Maximum Dose Difference Research Articles

SU-E-T-517: Ray Tracing and Mote Carlo Based Dose Computation for Chest Wall and Ribs for Lung Cancer Patients Treated with Cyberknife

Purpose: Chest wall and ribs are considered organs at risk while treating lung tumors adjacent to chest wall. This study was to evaluate the dosimetric differences in chest wall and ribs irradiation for Cyberknife stereotactic body radiation therapy (SBRT) when treatment plans were computed using Ray Tracking Algorithm (RTA) and Monte Carlo Algorithm (MCA). Methods: Twenty eight lung cancer patients who had tumors abutting the chest wall were selected for this study. A uniform 2cm thickness of chest wall and appropriate portions of rib were contoured for evaluating dose distribution using RTA and MCA of Cyberknife MultiPlan software (version‐3.5.2). Treatment plans were first computed using RTA and then were recomputed using MCA without changing the total monitor unit (MU). Range of prescription dose was 50Gy‐60Gy (mean=54.6Gy). Studied dosimetric parameters were maximum point dose and doses to 1cc, 5cc, 10cc, 30cc, 50cc, 70cc, and 100cc of chest wall. Maximum point dose and doses to 1cc, 2cc, 5cc, and 10cc of ribs were also studied. These parameters were considered as clinically more relevant. Results: Dosimetric computation results revealed that RTA overestimated dose to both the chest wall and the ribs for all the studied parameters. Increase of average dose was in the range of 7.5–12.1% (p‐value 0.05), and 10.3–13.4% (p‐value <0.05) for chest wall and ribs, respectively. The lower the volume of interest, the larger was the difference in dose for the two algorithms. Average difference in maximum point dose for chest wall was 11.2% (p‐value <0.001) and that was 13.4% for ribs (p‐value <0.002). Conclusion: As compared to Monte Carlo algorithm, Ray Tracing algorithm significantly overestimates dose to chest wall and ribs. Although treatment planning with Monte Carlo is more involved, it should be used because of accuracy and significant difference in dose distribution in chest wall and ribs.

TH‐C‐WAB‐11: MRI‐Based Treatment Planning with Pseudo CT Generated Through Atlas Registration

Purpose: To develop a magnetic resonance imaging (MRI)‐based treatment planning method without the need of acquiring patient‐specific computed tomography (CT) dataset. Methods: A pseudo CT, providing electron density information for dose calculation, was generated by deforming atlas CT images of other patients. Two approaches, arithmetic averaging and Gaussian process regression (Hofmann 2008), were used to combine the deformed images into a single dataset. The required deformation was derived from registering corresponding atlas MR images to those of the subject of interest. We tested this method on 7 patients aged 8–12 years with common pediatric brain tumors. We used a leave‐one‐out procedure, i.e., the 6 pairs of MR/CT images other than those of each patient of interest served as atlas images. The original patient‐specific CT was replaced with the pseudo CT and the dose distribution was recalculated to quantify the difference. We used Chi‐evaluation (Bakai 2003) for dose distribution comparison. Doses to the planning target volume (PTV) and organs at risk (OARs) were compared. Results: The pseudo CT generated by arithmetic averaging showed a high similarity to the patient‐specific CT (correlation coefficient, 0.75–0.85). The calculated dose distribution was in close agreement with original dose. Nearly the entire patient volume (98.4 – 99.8%) satisfied the criteria of Chi‐evaluation (&lt;3mm and 3% maximum dose). Dose indices of V95 and D95 in PTV differed by up to 2.2%. The average differences in maximum and mean doses to OARs ranged from 0.3% to 1.0%. The pseudo CT by Gaussian process showed similar performances for the images included in this study (P=0.27, Chi‐evaluation) despite longer computation time. Conclusion: MR‐based treatment planning with pseudo CT generated through atlas registration is feasible for pediatric brain tumor patients. The doses calculated from pseudo CT agreed well with those from patient‐specific CT.

SU‐E‐T‐164: Comparing Measurement Derived (3DVH) and Machine Log File Derived Dose Reconstruction Methods for VMAT QA in Heterogeneous Patient Geometries

