. The study involved designing and creating inclined applicators at 0° and 45° angles, which were capable of producing flattened and unflattened beams and shielding non-target areas from radiation. The applicator was evaluated based on dose uniformity, percentage depth dose, leakage, and dose rate. The findings showed that the applicator provided a uniform beam at various depths (1.23 to 1.30 for the flattened beam and 1.25 to 1.32 for the unflattened beam) and effectively shielded against leakage, with no leakage dose measured at 1 mm from the surface. Additionally, the dose rates after 3 min of irradiation for flattened and unflattened beams were 51.0 and 52.9 cGy for 0° and 48.5 and 54.8 cGy for 45° applicators, respectively. This suggests that the applicator can efficiently deliver IORT while minimizing exposure to normal tissues, particularly in treating chest and abdominal lesions near or on critical organs. The applicator has undergone dose calibration and has moved forward to clinical application.

Comparison and estimation of photoneutron dose produce between 10 MV flattened and unflattened beam in Elekta Versa HD™ medical linac.

In a high-energy medical linear accelerator (linac), if the interaction of photon energy is higher than the neutron binding energy of high atomic material, it emits a neutron field through photonuclear (γ, n) reaction. The current study, evaluates the photoneutron dose equivalent (PNDE) produced between the 10 MV flattened and unflattened beams as a function of field sizes in the Elekta Versa HD™ linac. The PNDE produced from Versa linac was recorded along the patient plane using the bubble detector personal neutron dosimeter and from the measured PNDE values, the theoretical PNDE values were simulated for various field sizes using nonlinear least-squares curve-fitting as a function of a polynomial. The percentage of deviation (PoD) and Chi-square (χ2) tests were performed between the measured and simulated PNDE values to study the reliability and validity. The results show that the mean PoD between the measured and simulated PNDE values for respective positions of a field size of FF beam was found to be -1.99% for 0.3×0.3, -4.39% for 5×5, -3.868% for 10×10, 0.590% for 15×15, 9.18% for 20×20, -4.133% for 25×25, and 0.467% for 30×30 cm2. Similarly, the mean PoD between the measured and simulated PNDE values for flattening filter-free (FFF) beam was found to be 1.36% for 0.3×0.3, -1.39% for 5×5, -5.38% for 10×10, 4.41% for 15×15, 3.84% for 20×20, 5.69% for 25×25, and -1.75% for 30×30 cm2. The maximum deviation between the measured and simulated PNDE values lies within the range ± 5%. From the study, it is observed that the FFF beam produces lesser neutron contamination than the FF beam.

Comparative dosimetric analysis of volumetric modulated arc therapy based craniospinal irradiation plans between Halcyon ring gantry and TrueBeam C-arm linear accelerator

This study evaluates the volumetric modulated arc therapy (VMAT) dosimetric comparison between Halcyon ring gantry and TrueBeam c-arm linear accelerators for craniospinal irradiation (CSI) of the neuro-axis. 25 patients, who received treatment for medulloblastoma and primitive neuro-ectodermal tumors between 2018 and 2021, were planned for VMAT in True Beam (TB), and Halcyon (HAL) linear accelerators using 6 MV unflattened (FFF) photon beams (HALFFF and TBFFF). Dose-volume statistics for the target and organs at risk (OARs) and the total number of monitoring units (MUs) in the treatment plans were compared which included dose received by 95% PTV volume (V95%), volume receiving ≥ 107% dose, homogeneity index (HI), conformity index (PI), MU and dose spillage (D10%, D30%, D50%, D70%, D90%). In all 26 OARs were considered of which five were serial and the remaining were parallel structures. For the former, the dose received by 0.2 cm3, volume = D0.2 cm3) were evaluated and for the latter mean dose were evaluated. Both arms were statistically compared with paired sample t-test with a significant value of ≤ 0.05. 11 patients received treatment with the Halcyon and the rest 14 in the TrueBeam C-arm linear accelerator. Patients in the low- and intermediate-risk category (n = 13) received 23.4 Gy in 13 fractions. The remaining patients were in the high-risk category and received 35 Gy in 21 fractions or 36 Gy in 20 fractions. For HALFFF and TBFFF, PTVV95% were 97.5 ± 0.8% and 97.4 ± 0.9% respectively (p = 0.371) while the V107% were 0.6 ± 0.4% and 0.5 ± 0.5 respectively (p = 0.504). However, the number of monitoring units showed statistical significance (p < 0.001) with values of 1331.9 ± 243.4 MU and 1089 ± 206.7 MU respectively for the HAL and TB plans. The differences in spillage dose were also statistically significant, favouring HAL plans at D30% (p = 0.002), D50% (p < 0.001), D70% (p = 0.039), and D90% (p = 0.01) level except for D10% (p = 0.090). Conformity index also showed statistical significance with PI_HAL = 0.9 ± 0.02 and PI_TB = 0.89 ± 0.03 (p = 0.029). For 10 of the 21 parallel structures, the mean dose differences were statistically significant in favouring of HAL plans. Halcyon based VMAT CSI plans are dosimetrically superior in terms of organ dose, especially for the large organs, and offer lower spillage doses than the TrueBeam plans. Plans generated by both linear accelerators are suitable for the patients’ treatments.

