SBRT Treatment Plans Research Articles

Linac- and CyberKnife-based MRI-only treatment planning of prostate SBRT using an optimized synthetic CT calibration curve.

CT Hounsfield Units (HUs) are converted to electron density using a calibration curve obtained from physical measurements of an electron density phantom. HU values assigned to an MRI-derived synthetic computed tomography (sCT) may present a different relationship with electron density compared to CT HU. Correct assignment of sCT HU values is critical for accurate dose calculation and delivery. The goals of this work were to develop a sCT calibration curve using patient data acquired on a clinically commissioned CT scanner and assess for CyberKnife- and volumetric modulated arc therapy (VMAT)-based MR-only treatment planning of prostate SBRT. Same-day CT and MRI simulation in the treatment position were performed on 10 patients treated with SBRT to the prostate. Dixon in-phase and out-of-phase MRIs were acquired on a 3T scanner using a 3D T1-weighted gradient-echo sequence to generate sCTs using a commercial sCT algorithm. CT and sCT datasets were co-registered and HU values compared using mean absolute error (MAE). An optimized HU-to-density calibration curve was created based on average HU values across an institutional patient database for each of the four sCT tissue types. Clinical CyberKnife and VMAT treatment plans were generated on each patient CT and recomputed onto corresponding sCTs. Dose distributions computed using CT and sCT were compared using gamma criteria and dose-volume-histograms. For the optimized calibration curve, HU values were -96, 37, 204, and 1170 and relative electron densities were 0.95, 1.04, 1.1, and 1.7 for adipose, soft tissue, inner bone, and outer bone, respectively. The proposed sCT protocol produced total MAE of 94±20HU. Gamma values mean±std (min-max) were 98.9%±0.9% (97.1%-100%) and 97.7%±1.3% (95.3%-99.3%) for VMAT and CyberKnife plans, respectively. MRI-derived sCT using the proposed approach shows excellent dosimetric agreement with conventional CT simulation, demonstrating the feasibility of MRI-derived sCT for prostate SBRT treatment planning.

1369: Impact of Target Volume on the Quality of Automatically Generated O-Ring linac SBRT Treatment Plans

1369: Impact of Target Volume on the Quality of Automatically Generated O-Ring linac SBRT Treatment Plans

Simulating an intra-fraction adaptive workflow to enable PTV margin reduction in MRIgART volumetric modulated arc therapy for prostate SBRT.

This study simulates a novel prostate SBRT intra-fraction re-optimization workflow in MRIgART to account for prostate intra-fraction motion and evaluates the dosimetric benefit of reducing PTV margins. VMAT prostate SBRT treatment plans were created for 10 patients using two different PTV margins, one with a 5 mm margin except 3 mm posteriorly (standard) and another using uniform 2 mm margins (reduced). All plans were prescribed to 36.25 Gy in 5 fractions and adapted onto each daily MRI dataset. An intra-fraction adaptive workflow was simulated for the reduced margin group by synchronizing the radiation delivery with target position from cine MRI imaging. Intra-fraction delivered dose was reconstructed and prostate DVH metrics were evaluated under three conditions for the reduced margin plans: Without motion compensation (no-adapt), with a single adapt prior to treatment (ATP), and lastly for intra-fraction re-optimization during delivery (intra). Bladder and rectum DVH metrics were compared between the standard and reduced margin plans. As expected, rectum V18 Gy was reduced by 4.4 ± 3.9%, D1cc was reduced by 12.2 ± 6.8% (3.4 ± 2.3 Gy), while bladder reductions were 7.8 ± 5.6% for V18 Gy, and 9.6 ± 7.3% (3.4 ± 2.5 Gy) for D1cc for the reduced margin reference plans compared to the standard PTV margin. For the intrafraction replanning approach, average intra-fraction optimization times were 40.0 ± 2.9 seconds, less than the time to deliver one of the four VMAT arcs (104.4 ± 9.3 seconds) used for treatment delivery. When accounting for intra-fraction motion, prostate V36.25 Gy was on average 96.5 ± 4.0%, 99.1 ± 1.3%, and 99.6 ± 0.4 for the non-adapt, ATP, and intra-adapt groups, respectively. The minimum dose received by the prostate was less than 95% of the prescription dose in 84%, 36%, and 10% of fractions, for the non-adapt, ATP, and intra-adapt groups, respectively. Intra-fraction re-optimization improves prostate coverage, specifically the minimum dose to the prostate, and enables PTV margin reduction and subsequent OAR sparing. Fast re-optimizations enable uninterrupted treatment delivery.