Purpose: To extend the 3DVH analysis to heterogeneous patient geometries for VMAT delivery and comparing its accuracy using machine log file derived dose reconstruction method Methods: A total of 10 patient plans were selected from a regular fractionation plan to complex SBRT plans. Treatment sites in the lung and abdomen were chosen to explore the effects of tissue heterogeneity on the respective dose reconstruction algorithms. Delivered plan in the patient geometry was reconstructed by using ArcCheck Planned Dose Perturbation (ACPDP) within 3DVH software, and by converting the machine logfile to Pinnacle3 9.0 treatment plan format. In addition, delivered gantry angles between machine logfile and 3DVH 4D measurement was also compared to evaluate the accuracy of the virtual inclinometer within the 3DVH. Results: Measured ion chamber and 3DVH derived isocenter dose agreed with planned dose within 0.3±0.7 Gy and −0.5±0.8 Gy respectively. Machine log file reconstructed doses and TPS dose agreed to within 2 Gy in PTV and OARs over the entire treatment course. 3DVH reconstructed dose showed a difference of up to 3.2 Gy in maximum PTV doses compared to planned dose in hypo lung patients due to plan heterogeneity. For majority of normal structures, dose differences were within 1 Gy except for few cases, where a maximum point dose difference of up to 2.2 Gy in proximal bronchial tree dose for a hypo lung patient was seen. The average Virtual Inclinometer Error (VIE) was − 0.65±1.6° for all patients, with a maximum error of −5.16±4.54° for an SRS case. Conclusion: Both methods are capable of taking into account the plan delivery errors. 3DVH is more sensitive to these errors compared to machine log file.

SU-E-J-207: Dosimetric Effects of Diaphragm Motion On Spine SBRT Treatments

Purpose: To evaluate the dosimetric effects of diaphragm motion on spine SBRT treatments with IMRT and VMAT deliveries. Methods: Five 4D-CTs were acquired for lung and liver SBRT patients. The vertebral body closest to the diaphragm level was contoured as the CTV. The thecal sac was used as a surrogate for the spinal cord PRV. A 3 mm expansion was used for the PTV with the PTV cropped to the cord PRV. Plans were optimized for PTV maximal coverage at 1800 cGy and cord PRV Dmax was no more than 1200 cGy. Plans were created for 7 and 11 field IMRT and partial arc VMAT modalities, all on the average intensity CT (AvgCT) and recalculated on the 4D CT data set. D95, D1, and Davg were recorded for the different delivery methods and all 11 CT sets (AvgCT+10 phases). Dose differences were calculated for each phase versus the planning AvgCT and for the maximum difference amongst the phases to evaluate worst-case scenarios. Results: Regarding the PTV coverage, the IMRT-7 plans always exhibited the largest dosimetric differences amongst metrics with Davg having the largest range (Davg[max-min]: 63 cGy) versus the IMRT-11 (51 cGy) and VMAT (49 cGy) out of the prescription dose of 1800 cGy. All other dosimetric points had smaller differences for PTV metrics. The maximum difference always occurred between exhale and inhale phases. The cord PRV D1% had smaller differences than PTV metrics with the maximum dose difference of 44 cGy for the IMRT-7 plans, and IMRT-11 had a maximum of 17 cGy and VMAT 23 cGy. Conclusions: Clinically relevant spine SBRT plans display less than 4% maximum difference in PTV dose (Davg) and 3% variability in the cord PRV D1% metric between inhale and exhale phases for IMRT-7 plans in the worst case scenario.

Use of treatment log files in spot scanning proton therapy as part of patient-specific quality assurance

The purpose of this work was to assess the monitor unit (MU) values and position accuracy of spot scanning proton beams as recorded by the daily treatment logs of the treatment control system, and furthermore establish the feasibility of using the delivered spot positions and MU values to calculate and evaluate delivered doses to patients. To validate the accuracy of the recorded spot positions, the authors generated and executed a test treatment plan containing nine spot positions, to which the authors delivered ten MU each. The spot positions were measured with radiographic films and Matrixx 2D ion-chambers array placed at the isocenter plane and compared for displacements from the planned and recorded positions. Treatment logs for 14 patients were then used to determine the spot MU values and position accuracy of the scanning proton beam delivery system. Univariate analysis was used to detect any systematic error or large variation between patients, treatment dates, proton energies, gantry angles, and planned spot positions. The recorded patient spot positions and MU values were then used to replace the spot positions and MU values in the plan, and the treatment planning system was used to calculate the delivered doses to patients. The results were compared with the treatment plan. Within a treatment session, spot positions were reproducible within ±0.2 mm. The spot positions measured by film agreed with the planned positions within ±1 mm and with the recorded positions within ±0.5 mm. The maximum day-to-day variation for any given spot position was within ±1 mm. For all 14 patients, with ∼1 500 000 spots recorded, the total MU accuracy was within 0.1% of the planned MU values, the mean (x, y) spot displacement from the planned value was (-0.03 mm, -0.01 mm), the maximum (x, y) displacement was (1.68 mm, 2.27 mm), and the (x, y) standard deviation was (0.26 mm, 0.42 mm). The maximum dose difference between calculated dose to the patient based on the plan and recorded data was within 2%. The authors have shown that the treatment log file in a spot scanning proton beam delivery system is precise enough to serve as a quality assurance tool to monitor variation in spot position and MU value, as well as the delivered dose uncertainty from the treatment delivery system. The analysis tool developed here could be useful for assessing spot position uncertainty and thus dose uncertainty for any patient receiving spot scanning proton beam therapy.