Target volume size effect on comparison of dynamic arc treatment plans computed using flattened and unflattened 6MV beams

Introduction/backgroundIn conventional linear accelerators, to obtain flat profiles leading to uniform dose distribution in homogeneous medium, the flattening filter is usually applied on the beam path. In recent years, to obtain higher dose rates, there have been the options of flattening filter free (FFF) beams and it has been noticed that these have many advantages. The aim of this study was to clearly underline the advantages and the drawbacks of flattened filter free (FFF) beams in comparison with the flattening filter (FF) beams for different clinical contexts (planning target volumes locations). MethodsTwo groups (planned with auto-planning VMAT, full and partial arcs) of eight patients each were analyzed: Group I (small planning target volume PTV, with average volume 48.9 ±44.4 cm3), Group II (large PTV, with average volume 532.4 ±368.8 cm3). Both beam modalities 6MV and 6MVFFF were compared in terms of Dmax, D95%, D1cc, D2cc, homogeneity index (HI), number of monitor units (MU), treatment delivery time. ResultsUsing the 6MVFFF, the treatment delivery time was significantly reduced (p<0.05). For larger PTVs, the number of MU increased by more than twice, and the p-value shown a significant difference (p= 0.008). The value of Dmax increased by 4%. On the contrary, for small volumes, the results were quite similar from 6MVFFF to 6MV except some differences in terms of MU. ConclusionIt is recommended to use 6MVFFF beam with small PTV volumes. Dose distributions are almost the same as with 6MV and there is a significant reduction of the treatment delivery time up to 57%. Due to the dose profile shape in FFF mode, the dose is lowered beyond the central axis for the FFF beams, and the additional MU allows the dose to be delivered away from the beam axis.

Investigation of Electron Contamination on Flattened and Unflattened Varian Clinac iX 6X and 15X Photon Beam Based on Monte Carlo Simulation

The aim of this study was to characterize electron contamination of a flattened (FF) and an unflattened (FFF) Varian Clinac iX 6X and 15X photon beams using Monte Carlo (MC) simulation. EGSnrc MC technique was used to model the flattened and unflattened head and simulate dose distribution of 6X and 15X of FF and FFF photon beam in water phantom. The materials and geometrical data of FF linac were provided by Tan Tock Seng Hospital (TTSH) Singapore. The FFF linac was modeled by removing the flattening filter component in the FF linac. Phase space files were scored after flattening filter and in the phantom surface. The phsp files were analyzed to characterize the particles produced by the linac head using BEAMDP. The contaminants contribute around 1 % and 2 % in the phsp1 for flattened and unflattened beams, respectively. The photons are scattered in small-angle in the range of 0 – 4 o . The contaminant electron contributes up to one hundredth compared to the photons. The increase of field area affects the increase in contaminants and penumbra width due to the increasing number of particle scattered out of the field area. The unflattened beam affects the increase in the number of electron contamination and surface dose. The penumbra width of the flattened beams was smaller than the unflattened beams for the same field size and energy.