PO-1805 Clinical impact of moving from type-b to type-c algorithms in lung SBRT treatment planning.

PO-1805 Clinical impact of moving from type-b to type-c algorithms in lung SBRT treatment planning.

Evaluating on-board kVCT- and MVCT-based dose calculation accuracy using a thorax phantom for helical tomotherapy treatments

Purpose. To evaluate the impact of CT number calibration and imaging parameter selection on dose calculation accuracy relative to the CT planning process in thoracic treatments for on-board helical CT imaging systems used in helical tomotherapy. Methods and Materials. Direct CT number calibrations were performed with appropriate protocols for each imaging system using an electron density phantom. Large volume and SBRT treatment plans were simulated and optimized for planning CT scans of an anthropomorphic thorax phantom and transferred to registered kVCT and MVCT scans of the phantom as appropriate. Relevant DVH metrics and dose-difference maps were used to evaluate and compare dose calculation accuracy relative to the planning CT based on a variation in imaging parameters applied for the on-board systems. Results. For helical kVCT scans of the thorax phantom, median differences in DVH parameters for the large volume treatment plan were less than ±1% with dose to the target volume either over- or underestimated depending on the imaging parameters utilized for CT number calibration and thorax phantom acquisition. For the lung SBRT plan calculated on helical kVCT scans, median dose differences were up to –2.7% with a more noticeable dependence on parameter selection. For MVCT scans, median dose differences for the large volume plan were within +2% with dose to the target overestimated regardless of the imaging protocol. Conclusion. Accurate dose calculations (median errors of <±1%) using a thorax phantom simulating realistic patient geometry and scatter conditions can be achieved with images acquired with a helical kVCT system on a helical tomotherapy unit. This accuracy is considerably improved relative to that achieved with the MV-based approach. In a clinical setting, careful consideration should be made when selecting appropriate kVCT imaging parameters for this process as dose calculation accuracy was observed to vary with both parameter selection and treatment type.

Knowledge-based planning algorithm for lung SBRT with robust Bayesian stochastic frontier analysis and missing data management.

A knowledge-based planning technique is developed based on Bayesian stochastic frontier analysis. A novel missing data management is applied in order to handle missing organs-at-risk and work with a completedataset. Geometric metrics are used to predict DVH metrics for lung SBRT with a retrospective database of 299 patients. In total, 16 DVH metrics were predicted for the main bronchus, heart, esophagus, spinal cord PRV, great vessels, and chest wall. The predictive model is tested on a test group of 50patients. Mean difference between the observed and predicted values ranges between 1.5 ± 1.9 Gy and 4.9 ± 5.3 Gy for the spinal cord PRV D0.35cc and the main bronchus D0.035cc,respectively. The missing data model implanted in the predictive model is robust in the estimation of the parameters. Bayesian stochastic frontier analysis with missing data management can be used to predict DVH metrics for lung SBRT treatmentplanning.

Validation of spine SBRT using a 3D printed Anthropomorphic phantom

A segment of a spine was 3D-printed based on real patient anatomy, using metal-doped high density plastic to radiographically mimic bone. This spine was submerged in a water tank to create an anthropomorphic phantom. The spine print incorporated a slot for Gafchromic EBT3 film dosimeters and fiducials for alignment of measured and calculated dose distributions. Spine SBRT treatment plans were generated for both 6 MV and 10FFF energies based on oncologist-drawn contours transferred from real anatomy. Plans were delivered under image guidance using our clinical procedures, to evaluate the dosimetric accuracy of our planning system in high density inhomogeneities and the geometric accuracy of delivery. Results show that the Acuros XB algorithm (dose-to-water) agrees well with film measurements throughout the measured region, including within the bone substitute material. Alignment of the steep dose gradients in planned and measured doses was within 0.5 mm in the ANT-POST direction and within 0.9 mm in the SUP-INF direction, both within machine tolerances. Our results give us confidence in our ability to plan and accurately deliver spinal SBRT treatments.