A Monte Carlo study on dose distribution validation of GZP6 60Co stepping source

AimStepping source in brachytherapy systems is used to treat a target lesion longer than the effective treatment length of the source. Cancerous lesions in the cervix, esophagus and rectum are examples of such a target lesion. BackgroundIn this study, the stepping source of a GZP6 afterloading intracavitary brachytherapy unit was simulated using Monte Carlo (MC) simulation and the results were used for the validation of the GZP6 treatment planning system (TPS). Materials and methodsThe stepping source was simulated using MCNPX Monte Carlo code. Dose distributions in the longitudinal plane were obtained by using a matrix shift method for esophageal tumor lengths of 8 and 10cm. A mesh tally has been employed for the absorbed dose calculation in a cylindrical water phantom. A total of 5×108 photon histories were scored and the MC statistical error obtained was at the range of 0.008–3.5%, an average of 0.2%. ResultsThe acquired MC and TPS isodose curves were compared and it was shown that the dose distributions in the longitudinal plane were relatively coincidental. In the transverse direction, a maximum dose difference of 7% and 5% was observed for tumor lengths of 8 and 10cm, respectively. ConclusionConsidering that the certified source activity is given with ±10% uncertainty, the obtained difference is reasonable. It can be concluded that the accuracy of the dose distributions produced by GZP6 TPS for the stepping source is acceptable for its clinical applications.

An Integrated 2-dimensional Dosimeter and Electronic Portal Imaging Device for In Vivo Dosimetry: A Feasibility Study

Purpose/Objective(s): As radiation therapy becomes more conformal and the delivery technology becomes more complex, in-vivo dosimetry is becoming more necessary but at the same time more technically challenging. This work investigates the feasibility of a novel integrated in-vivo imaging and dosimetry device to meet the current and future requirements of radiation therapy treatment verification whereby imaging and dosimetry could be achieved concurrently. Materials/Methods: The dose response of a commercial 2-D ion-chamber array, in two configurations was investigated; (1) the experimental configuration with the ion-chamber array positioned directly under an electronic portal imaging device (EPID), (2) the standard dosimetry configuration with the ion-chamber array and 5cm solid water build-up. All measurements were conducted with the ion chamber array at isocentric distance and no additional objects in the beam. The measured dose was compared with the calculated dose from a treatment planning system (TPS). For the experimental configuration (1) the ion chamber array was positioned on the treatment couch with the EPID sitting directly on top of it. An additional 3.2 cm solid water was added to fill the air gap between the array and EPID. All the experiments were conducted at zero gantry angle. The central axis dose per Monitor unit and dose profiles for a 10x10 cm open field was measured in both configurations. The water equivalent depth of the experimental configuration (1) was determined by comparison with the TPS calculated dose. This depth was subsequently used for TPS dose calculations for further open beams, IMRT test patterns, and clinical IMRT beams. The field size factor and off-axis dose response measured with configuration (1) for open fields was compared with TPS data. The gamma analysis was performed with 2% and 3 mm criteria. Agreement between ion-chamber array and TPS was compared for both configurations. Results: The water equivalent depth of the ion chamber array in configuration (1) was 8.2 cm. The maximum dose difference between measured dose profiles with configuration (1) and TPS was 3.5% for 20x20 cm field size. The field size output factor with configuration (1) agreed within 2% with TPS. For a given set of gamma criteria the percentage of gamma pass agreed to with 0.5% for all the measurements. Conclusion: An ion chamber array integrated with an EPID maintains accurate water equivalent dose response. This configuration has the potential to provide a new technique of accurate 2D in vivo dosimetry. Further work is underway to assess the impact of the ion chamber array on imaging, and the performance of the device with an object in the beam to more closely simulate the clinical setting. Author Disclosure: S. Deshpande: None. L. Holloway: None. P. Metcalfe: None. P. Vial: None.