Monte Carlo computation of photon energy spectra in central axis of flattened and unflattened beams and doses in critical organs in a water phantom model of prostate radiotherapy

A thorough understanding of the energy spectra for clinical linear accelerators is essential in modern cancer treatment planning. We aimed to determine the radiation spectra and out-of-field doses in a water phantom under the conditions of prostate radiotherapy in beams with a flattening filter (FF) and without (FFF). Monte Carlo simulations were performed to determine the energy spectra and doses outside the irradiated volume. Doses were registered in locations corresponding to critical organs (kidneys, lungs, and thyroid) and the spectra was determined in the central axis (prostate gland). Simulations were conducted for 6 MV and 10 MV for FF and FFF beams. The out-of-field doses were calculated at selected points from the field edge for 6 MV photon beams (FF and FFF), and for 10 and 15 MV beams (FFF only). Compared to FF beams, we found that the mean energy for the photon spectra of FFF beams was lower. For 6 MV FF beams, the relative doses for the critical organs (kidneys, lungs and thyroid) were 0.7%, 1.2%, 0.6% versus 1.6%, 1.2%, 1.3% for the unflattened beams. For 10 MV FF beams, the out-of-field doses to the organs were 0.1%, 0.4% and 0.4% versus 0.4%, 0.2% and 0.3% with the FFF beam. FFF beams had a higher photon fluence, indicating less scattered radiation. Removal of the filter leads to lower doses in the areas surrounding the irradiated volumes, especially at large distances from the treatment field. These findings are consistent with previous reports, indicating that flattening filter free beams are likely to improve clinical outcomes. The reduction in scattered radiation lowers the probability of inducing second cancers.

Quantification of Nanoscale Dose Enhancement in Gold Nanoparticle-Aided External Photon Beam Radiotherapy.

Simple SummaryGold nanoparticles (AuNPs) have become common in radiation oncology research through the past decades. Their radiosensitization effect could offer a novel approach to cancer diagnosis and radiotherapy as well. The aim of this study is the assessment of dose enhancement attributed to the incubation of AuNPs in an irradiated target. The present work is focused on investigating the impact of AuNPs properties in dose increase under different irradiation conditions using 6 MV photon beams with and without a flattening filter via Monte Carlo simulations. Results from the simulated scenarios depict a sufficient dose raise especially at close distances to AuNPs, depending on the presence of a flattening filter in the path of the photon beam, AuNPs size, the type of modeled distribution and their concentration. Therefore, the obtained enhanced dose deposition due to AuNPs presence in an irradiated region could lead to more sufficient tumor cell destruction than irradiation alone.The recent progress in Nanotechnology has introduced Gold Nanoparticles (AuNPs) as promising radiosensitizing agents in radiation oncology. This work aims to estimate dose enhancement due to the presence of AuNPs inside an irradiated water region through Monte Carlo calculations. The GATE platform was used to simulate 6 MV photon histories generated from a TrueBeam® linear accelerator with and without a Flattening Filter (FF) and model AuNPs clusters. The AuNPs size, concentration and distribution pattern were examined. To investigate different clinical irradiation conditions, the effect of field size, presence of FF and placement of AuNPs in water were evaluated. The range of Dose Enhancement Factors (DEF = DoseAu/DoseWater) calculated in this study is 0.99 ± 0.01–1.26 ± 0.02 depending on photon beam quality, distance from AuNPs surface, AuNPs size and concentration and pattern of distribution. The highest DEF is reported for irradiation using un-flattened photon beams and at close distances from AuNPs. The obtained findings suggest that dose deposition could be increased in regions that represent whole cells or subcellular targets (mitochondria, cell nucleus, etc.). Nevertheless, further and consistent research is needed in order to make a step toward AuNP-aided radiotherapy in clinical practice.