Physical Confirmation of Biology-Guided Radiotherapy Directed at Static Targets With Varying Shapes and Background Contrast Environments

Biology-guided radiotherapy (BgRT), utilizes hardware that incorporates a PET detection system into a ring-gantry LINAC for real-time tracking delivery. For this study, we focus on the BgRT delivery performance in the case of static targets for dose accuracy measurement. During active beam delivery, the system operates by aiming beamlets of therapeutic radiation at malignant tumors in response to outgoing PET emissions with sub-second latency. Over a treatment fraction, these beamlets sum to the total intended dose prescribed by the physician. Physical demonstration of BgRT has not previously been reported. Here we report phantom experiments validating BgRT using static PET-avid targets of varying shapes and background PET environments.An FDG-fillable insert with different shaped targets was developed to mimic different potential radiotherapy targets. Spherical and C-shaped targets were filled with 18F-flurodeoxyglucose (FDG) to represent tumors and/or organs-at-risk. The background material in the insert was either a homogenous water medium or a water filled heterogeneous medium with a Styrofoam mesh simulating lung tissue surrounding the target. Targets and OARs were filled with FDG to achieve a target/OAR: background ratio of 8:1, while the background concentration varied from 4.5-10 kBq/ml to simulate typical patient background activity concentrations. For our set-ups were investigated: Spherical target in homogenous background, spherical target in heterogeneous background, C-shaped target in homogenous background, and spherical target in homogenous background with nearby PET-avid, C-shaped OAR. For each combination, SBRT and BgRT treatment plans were created on the RefleXion treatment planning system (TPS). Next, SBRT and BgRT plans were delivered on a pre-commercial version of the RefleXion system and dosimetric accuracy was measured using the AC phantom. Gamma criteria (3%/3mm) were used to compare delivered and calculated dose. Paired Student's t-test assessed differences in SBRT and BgRT gamma pass results.All BgRT deliveries met 3%/3mm gamma pass criteria greater than 95% (Range, 95.2% to 98.9%). Additionally, gamma passing rates were not significantly different than the companion SBRT plans performed on the same targets (P = 0.16).Dose accuracy results indicate that BgRT plans can be delivered accurately over a variety of test conditions (different target shapes, attenuating media and activity concentration levels) to FDG-avid targets, with performance similar to SBRT treatments.

Modelling of a novel technique to improve the visualisation of implanted fiducial markers for intra-fraction MV imaging of prostate VMAT targets

Purpose. This study explored a novel technique to improve the MV imaging based fiducial visibility for a cohort of prostate radiotherapy patients, without compromising the original treatment plan. The study also compared these results to visibility using single MLC control points, as well as short arcs. Methods. Geometric data from 68 prostate radiotherapy treatments, each with implanted gold fiducials, was retrospectively analysed. Fiducials were contoured for each patient, and conventional and SBRT treatment plans were generated using a VMAT technique. Using an in-house script, fiducial contours were projected onto the VMAT MLC control points. Resulting data was assessed to determine whether the fiducial contours were theoretically visible for single MLC control points and groups of MLC control points (short arcs), both being surrogates for intra-fraction MV imaging. Using this data, a theoretical quadrant technique was investigated that assessed the region surrounding each fiducial to determine if visualisation would theoretically improve. Results. Using a conventional treatment type, mean fiducial visibility for single MLC control points across the patient cohort ranged from 2.5% up to 17.8%. For SBRT, fiducial visibility ranged from 1.8% up to 19.7%. For short arcs, fiducial visibility for conventional treatment types ranged from 5.9% up to 20.7%. For SBRT, fiducial visibility ranged from 4.6% up to 23.1%. When the novel fiducial quadrant technique was used, theoretical visibility improved two-fold, from 22.7% up to 52.5% and from 24.7% up to 55.3% for conventional and SBRT treatment types respectively. Conclusions. Fiducial visibility was assessed for a cohort of VMAT prostate patients. Using the novel quadrant technique, it was demonstrated that theoretical visualisation and localisation of the implanted fiducials could be improved two-fold, without sacrificing treatment plan quality.