Partial Breast Extracranial Radiosurgery: A Dosimetric Evaluation of Treatment Delivery Accuracy

Partial breast extra-cranial radiosurgery was proposed to treat early stage breast cancer patient preoperatively. Cone-beam CT (CBCT) acquired prior to treatment represents the actual patient position during the treatment. This study evaluates dosimetric variations between the delivered plan (DP) based on CBCT and the original plan (OP) based on the planning CT. Seven patients treated to 15 Gy in the prone position were included in this study. The gross tumor volume (GTV) (volume: 0.5 cc to 2.7 cc) was defined based on fused CT and breast MR images. The clinical target volume (CTV) encompassed the GTV + a 1.5-cm isotropic margin. An additional 0.3-cm isotropic margin was added to form the planning target volume (PTV). Both CTV and PTV excluded the outermost 0.5 cm of the patient surface. A 3 mm thick region along the ipsilateral breast surface was defined as skin to analyze dose to the subcutaneous tissue. Four to five non-coplanar photon beams were arranged to avoid heart, lungs and contralateral breast and intensity modulated radiation therapy (IMRT) provided further optimization. The plans were normalized to deliver 100% of the prescription to at least 95% of the CTV. For treatment, target localization was achieved based on both skin surface and soft-tissue tumor and/or biopsy markers using CBCT. To standardize the CBCT images, the CT numbers were assigned - breast to -122 HU, lungs to -719 HU, and all others to 40 HU. The GTV, CTV, and PTV were copied to CBCT based on the target localization while the skin, breasts, and lungs were manually contoured. The delivered dose distribution was re-calculated on the CBCT. Dosimetric parameters of OP and DP were compared. The average CTV receiving 100% of the prescription (V100%) was 95.5% ± 0.4 vs 93.2% ± 5.7 (OP vs. DP). The difference in V100% between OP and DP was > 10% for 2 patients (DP ≤ OP). However, the difference in V96% between OP and DP was < 1% for all 7 patients. The average maximum dose to skin was 76.7% ± 13.9 vs. 77.4% ± 11.7. The difference in maximum skin dose between OP and DP was > 8% for 3 patients (DP ≥ OP for 2 patients and ≤ OP for 1 patient). The lung V5Gy was ≤ 1% for all patients in both OP and DP. The contralateral breast V1Gy was 8.1% vs 12.9% for 1 patient, 6 patients had V1Gy < 1% in both OP and DP. The heart V5Gy was zero in OP for all patients, and it was not delineated in CBCT due to difficulties with image quality. Partial breast radiosurgery in this cohort was delivered with a high level of accuracy to the target coverage (CTV V96% achieved for all patients within 1%). Similarly, doses to critical organs were essentially as planned. However, small differences in skin doses were seen due to the effect of set-up variation on breast shape.

A real‐time in vivo dosimetric verification method for high‐dose rate intracavitary brachytherapy of nasopharyngeal carcinoma

A real-time in vivo dosimetric verification method using metal-oxide-semiconductor field effect transistor (MOSFET) dosimeters has been developed for patient dosimetry in high-dose rate (HDR) intracavitary brachytherapy of nasopharyngeal carcinoma (NPC). The necessary calibration and correction factors for MOSFET measurements in (192)Iridium source were determined in a water phantom. With the detector placed inside a custom-made nasopharyngeal applicator, the actual dose delivered to the tumor was measured in vivo and compared to the calculated values using a commercial brachytherapy planning system. Five MOSFETs were independently calibrated with the HDR source, yielding calibration factors of 0.48 ± 0.007 cGy∕mV. The maximum sensitivity variation was no more than 7% in the clinically relevant distance range of 1-5 cm from the source. A total of 70 in vivo measurements in 11 NPC patients demonstrated good agreement with the treatment planning. The mean differences between the planned and the actually delivered dose within a single treatment fraction were -0.1% ± 3.8% and -0.1% ± 3.7%, respectively, for right and left side assessments. The maximum dose deviation was less than 8.5%. In vivo measurement using the real-time MOSFET dosimetry system is possible to evaluate the actual dose to the tumor received by the patient during a treatment fraction and thus can offer another line of security to detect and prevent large errors.

SU-E-T-392: A Software Tool of Extracting Dose Plane for Tomotherapy Delivery QA.

To develop a software tool to extract dose planes base on on the setup (red) laser positions for Tomotherapy delivery QA(DQA). We have a develop a simple tool to simplify a time-consuming step for tomotherapy DQA. The DQA analysis tools in Tomotherapy software was useful for film dosimetry but not convenient for 2D-array QA devices. Current DQA software requires the user to perform alignments twice: one for setting DQA phantom and the other for extracting dose plane. These two steps can be simplified into one if both alignments are based on the central axis of the detector plane. Once the setup (red) lasers are set for DQA, the dose plane can be extracted from the dicom dose file and the dicom plan file automatically. An open source software was written in C using dicomtk library. The accuracy of the program has been tested by comparing the extracted dose plane to the same dose plane but extracted by the manual method. The maximum dose difference is 0.5% and the mean difference is 0.2%. The differences are probably due to the variation between two manual alignments. The software extracts 2D-dose plane in a simple and automatic way. This not only cuts workload but also reduce the degree of human error because only one manual alignment is needed. A copy of this open source software is available upon request. We also hope Tomotherapy provides similar function in future upgrade. A software tool has been developed to simplify Tomotherapy DQA procedure.