Synchronous bilateral breast carcinoma irradiation: A comparative investigation between flattened and unflattened beams

Synchronous bilateral breast carcinoma (SBBC) radiotherapy is a very complicated and time-consuming process. Considering the advantages of unflattened (FFF) beams at a high dose rate, this work investigated the feasibility of FFF beam application in SBBC radiotherapy and compared the advantages between FFF and flattened (FF) beams. CT images of 13 patients with SBBC were retrospectively collected to design intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans using FF and FFF beams. Dosimetric verification was applied for each plan. The target volume metrics, dose received by organs at risk (OARs), and delivery parameter of the plans were documented. All plans met the universal tolerance limits in dosimetric verification. FFF decreased the volume to receive 5 Gy (V5Gy), V7Gy, and mean dose of the left lung but slightly increased the V30Gy of the liver for VMAT and decreased the V17Gy and V20Gy of the right lung for IMRT. No remarkable differences between FF and FFF were found in the other investigated OARs and all the investigated target volume metrics. The mean treatment times of FFF were 66 and 45 s shorter than those of FF for IMRT and VMAT, respectively. FFF beams were feasible and had advantages in reducing treatment time and protecting the lung while maintaining the target volume metrics in SBBC irradiation.

Flattening Filter-Free Volumetric-Modulated Arc Radiotherapy for Left-Sided Whole-Breast, Partial-Breast, and Postmastectomy Irradiations.

Purpose:Unflattened photon beams exhibit many benefits over traditional flattened beams for radiotherapy (RT), but comprehensive evaluations of dosimetric results and beam-on time using flattening filter-free (FFF) beams for all types of breast irradiations are still lacking. The purpose of this study was to investigate if FFF RT can maintain equal or better dose coverage than standard flattened-beam RT while reducing doses to organs at risk (OARs) and beam-on time for various types of breast cancer irradiations.Methods and Materials:FFF volumetric-modulated arc therapy (FFF-VMAT) and standard VMAT (STD-VMAT) treatment plans were created for 15 whole-breast irradiation (WBI) patients with 50 Gy/25 fractions, 13 partial-breast irradiation (PBI) patients with 38.5 Gy/10 fractions, and 9 postmastectomy irradiation (PMI) patients with 50 Gy/25 fractions. Planning target volume (PTV) coverage and dose to OARs were evaluated.Results:Both techniques produced clinically acceptable plans for all three types of irradiations. For WBI, FFF-VMAT plans exhibited similar PTV and OARs evaluation metrics as STD-VMAT. For PBI, FFF-VMAT plans showed significantly lower mean and maximum doses for ipsilateral and contralateral lungs, contralateral breast, and heart. For PMI, FFF-VMAT plans showed significantly lower mean dose and V5 for contralateral breast but significantly higher Dmean, Dmax, and V20 for ipsilateral lung and significantly higher Dmean, V22.5, and V30 for heart. FFF-VMAT techniques significantly reduced beam-on time than STD-VMAT for all cases.Conclusion:This work has shown that FFF beams are most beneficial for small-field irradiation such as PBI, and breast cancer patients could potentially benefit from the shortened beam-on time.

P111 - Influence of using unflattened photon beams in VMAT prostate treatments

P111 - Influence of using unflattened photon beams in VMAT prostate treatments

Evaluation of the dosimetric characteristics of 10 MV flattened and unflattened photon beams in a heterogeneous phantom

Evaluation of the dosimetric characteristics of 10 MV flattened and unflattened photon beams in a heterogeneous phantom

Portal dosimetry of small unflattened beams

We developed and validated a dedicated small field back–projection portal dosimetry model for pretreatment and in vivo verification of stereotactic plans entailing small unflattened photon beams. For this purpose an aSi–EPID was commissioned as a small field dosimeter. Small field output factors for 6 MV FFF beams were measured using the PTW microDiamond detector and the Agility 160–leaf MLC from Elekta. The back-projection algorithm developed in our department was modified to better model the small field physics. The feasibility of small field portal dosimetry was validated via absolute point dose differences w.r.t. small static beams, and 5 hypofractionated stereotactic VMAT clinical plans measured with the OCTAVIUS 1000 SRS array dosimeter and computed with the treatment planning system Pinnacle v16.2. Dose reconstructions using the currently clinically applied back–projection model were also computed for comparison. We found that the latter yields underdosage of about −8% for square beams with cross section near 10 mm x 10 mm and about −6% for VMAT treatments with PTV volumes smaller than about 2cm3. With the methods described in this work such errors can be reduced to less than the ±3.0% recommendations for clinical use. Our results indicate that aSi–EPIDs can be used as accurate small field radiation dosimeters, offering advantages over point dose detectors, the correct positioning and orientation of which is challenging for routine clinical QA.