Dosimetric feasibility of neurovascular bundle-sparing stereotactic body radiotherapy with periprostatic hydrogel spacer for localized prostate cancer to preserve erectile function

Objective:We aim to test the hypothesis that neurovascular bundle (NVB) displacement by rectal hydrogel spacer combined with NVB delineation as an organ at risk (OAR) is a feasible method for NVB-sparing stereotactic body radiotherapy.Methods:Thirty-five men with low- and intermediate-risk prostate cancer who underwent rectal hydrogel spacer placement and pre-, post-spacer prostate MRI studies were treated with prostate SBRT (36.25 Gy in five fractions). A prostate radiologist contoured the NVB on both the pre- and post-spacer T2W MRI sequences that were then registered to the CT simulation scan for NVB-sparing radiation treatment planning. Three SBRT treatment plans were developed for each patient: (1) no NVB sparing, (2) NVB-sparing using pre-spacer MRI, and (3) NVB-sparing using post-spacer MRI. NVB dose constraints include maximum dose 36.25 Gy (100%), V34.4 Gy (95% of dose) <60%, V32Gy <70%, V28Gy <90%.Results:Rectal hydrogel spacer placement shifted NVB contours an average of 3.1 ± 3.4 mm away from the prostate, resulting in a 10% decrease in NVB V34.4 Gy in non-NVB-sparing plans (p < 0.01). NVB-sparing treatment planning reduced the NVB V34.4 by 16% without the spacer (p < 0.01) and 25% with spacer (p < 0.001). NVB-sparing did not compromise PTV coverage and OAR endpoints.Conclusions:NVB-sparing SBRT with rectal hydrogel spacer significantly reduces the volume of NVB treated with high-dose radiation. Rectal spacer contributes to this effect through a dosimetrically meaningful displacement of the NVB that may significantly reduce RiED. These results suggest that NVB-sparing SBRT warrants further clinical evaluation.Advances in knowledge:This is a feasibility study showing that the periprostatic NVBs can be spared high doses of radiation during prostate SBRT using a hydrogel spacer and nerve-sparing treatment planning.

Evaluation of IGRT-Induced Imaging Doses and Secondary Cancer Risk for SBRT Early Lung Cancer Patients In Silico Study.

Objectives:This study performed dosimetry studies and secondary cancer risk assessments on using electronic portal imaging device (EPID) and cone beam computed tomography (CBCT) as image guided tools for the early lung cancer patients treated with SBRT.Methods:The imaging doses from MV-EPID and kV-CBCT of the Edge accelerator were retrospectively added to sixty-one SBRT treatment plans of early lung cancer patients. The MV-EPID imaging dose (6MV Photon beam) was calculated in Pinnacle TPS, and the kV-CBCT imaging dose was simulated and calculated by modeling of the kV energy beam in TPS using Pinnacle automatic modeling program. Three types of plans, namely PlanEPID, PlanCBCT and Planorigin, were generated with incorporating doses of EPID, CBCT and no imaging, respectively, for analysis. The effects of imaging doses on dose-volume-histogram (DVH) and plan quality were analyzed, and the excess absolute risk (EAR) of secondary cancer for ipsilateral lung was evaluated.Results:The regions that received less than 50 cGy were significantly impacted by the imaging doses, while the isodose lines greater than 1000 cGy were barely changed. The DVH values of ipsilateral lung increased the most in PlanEPID, followed by PlanCBCT. Compared to Planorigin on the average, the estimated EAR of ipsilateral lung in PlanEPID increased by 3.43%, while the corresponding EAR increase in PlanCBCT was much smaller (about 0.4%). Considering only the contribution of the imaging dose, the EAR values for the ipsilateral lung due to the MV-EPID dose in 5 years,10 years and 15 years were 1.49 cases, 2.09 cases and 2.88 cases per 104PY respectively, and those due to the kV-CBCT dose were about 9 times lower, correspondingly.Conclusions:The imaging doses produced by MV-EPID and kV-CBCT had little effects on the target dose coverage. The secondary cancer risk caused by MV-EPID dose is more than 8.5 times that of kV-CBCT.