SU-E-T-485: Validation of Acuros XB Dose Calculations in SBRT Lung Planning with Monte Carlo Methods.

The Acuros XB (AXB) Advanced Dose algorithm (Varian Medical Systems) represents a dramatic shift in clinical photon dose calculation methodology from pencil-beam superposition/convolution methods. Early studies evaluating the accuracy of the algorithm in lung have found good agreement with both measurement and Monte Carlo based dose calculations. In this study, a dosimetric validation of Acuros is performed for clinical SBRT lung planning cases using Monte Carlo (MC) calculations as a benchmark. MC simulations using BEAMnrc/DOSXYZnrc were carried out for 8 AXB calculated 6/10 MV arc plans delivered on a TrueBeamTM STx linac in high-dose-rate flattening filter free mode. Clinical planning constraints were applied in each case with plans normalized to achieve 95% PTV coverage. Metrics used in the evaluation include: maximum and minimum GTV/PTV dose, PTV isodose coverage, conformity and dose profile comparisons. To understand the impact of moving toward to AXB calculations in SBRT lung planning, calculations using the Analytical Anisotropic Algorithm (AAA) are presented for each plan. For both 6 and 10MV energies, consistent mean GTV dose and PTV isodose coverage was observed for AXB and MC calculations. GTV mean dose was observed to deviate by <2% for all cases. Isodose coverage for MC simulations ranged from 92%-98%. AAA was also in agreement with MC simulations within the GTV to within 2%. AXB and MC maximum and minimum PTV dose differences were larger (up to 9%) but not of clinical concern. In several cases, AXB exhibited a significant improvement in dose calculation accuracy in the lung region surrounding the GTV over AAA, particularly with lung densities < 0.1 g/cc. AcurosXB provides increased accuracy in modelling dose deposition for SBRT lung over AAA and is found to be in good agreement with MC calculations.

TH‐A‐BRA‐08: Retrospective Accuracy Estimation for Motion Compensated Robotic Radiosurgery of the Liver

Purpose: The CyberKnife™ compensates translational target motion by moving the beams synchronously. While the system was found to operate with sub‐millimeter accuracy in phantoms, determining the clinical accuracy is challenging. Measuring the delivered dose distribution inside a patient is impractical. Hence an analysis of treatment data is typically used to estimate residual errors. Methods: We implant 3‐5 fiducials for target tracking and treat liver tumors in 3‐5 fractions with 45Gy at 80% to the PTV (CTV+3mm). Patients are aligned based on X‐ray images in expiration breath hold. During delivery, X‐ray images are acquired every 60‐90s, and the translational and rotational misalignment is computed. We grouped this data into 10 respiratory phases. The mean misalignment for each phase was used to simulate the translation and rotation of the target with respect to the alignment center. The resulting dose distribution was computed and compared to the planned dose. Additionally, the quality of motion prediction was evaluated. Results: We analyzed 5 cases with a total of 17 fractions. The maximal target motion per fraction ranged from 9.2mm to 25.7mm (3D trajectory). The mean error for each patient ranged from ‐0.76/‐0.01/‐ 0.32mm to 0.35/0.17/0.10mm (Translation IS/LR/AP) and ‐0.94/‐0.82/‐2.07 degrees to 0.24/1.95/2.36 degrees (Rotation roll/pitch/yaw). The dose simulation showed point dose difference for each patient ranging from ‐ 0.10Gy to ‐0.76Gy (Mean) and ‐1.13Gy to ‐5.05Gy (Max). The resulting reduction in coverage ranged from 0.37% to 4.19% (PTV) and ‐0.43% to +0.94% (CTV). Finally, the mean prediction error over all fractions was 0.33mm. Conclusions: We demonstrated that while maximum point dose differences can be considerable, the coverage of the CTV is maintained even in the presence of substantial respiratory motion. The results indicate that the standard 3mm system uncertainty margin can account for errors due to rotation and deformation during robotic radiosurgery for tumors in the liver.

SU-E-T-618: Error Compensated Sparse Optimization for Fast Radiosurgery Treatment Planning.