EPID-based 3D dosimetry for pre-treatment FFF VMAT stereotactic body radiotherapy plan verification using dosimetry CheckTM.

The software Dosimetry Check (DC) reconstructs the 3D dose distribution on CT images data set by using EPID measured signal. This study aimed to evaluate DC for stereotactic body radiotherapy (SBRT) with unflattened photon beams (FFF) for dosimetric independent plan verification in pre-treatment modality. DC v.4.1 was configured for Varian TrueBeam STx FFF beams equipped with EPID aS-1200. The DC FFF models were tested using arc open fields (from 1×1cm2 to 15×15cm2) and VMAT (Volumetric Modulated Arc Therapy) SBRT plans on phantom and patient CTs. DC dose distributions (DDC) were compared with that calculated by Eclipse with Acuros XB algorithm (DAXB) and one measured by Octavius 1000 SRS detector (DOCT). All differences were quantified in terms of the local 3D gamma passing rate (%GP), DVH and point dose differences. DC was configured for FFF VMAT using an appropriate correction procedure. %GP2%2mm (mean±standard deviation) of DOCT-DDC was 96.3±2.7% for open fields whereas it was 90.1±5.9% for plans on homogeneous phantom CT. However, average %GP3%3mm of DAXB-DDC was 95.0±4.1 for treatments on patient CT. The fraction of plans passing the %GP3%3mm DQA tolerance level [10% (50%) of maximum dose threshold] were 20/20 (14/20) and 18/20 (16/20) for OCT on phantom CT and DC on patient CT, respectively. DC characterization for FFF beams was performed. For stereotactic VMAT plan verifications DC showed good agreement with TPS whereas underlined discrepancies with Octavius in the high dose regions. A customized tolerance level is required for EPID-based VMAT FFF pre-treatment verification when DC system is applied.

Dosimetric performance of continuous EPID imaging in stereotactic treatment conditions.

This study aims at investigating the dosimetric characteristics of a Varian aS1000 EPID, focusing on its continuous acquisition mode under the challenging conditions that can be met in stereotactic radiotherapy verification. An aS1000 EPID installed on a Varian TrueBeamSTx was irradiated with 6 and 10 MV unflattened and flattened photon beams. In order to avoid detector saturation, the source-to-detector distance (SDD) was set to 150 or 180cm depending on the dose rate. EPID image sets were acquired in continuous mode (CM) and also in the commonly used integrated mode (IM) for comparison, to evaluate dose linearity (including dose rate dependence), repeatability, reproducibility, stability, ghosting effect and field size dependence. CM response linearity was found to be within 0.8% of IM and independent of dose rate. Response repeatability was slightly better for IM and FF beams, being in all cases within 0.9%. Reproducibility was within 0.6% for both modes and all beam qualities. Response stability between continuous frames varied within 1% for dynamic and static irradiations and for all the beam qualities, showing its independence from these parameters. Ghosting effect was not significant, being comparable to signal variations between continuous frames (±1%). Field size dependence in both modes agreed within 1%. The dosimetric response of the aS1000 EPID in CM with FFF beams and high dose rates is comparable to that in IM and for flattened beams provided that the appropriate SDD is used. aS1000 EPID in continuous acquisition mode is therefore suitable for stereotactic applications.

Image quality of 4D in-treatment CBCT acquired during lung SBRT using FFF beam: a phantom study