The feasibility and efficacy of new SBRT technique HyperArc for recurrent nasopharyngeal carcinoma: noncoplanar cone-based robotic system vs. noncoplanar high-definition MLC based Linac system

The purpose of this study was to evaluate the feasibility and efficacy of HyperArc (HA) for recurrent nasopharyngeal cancer (NPC) by comparing it with the CyberKnife system (CK). Fifteen patients with recurrent nasopharyngeal cancer who were treated using the noncoplanar cone-based robotic CK system were enrolled. CK was delivered with a median dose of 37.5 Gy in 5 fractions. The delivered CK treatment plans were the sources for the corresponding homogeneous HA (HA-H) and inhomogeneous HA (HA-IH) plans. The HA-H plans were generated to meet the corresponding treatment plan criteria for the CK plans. The HA-IH plans were designed to emulate the corresponding inhomogeneous CK isodose distributions. These three SBRT treatment plans were compared with target coverage, sparing of organs at risk (OARs), and dose distribution metrics. The HA-H and HA-IH plans consistently exhibited CTV and PTV coverage levels similar or better to those of the CK plans but significantly reduced the dose to OARs. Using the HA techniques (both HA-H and HA-IH plans), the mean maximal doses to the spinal cord, brainstem, optic nerves, and optic chiasm were reduced by approximately 60%, compared to the CK plans. The high dose spillage, conformity, and homogeneity indices of the HA-H plans were significantly better than those of the CK plans. The HA-IH plans showed faster dose falloff and similar conformity of the HA-H plans and dose heterogeneity of the CK plans. Here we demonstrated the HA treatment plan system for recurrent NPC is feasible, either homogeneous or inhomogeneous delivery. Excellent sparing of OARs and dosimetric distribution and very efficient delivery make HA an attractive SBRT technique for recurrent NPC treatment.

Experimental evaluation of the dosimetric impact of intrafraction prostate rotation using film measurement with a 6DoF robotic arm.

Tumor motion during radiotherapy can cause a reduction in target dose coverage and an increase in healthy tissue exposure. Tumor motion is not strictly translational and often exhibits complex six degree-of-freedom (6DoF) translational and rotational motion. Although the dosimetric impact of prostate tumor translational motion is well investigated, the dosimetric impact of 6DoF motion has only been studied with simulations or dose reconstruction. This study aims to experimentally quantify the dose error caused by 6DoF motion. The experiment was designed to test the hypothesis that 6DoF motion would cause larger dose errors than translational motion alone through gamma analyses of two-dimensional film measurements. Four patient-measured intrafraction prostate motion traces and four VMAT 7.25Gy/Fx SBRT treatment plans were selected for the experiment. The traces represented typical motion patterns, including small-angle rotations (<4°), transient movement, persistent excursion, and erratic rotations (>6°). Gafchromic film was placed inside a custom-designed phantom, held by a high-precision 6DoF robotic arm for dose measurements in the coronal plane during treatment delivery. For each combination of the motion trace and treatment plan, two film measurements were made, one with 6DoF motion and the other with the three-dimensional (3D) translation components of the same trace. A gamma pass rate criteria of 2% relative dose/2mm distance-to-agreement was used in this study and evaluated for each measurement with respect to the static reference film. Two test thresholds, 90% and 50% of the reference dose, were applied to investigate the difference in dose coverage for the PTV region and surrounding areas, respectively. The hypothesis was tested using a Wilcoxon signed-rank test. For each of the 16 plan and motion trace pairs, a reduction in the gamma pass rate was observed for 6DoF motion compared with 3D translational motion. With 90% gamma-test threshold, the reduction was 5.8%±7.1% (P<0.01). With 50% gamma-test threshold, the reduction was 4.1%±4.8% (P<0.01). For the first time, the dosimetric impact of intrafraction prostate rotation during SBRT treatment was measured experimentally. The experimental results support the hypothesis that 6DoF tumor motion causes higher dose error than translation motion alone.