Radiosurgical treatment planning requires a good approximation of the dose distribution which is typically computed on a high resolution grid. However, the resulting optimization problem is large, and leads to substantial runtime. We study a sparse grid approach, for which we estimate and compensate for the expected deviations from the bounds. We buildup an estimate of the hotspot error distribution by measuring the maximum dose deviation within a voxel for a large number of randomly generated beam configurations. This results in a conservative estimation of overdosage as a function of upper bound reduction for different grid sizes. We adjust the bounds for voxels inside the target volume (PTV) according to our estimation thus maintaining the likelihood of dose deviations within acceptable limits. The approach was applied to a prostate case, where the volumes of interest are large and close to each other. Our planning objective is a prescribed dose of 36.25 Gy to the 87% isodose. We employed constrained optimization to optimize the lower PTV bound on 2, 4, and 8mm isotropic grids. Results were computed on 1mm grid. The initial coverage was 93.7%, 92%, and 91%, and the volume exceeding the upper bound was 0.74%, 1.71%, and 9% for grid sizes of 2, 4, and 8mm, respectively. Changing the upper bound by 0.5% and 2.5% for the 4 and 8 mm grids resulted in only 0.75% and 2.2% of the volume exceeding the bound. The coverage did not change. Mean optimization times were 141.1, 22.6 and 3.4 minutes using the 2, 4 or 8mm grid, respectively. Experiments show that planning on a sparse grid can achieve comparable results with those of a high resolution grid, as long as the bounds are carefully balanced. This leads to substantially lower optimization times which facilitates interactive planning. This work was supported by the Graduate School for Computing in Medicine and Life Sciences funded by Germany’s Excellence Initiative [DFG GSC 235/1].

Biological consequences of MLC calibration errors in IMRT delivery and QA

The purpose of this work is threefold: (1) to explore biological consequences of the multileaf collimator (MLC) calibration errors in intensity modulated radiotherapy (IMRT) of prostate and head and neck cancers, (2) to determine levels of planning target volume (PTV) and normal tissue under- or overdose flagged with clinically used QA action limits, and (3) to provide biologically based input for MLC QA and IMRT QA action limits. Ten consecutive prostate IMRT cases and ten consecutive head and neck IMRT cases were used. Systematic MLC offsets (i.e., calibration error) were introduced for each control point of the plan separately for X1 and X2 leaf banks. Offsets were from - 2 to 2 mm with a 0.5 mm increment. The modified files were imported into the planning system for forward dose recalculation. The original plan served as the reference. The generalized equivalent uniform dose (gEUD) was used as the biological index for the targets, rectum, parotid glands, brainstem, and spinal cord. Each plan was recalculated on a CT scan of a 27 cm diameter cylindrical phantom with a contoured 0.6 cc ion chamber. Dose to ion chamber and 3D gamma analysis were compared to the reference plan. QA pass criteria: (1) at least 95% of voxels with a dose cutoff of 50% of maximum dose have to pass at 3 mm/3% and (2) dose to chamber within 2% of the reference dose. For prostate cases, differences in PTV and rectum gEUD greater than 2% were identified. However, a larger proportion of plans leading to greater than 2% difference in prostate PTV gEUD passed the ion chamber QA but not 3D gamma QA. A similar trend was found for the rectum gEUD. For head and neck IMRT, the QA pass criteria flagged plans leading to greater than 4% differences in PTV gEUD and greater than 5% differences in the maximum dose to brainstem. If pass criteria were relaxed to 90% for gamma and 3% for ion chamber QA, plans leading to a 5% difference in PTV gEUD and a 5%-8% difference in brainstem maximum dose would likely pass IMRT QA. A larger proportion of head and neck plans with greater than 2% PTV gEUD difference passed 3D gamma QA compared to ion chamber QA. For low modulation plans, there is a better chance to catch MLC calibration errors with 3D gamma QA rather than ion chamber QA. Conversely, for high modulation plans, there is a better chance to catch MLC calibration errors with ion chamber QA rather than with 3D gamma QA. Ion chamber and 3D gamma analysis IMRT QA can detect greater than 2% change in gEUD for PTVs and critical structures for low modulation treatment plans. For high modulation treatment plans, ion chamber and 3D gamma analysis can detect greater than 2% change in gEUD for PTVs and a 5% change in critical structure gEUD since either QA methods passes the QA criteria. For gEUD changes less than those listed above, either QA method has the same proportion of passing rate.