BackgroundRotational beam delivery enables concurrent acquisition of cone-beam CT (CBCT), thereby facilitating further geometric verification of patient setup during radiation treatment. However, it is challenging to acquire CBCT during stereotactic body radiation therapy (SBRT) using flattening-filter free X-ray beams, in which a high radiation dose is delivered. This study presents quantitative evaluation results of the image quality in four-dimensional (4D) in-treatment CBCT acquired during SBRT delivery.MethodsThe impact of megavoltage (MV) scatter and acquisition parameters on the image quality was evaluated using Catphan 503 and XSight lung tracking phantoms. The in-treatment CBCT images of the phantoms were acquired while delivering 16 SBRT plans. The uniformity, contrast, and contrast-to-noise ratio (CNR) of the in-treatment CBCT images were calculated and compared to those of CBCT images acquired without SBRT delivery. Furthermore, the localizing accuracy of the moving target in the XSight lung phantom was evaluated for 10 respiratory phases.ResultsThe CNR of the 3D-reconstucted Catphan CBCT images was reduced from 6.3 to 2.6 due to the effect of MV treatment scatter. Both for the Catphan and XSight phantoms, the CBCT image quality was affected by the tube current and monitor units (MUs) of the treatment plan. The lung target in the XSight tracking phantom was most visible for extreme phases; the mean CNRs of the lung target in the in-treatment CBCT images (with 40 mA tube current) across the SBRT plans were 3.2 for the end-of-exhalation phase and 3.0 for the end-of-inhalation phase. The lung target was localized with sub-millimeter accuracy for the extreme respiratory phases.ConclusionsFull-arc acquisition with an increased tube current (e.g. 40 mA) is recommended to compensate for degradation in the CBCT image quality due to unflattened MV beam scatter. Acquiring in-treatment CBCT with a high-MU treatment beam is also suggested to improve the resulting CBCT image quality.

An assessment of the ExacTrac intrafraction imaging capabilities for flattening filter free prostate stereotactic body radiotherapy

The uncertainties associated with image matching using the ExacTrac® system (BrainLab, Munich, Germany) have been the subject of investigation in the literature for extra-cranial sites. However, the uncertainties involved in the use of ExacTrac in the presence of higher scatter conditions like that for intrafraction imaging of prostate stereotactic radiotherapy utilising unflattened beams is yet to be determined. A prostate phantom was created with 3 implanted gold fiducial markers. This phantom was shifted by 1 mm and 2 mm amounts in the translational planes and by 1° and 2° amounts in the rotational planes and subsequently imaged by ExacTrac during delivery of a clinical SBRT plan. ExacTrac auto-match results were compared to the known offsets with uncertainties calculated. Calculated shifts were shown to be accurate within one standard deviation of the known offsets. Uncertainties were found to vary considerably among the 6 dimensions with matching in the vertical and angle vertical directions having standard deviations of 0.7 mm and 1.3°, respectively. These results agreed with the literature cases for pre-treatment setup and lower scatter condition IMRT intrafraction delivery. Based on these values, probabilities of intrafraction inhibits were calculated based on patient movement and possible fusion tolerances. While the measured uncertainties are adequately defined in order to calculate appropriate target margins, their relatively large magnitudes made choice of intrafraction fusion tolerances problematic. A degree of compromise between the rate of false positives and false negatives is required when implementing ExacTrac into a SBRT prostate protocol.

Technical Note: Flattening filter free beam from Halcyon linac: Evaluation of the profile parameters for quality assurance.

The use of flattening filter free (FFF) beams generated by standard linear accelerators is increasing in the clinical practice. The radiation intensity peaked toward the beam central axis is properly managed in the optimization process of treatment planning through intensity modulation. Specific FFF parameters for profile analysis, as unflatness and slope for FFF beams, based on the renormalization factor concept has been introduced for quality assurance purposes. Recently, Halcyon, an O-ring based linear accelerator equipped with a 6MV FFF beam only has been introduced by Varian. Renormalization factors and related fit parameters according to Fogliata et al. ["Definition of parameters for quality assurance of FFF photon beams in radiation therapy," Med. Phys. 39, 6455-6464 (2012)] have been evaluated for the 6MV FFF beam generated by Halcyon units. The Halcyon representative beam data provided by Varian were used. Dose fall-off at the field edges was matched with an unflattened beam generated by a 6MV from a TrueBeam linac. Consistency of the results was evaluated against measurements on a clinical Halcyon unit, as well as a TrueBeam 6MV FFF for comparison. The five parameters in the analytical equation for estimating the renormalization factor were determined with an R2 of 0.997. The comparison of the unflatness parameters between the Halcyon representative and hospital beam data was consistent within a range of 0.6%. Consistently with the computed parameters, the Halcyon profiles resulted in a less pronounced peak than TrueBeam. Renormalization factors and related fit parameters from the 6MV FFF beam generated by the Varian Halcyon unit are provided.