Dose deviations induced by respiratory motion for radiotherapy of lung tumors: Impact of CT reconstruction, plan complexity, and fraction size.

A thorax phantom was used to assess radiotherapy dose deviations induced by respiratory motion of the target volume. Both intensity modulated and static, non‐modulated treatment plans were planned on CT scans of the phantom. The plans were optimized using various CT reconstructions, to investigate whether they had an impact on robustness to target motion during delivery. During irradiation, the target was programmed to simulate respiration‐induced motion of a lung tumor, using both patient‐specific and sinusoidal motion patterns in three dimensions. Dose was measured in the center of the target using an ion chamber. Differences between reference measurements with a stationary target and dynamic measurements were assessed. Possible correlations between plan complexity metrics and measured dose deviations were investigated. The maximum observed motion‐induced dose differences were 7.8% and 4.5% for single 2 Gy and 15 Gy fractions, respectively. The measurements performed with the largest target motion amplitude in the superior–inferior direction yielded the largest dosimetric deviations. For 2 Gy fractionation schemes, the summed dose deviation after 33 fractions is likely to be less than 2%. Measured motion‐induced dose deviations were significantly larger for one CT reconstruction compared to all the others. Static, non‐modulated plans showed superior robustness to target motion during delivery. Moderate correlations between the modulation complexity score applied to VMAT (MCSv) and measured dose deviations were found for 15 Gy SBRT treatment plans. Correlations between other plan complexity metrics and measured dose deviations were not found.

Erratum: "Investigating the impact of patient arm position in an MR-linac on liver SBRT treatment plans" [Med. Phys. Vol 46, 5144-5151 (2019)

Erratum: "Investigating the impact of patient arm position in an MR-linac on liver SBRT treatment plans" [Med. Phys. Vol 46, 5144-5151 (2019)

MRI-based treatment planning for liver stereotactic body radiotherapy: validation of a deep learning-based synthetic CT generation method.

The purpose of this work is to develop and validate a learning-based method to derive electron density from routine anatomical MRI for potential MRI-based SBRT treatment planning. We proposed to integrate dense block into cycle generative adversarial network (GAN) to effectively capture the relationship between the CT and MRI for CT synthesis. A cohort of 21 patients with co-registered CT and MR pairs were used to evaluate our proposed method by the leave-one-out cross-validation. Mean absolute error, peak signal-to-noise ratio and normalized cross-correlation were used to quantify the imaging differences between the synthetic CT (sCT) and CT. The accuracy of Hounsfield unit (HU) values in sCT for dose calculation was evaluated by comparing the dose distribution in sCT-based and CT-based treatment planning. Clinically relevant dose-volume histogram metrics were then extracted from the sCT-based and CT-based plans for quantitative comparison. The mean absolute error, peak signal-to-noise ratio and normalized cross-correlation of the sCT were 72.87 ± 18.16 HU, 22.65 ± 3.63 dB and 0.92 ± 0.04, respectively. No significant differences were observed in the majority of the planning target volume and organ at risk dose-volume histogram metrics ( p > 0.05). The average pass rate of γ analysis was over 99% with 1%/1 mm acceptance criteria on the coronal plane that intersects with isocenter. The image similarity and dosimetric agreement between sCT and original CT warrant further development of an MRI-only workflow for liver stereotactic body radiation therapy. This work is the first deep-learning-based approach to generating abdominal sCT through dense-cycle-GAN. This method can successfully generate the small bony structures such as the rib bones and is able to predict the HU values for dose calculation with comparable accuracy to reference CT images.