RapidArc patient specific mechanical delivery accuracy under extreme mechanical limits using linac log files

To assess the accuracy of RapidArc (RA) delivery for treatment machine operation near allowable mechanical limits in dynamic multileaf collimator (DMLC) leaf velocities, gantry speeds, and dose rates. Thirty RA patient plans were created for treatment of lung, gastrointestinal, and head and neck cancers on a Trilogy unit. For each patient, three RA plans were generated; one with medium MLC velocities, highest gantry speeds, and dose rates (case A); one with maximal allowable MLC leaf velocities (case B); and one with lowest gantry speeds (case C). Combinations of dose rates (140-600 MU/min), gantry speeds (2-5.4°/s), and DMLC leaf velocities (1.3-2.4 cm/s) were utilized to test the RapidArc delivery accuracy. Linac delivery log files were acquired after delivery of each plan. In-house developed software was used to read in the original RapidArc DICOM plan and update the plan to reflect the delivered plan by using the leaf position (L), gantry position (G), and MU dose values (D) extracted from the linac log files. This modified DICOM RT plan was imported back to ECLIPSE and the delivered 3D dose map recomputed. Finally, the planned and delivered 3D isodose maps were compared under three criteria to evaluate the dosimetric differences: maximum percentage dose difference, 3D gamma analysis criteria for 3%/3mm DTA, number of dose voxels having a dose difference that is greater than 1%, 2%, or 3% of the maximum dose, and their respective percentages. For the three cases indicated above, MLC leaf position discrepancies between planned and delivered values are 0.8 ± 0.2, 1.2 ± 0.2, and 0.8 ± 0.2 mm; the maximum gantry position discrepancies are 0.9° ± 0.2°, 0.9° ± 0.2°, and 0.6° ± 0.1°, and the maximum differences in delivered MU per control point are 0.2 ± 0.1, 0.2 ± 0.1, and 0.04 ± 0.01, respectively. Maximum percentage dose difference observed is 6.7%, for a case where 1 cm MLC leaves were used with high MLC leaf velocity. Maximum number (percentage) of dose voxels having a dose difference that is greater than 1%, 2%, and 3% of the maximum dose were 4761 (0.35%), 897 (0.07%), and 188 (0.01%). This also corresponds to the plan utilizing the most number of 1 cm MLC leaves. The 3D Gamma factor acceptance rates are better than 99%. This work shows that the accuracy of RapidArc delivery holds across the full range of gantry speeds, leaf velocities, and dose rates with small dosimetric uncertainties for 0.5 cm MLC leaves. However, caution should be exercised when using large MLC leaves in RapidArc. A novel technique to obtain the delivered 3D dose distributions using machine log files is also presented.

Respiratory liver motion estimation and its effect on scanned proton beam therapy

Proton therapy with active scanning beam delivery has significant advantages compared to conventional radiotherapy. However, so far only static targets have been treated in this way, since moving targets potentially lead to interplay effects. For 4D treatment planning, information on the target motion is needed to calculate time-resolved dose distributions. In this study, respiratory liver motion has been extracted from 4D CT data using two deformable image registration algorithms. In moderately moving patient cases (mean motion range around 6 mm), the registration error was no more than 3 mm, while it reached 7 mm for larger motions (range around 13 mm). The obtained deformation fields have then been used to calculate different time-resolved 4D treatment plans. Averaged over both motion estimations, interplay effects can increase the D5–D95 value for the clinical target volume (CTV) from 8.8% in a static plan to 23.4% when motion is considered. It has also been found that the different deformable registration algorithms can provide different motion estimations despite performing similarly for the selected landmarks, which in turn can lead to differing 4D dose distributions. Especially for single-field treatments where no motion mitigation is used, a maximum (mean) dose difference (averaged over three cases) of 32.8% (2.9%) can be observed. However, this registration ambiguity-induced uncertainty can be reduced if rescanning is applied or if the treatment plan consists of multiple fields, where the maximum (mean) difference can decrease to 15.2% (0.57%). Our results indicate the necessity to interpret 4D dose distributions for scanned proton therapy with some caution or with error bars to reflect the uncertainties resulting from the motion estimation. On the other hand, rescanning has been found to be an appropriate motion mitigation technique and, furthermore, has been shown to be a robust approach to also deal with these motion estimation uncertainties.

Reducing Gated IMRT Delivery Times: Dual-gated Delivery Optimization and Implementation

Reducing Gated IMRT Delivery Times: Dual-gated Delivery Optimization and Implementation

In vivo measurement of radiation dose during radiotherapy in breast cancer patients using MOSFET dosimeter