140. Characterization of a 2D chamber array for high modulated VMAT plan verification

PurposeHigh modulated VMAT plan can be achieved by the last generation of Treatment Planning System (TPS). The aim of this study is to evaluate if a 2D chamber array is adequate to check this complex plans. The response function of the ion chamber and the spatial frequency of the dose distribution are analyzed to quantify the loose of information due to the aliasing effect [1]. Methods and materialsAlmost 200 VMAT plans of different anatomic district (Table 1) were performed with TPS Monaco; the plans were delivered with Elekta agility LINAC and for the patient pre-treatment QA a PTW OCTAVIUS Detector 729 was employed [2]. Gafchromic EBT3 were used as reference dosimetric system due to the higher spatial resolution. A home made program in Matlab was elaborated to analyze the spatial frequency of each line and column of the dose distribution maps. ResultsEach patient plan was recalculate in a specific phantom for pre-treatment verification. This method often involves a loss of spatial frequencies (Fig. 1). The highest detectable spatial frequency that can be obtained with the “OCTAVIUS Detector729” is 0.05 mm−1 (single acquisition). The large sampling distance may result in under-sampling of VMAT fields with high frequency contents. Head & Neck and Lung stereotactic treatment maps show frequency contribution beyond the limit 0.05 mm−1 with loss of information and possible artefacts in case of reconstructed distribution (aliasing). ConclusionThe 2D Array is a dosimetrically accurate and sensitive tool when used for QA and verification of most clinical radiotherapy beams. However, verification for Head and Neck or SBRT treatment plans require more closely spaced ion chambers or multiple merged acquisition should be considered in order to fulfil Nyquist theorem.

143. VMAT pre-treatment verifications using Gafchromic™ EBT3 and FILM QA software

Purpose The aim of this work is to optimize the procedures for VMAT patient-specific quality assurance for conventionally fractionated and SBRT treatments using Gafchromic™ EBT3 film dosimetry and FilmQA software. Methods 16 VMAT treatment plans (8 conventionally fractionated and 8 SBRT) calculated with EclipseTM TPS and delivered with TrueBeam 2.5 have been analyzed. Plans were verified with two different QA methods: routinely pre-treatment verification system, based on EPID images and EPIQA software and dose map measurements by Gafchromic™ EBT3 film dosimetry digitized with Epson 10000 XL and analyzed with FILM QA software. 6 MV, 10 MV and 10 FFF photon beams were used to obtain calibration dose curves from 0 to 25 Gy and to study energy dependence. Triple-channel method was used to improve film dosimetry accuracy. To eliminate scan-to-scan variability and uncertainty therefrom, EBT3 films exposed to each VMAT arc were coupled (and digitalized in a single scan) to two reference films: one film exposed to a known dose and one unexposed film [1] . This allows to obtain a linear dose scaling on the calibration curve and therefore a more accurate dose map. The choice of the reference dose value was made on the basis of its density distribution on the plane delivered for each individual arc (Fig. 1). Pre-treatment verifications were quantified using the global gamma index approach with 3%/3 mm criteria and 95% passing rate. Results For conventionally fractionated plans the average passing rate was 98.8% and 97.6%, for EBT3F instead for SBRT plans the average was 97.6% and 96.3% respectively (Table 1). Conclusions Under the experimental conditions described above, the use of EBT3 film dosimetry analyzed with FilmQA software shows to be an accurate method for VMAT conventionally fractionated and SBRT treatment plans verifications.

Evaluating Safety and Dosimetry of SBRT following 90Y for Hepatocellular Carcinoma

Liver directed therapy is fundamental in the treatment of HCC. SBRT is the only non-invasive local therapy characterized by precise target delineation and accurate, quantifiable dosimetry. Often, SBRT is utilized following interventional therapy; however, there is a lack of data reporting safety and outcomes of patients receiving liver SBRT after yttrium-90 (90Y). We performed a safety and dosimetric evaluation in our series of HCC patients who received SBRT after 90Y by compiling post 90Y PET imaging fusions with SBRT treatment plans for accurate quantification of liver exposed to both SBRT and 90Y. An institutional review board-approved liver SBRT registry of 197 patients was used for data collection. All patients had post-90Y PET (measuring Bremsstrahlung component of 90Y) fused to SBRT treatment planning CT creating composite plans for dosimetric analysis. Volume of ablated liver was estimated using the sharp gradient edge of post-90Y PET, and the 30 Gy isodose line for SBRT (V30). Clinical and laboratory follow up 3 months post SBRT were used to evaluate safety. Changes in chemistry and hematology related to hepatic function, using CTCAE v4.0, were analyzed. Sixteen patients with non-metastatic HCC and prior 90Y were treated with SBRT to 18 intrahepatic sites. Patients received 23 prior 90Y administrations; six patients were treated twice and one patient treated three times. Child-Pugh (CP) category at SBRT was A in 12, and two each of B and C. SBRT dose was 35-50 Gy in 3-5 fractions (median 50 Gy/5). Median time between 90Y and SBRT was 4.6 months (range 1.3-34.8). Mean follow up was 14 months, and 11 patients were alive at analysis. Treatment intent was definitive for 13 patients and downstaging in 3. Mean total liver volume was 2,000 cc (range 986-3304), SBRT PTV 209 cc (range 26-494), and individual 90Y treatment volume 308 cc (range 36-1477). Liver ablative results summarized in Table 1.Abstract SU_13_2123; Table 1Mean valueMean %RangeV30 liver (SBRT)273 cc13.7%36-739 cc (3-34.8%)90Y volume (sum)436 cc22.6%95-1512 cc (3.5-52.6%)V30 volume within prior 90Y93 cc33.2%0-306 cc (0-75%) Open table in a new tab Laboratory data was available for 75% of patients. At 3 months post SBRT there were no new grade ≥2 changes from baseline in bilirubin, hemoglobin, INR, AST, ALT, and albumin in CP-A/B patients. The two CP-C patients, who had higher 90Y volumes and more overlap of 90Y with V30, demonstrated clinically significant hepatic decompensation. Although toxicity was noted, both patients were successfully down-staged within Milan then rescued with transplant 10 and 15 weeks post SBRT. This is the first series to incorporate PET delineation of 90Y ablation distribution in a composite plan with SBRT. Our experience of CP-A /B patients demonstrates minimal toxicity, but notable decompensation in CP-C patients. SBRT can be an effective tool to downstage within Milan but extra caution is warranted in this group, such that risk matches treatment intent, especially in patients that may not be rescued with transplant.

P281] Choice of reference dose value for dose plan comparison procedures using gafchromictm EBT3 and film QA software

Purpose In VMAT patient-specific quality assurance procedures using Gafchromic™ EBT3 film dosimetry and FilmQA software, to eliminate scan-to-scan variability and uncertainty therefrom, EBT3 films exposed to each VMAT arc are coupled (and digitalized in a single scan) to two reference films: one film exposed to a known dose and one unexposed film. The aim of this work is give indications on how to choose reference dose value based on some experimental evidence. Methods 6MV, 10MV and 10FFF photon beams were used to obtain calibration dose curves from 0 to 25 Gy and to study energy dependence on Gafchromic™ EBT3 film dosimetry digitized with Epson 10000 XL and analyzed with FILM QA software. Triple-channel method was used to improve film dosimetry accuracy. 16 VMAT treatment plans (8 conventionally fractionated and 8 SBRT) calculated with Eclipse™ TPS and delivered with TrueBeam 2.5 were imported as DICOM files in FILM QA software. This data file include all plan data including dose range and isocenter. The density distribution on the plane delivered for each individual arc was calculated. The choice of the reference know dose value was made on the basis of such density distribution. This allows to obtain a linear dose scaling on the calibration curve and therefore a more accurate dose map. Pre-treatment verifications were quantified using the global gamma index approach with 3%/3 mm criteria and 95% passing rate. Results The selection criteria identified were two: (1) the reference dose value is within the range 70% −90% of the cumulative distribution density; (2) the value of reference dose is closely near of the maximum distribution density value. For conventionally fractionated plans the average passing rate was 98.8% and for SBRT plans the average was 97.6%. Conclusion This approach allowed to establish a single criterion of choice for reference know dose value to eliminate scan-to-scan variability and uncertainty therefrom also for different types of treatment: conventionally fractionated and SBRT. Under the experimental conditions described above, the use of EBT3 film dosimetry analyzed with FilmQA software shows to be an accurate method for VMAT conventionally fractionated and SBRT treatment plans verifications.