Objective The purpose of the study was to observe and analysis the actual dosage of patients with breast cancer using metal oxide semiconductor field effect transistor (MOSFET) detector.Methods First, Phantom measurements were performed to investigate dose distribution in the area of the junction in a half-field matching method and the influence of factors related to the accelerator. In vivo dose measurements were performed for patients with breast cancer to investigate the skin dose and the junction of supraclavicular-axillary field and tangential field in 6 MV X-ray beams. Results Phantom measurements showed that the relative deviation in the junction were within + 3％, and the dose distributions in the junction area depended on the matching field direction (x or y). In vivo measurement of tangential region for patients showed that, the maximum dose deviation between measurement and calculation was -30. 39％,the minimum deviation was - 18. 85％, the average dose deviation was -24. 76％. The dose deviation of tangential fields for patients with breast-conserving surgery was larger than that patients with radical surgery (t =2. 40 ,P＜0. 05), while dose deviation of supraclavicular-axillary fields was not significantly different. The average values of 15 fraction in the junction area showed more stable than one individual measurement.Conclusions It is important to real-time, in vivo measurement of radiation dose during radiotherapy in patients with breast cancer, and change treatment plan in time, to ensure the accuracy of target dose. Key words: Metal oxide semiconductor field effect transistor detector; Dosimetry; Adjacent half-fields; Phantom; Breast neoplasm

Dosimetric impact of target definitions on normal structures in head and neck cancer

Background and PurposePrevious work by our group suggests smaller target volumes may result in equivalent locoregional control for head and neck cancer. We evaluated whether smaller target volumes may also result in improved normal tissue sparing.Methods and MaterialsTen patients with Stage III-IV head and neck cancer were contoured and planned according to target definitions in RTOG 0522 in a two dose level plan (RTOG), as well as a three dose level plan, using smaller target volumes and an intermediate dose prescription (3Dose). Plans were compared for coverage of targets and sparing of normal tissuesResultsThe high dose target, elective nodal target, and total volume targeted were significantly smaller in 3Dose plans (p < 0.001). There was no difference in volume receiving 100% of each prescription level in RTOG or 3Dose plans. Mean dose to contralateral parotid, mandible, larynx, and inferior pharyngeal constrictor, and maximum dose to brainstem were significantly lower in 3Dose plans. There was no significant difference in maximum dose to spinal cord or volume of tissue not otherwise specified receiving 70 Gy.ConclusionsSmaller target volumes with the addition of an intermediate dose volume results in improved sparing of most normal tissues.

Modeling the TrueBeam linac using a CAD to Geant4 geometry implementation: Dose and IAEA‐compliant phase space calculations

To create an accurate 6 MV Monte Carlo simulation phase space for the Varian TrueBeam treatment head geometry imported from CAD (computer aided design) without adjusting the input electron phase space parameters. GEANT4 v4.9.2.p01 was employed to simulate the 6 MV beam treatment head geometry of the Varian TrueBeam linac. The electron tracks in the linear accelerator were simulated with Parmela, and the obtained electron phase space was used as an input to the Monte Carlo beam transport and dose calculations. The geometry components are tessellated solids included in GEANT4 as GDML (generalized dynamic markup language) files obtained via STEP (standard for the exchange of product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP-GDML converter. The linac has a compact treatment head and the small space between the shielding collimator and the divergent are of the upper jaws forbids the implementation of a plane for storing the phase space. Instead, an IAEA (International Atomic Energy Agency) compliant phase space writer was implemented on a cylindrical surface. The simulation was run in parallel on a 1200 node Linux cluster. The 6 MV dose calculations were performed for field sizes varying from 4 x 4 to 40 x 40 cm2. The voxel size for the 60 x 60 x 40 cm3 water phantom was 4 x 4 x 4 mm3. For the 10 x 10 cm2 field, surface buildup calculations were performed using 4 x 4 x 2 mm3 voxels within 20 mm of the surface. For the depth dose curves, 98% of the calculated data points agree within 2% with the experimental measurements for depths between 2 and 40 cm. For depths between 5 and 30 cm, agreement within 1% is obtained for 99% (4 x 4), 95% (10 x 10), 94% (20 x 20 and 30 x 30), and 89% (40 x 40) of the data points, respectively. In the buildup region, the agreement is within 2%, except at 1 mm depth where the deviation is 5% for the 10 x 10 cm2 open field. For the lateral dose profiles, within the field size for fields up to 30 x 30 cm2, the agreement is within 2% for depths up to 10 cm. At 20 cm depth, the in-field maximum dose difference for the 30 x 30 cm2 open field is within 4%, while the smaller field sizes agree within 2%. Outside the field size, agreement within 1% of the maximum dose difference is obtained for all fields. The calculated output factors varied from 0.938 +/- 0.015 for the 4 x 4 cm2 field to 1.088 +/- 0.024 for the 40 x 40 cm2 field. Their agreement with the experimental output factors is within 1%. The authors have validated a GEANT4 simulated IAEA-compliant phase space of the TrueBeam linac for the 6 MV beam obtained using a high accuracy geometry implementation from CAD. These files are publicly available and can be used for further research.