Nicolas Ploquin

Phantom evaluation of a commercially available three modality image guided radiation therapy system

The authors describe a detailed evaluation of the capabilities of imaging and image registration systems available with Varian linear accelerators for image guided radiation therapy (IGRT). Specifically, they present modulation transfer function curves for megavoltage planar, kilovoltage (kV) planar, and cone beam computed tomography imaging systems and compare these with conventional computed tomography. While kV planar imaging displayed the highest spatial resolution, all IGRT imaging techniques were assessed as adequate for their intended purpose. They have also characterized the image registration software available for use in conjunction with these imaging systems through a comprehensive phantom study involving translations in three orthogonal directions. All combinations of imaging systems and image registration software were found to be accurate, although the planar kV imaging system with automatic registration was generally superior, with both accuracy and precision of the order of 1 mm, under the conditions tested. Based on their phantom study, the attainable accuracy for rigid body translations using any of the features available with Varian equipment will more likely be limited by the resolution of the couch readouts than by inherent limitations in the imaging systems and image registration software. Overall, the accuracy and precision of currently available IGRT technology exceed published experience with the accuracy and precision of contouring for planning.

A comparison of techniques for simulating set-up error and uncertainty in head and neck IMRT

We have compared four computational methods for quantifying the effect of set-up error and uncertainty on delivered doses to targets and organs at risk in the intensity modulated radiation therapy treatment of head and neck cancer. These four methods were direct simulation, simple convolution, plus two modified convolution approaches. Discrepancies of up to 20% in the equivalent uniform dose (EUD) between direct simulation and simple convolution were estimated for the relatively superficial parotid gland at a systematic set-up error of 6 mm standard deviation and a random uncertainty of 2 mm standard deviation. Truncated convolution agreed with direct simulation to within 6% for all situations studied. However, of the four methods, only direct simulation can quantify the range of outcomes (EUD) associated with a finite number of courses and fractions. Our results are particularly relevant to the design of dose escalation studies in head and neck cancer.

Evaluation of linear accelerator performance standards using an outcome oriented approach.

Radiation therapy, along with other branches of medicine, is moving towards a firmer basis in evidence to optimally utilize resources. As new treatment technology and strategies place greater demands on quality assurance resources, the need to objectively evaluate equipment and process performance standards from the perspective of predicted clinical impact becomes more urgent. This study evaluates the appropriateness of recommended quality control tolerance and action levels for linear accelerators based on the calculated dosimetric impact of suboptimal equipment performance. A method is described to quantify the dosimetric changes, as reflected by the changes in the outcome surrogate, equivalent uniform dose (EUD), of machine performance deviations from the optimal, specifically in the range of tolerance and action levels promulgated by the Canadian Association of Provincial Cancer Agencies (CAPCA). Linear accelerator performance deviations were simulated for the treatment of prostate, breast, lung, and brain using 3D conformal techniques, and the impact evaluated in terms of the changes in the EUD of the target volumes and two principal organs at risk (OARs) per site. The eight key performance characteristics examined are: Output constancy, beam flatness, gantry angle, collimator angle, field size indicator, laser alignment (three directions) and, by inference, the optical distance indicator. Currently accepted CAPCA tolerance levels for these eight performance characteristics are shown to maintain average EUD deviations to within 2% for the targets and 2 Gy for the OARs. However, within the 2% or 2 Gy range, the recommended tolerance levels are found to have markedly different effects on the EUDs of the structures of interest.

Sci-Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques - 05: A novel respiratory motion simulation program for VMAT treatment plans: a phantom validation study

Purpose: To develop and validate a computational method to simulate craniocaudal respiratory motion in a VMAT treatment plan. Methods: Three 4DCTs of the QUASAR respiratory motion phantom were acquired with a 2cm water-density spherical tumour embedded in cedar to simulate lung. The phantom was oscillating sinusoidally with an amplitude of 2cm and periods of 3, 4, and 5 seconds. An ITV was contoured and 5mm PTV margin was added. High and a low modulation factor VMAT plans were created for each scan. An in-house program was developed to simulate respiratory motion in the treatment plans by shifting the MLC leaf positions relative to the phantom. Each plan was delivered to the phantom and the dose was measured using Gafchromic film. The measured and calculated plans were compared using an absolute dose gamma analysis (3%/3mm). Results: The average gamma pass rate for the low modulation plan and high modulation plans were 91.1% and 51.4% respectively. The difference between the high and low modulation plans gamma pass rates is likely related to the different sampling frequency of the respiratory curve and the higher MLC leaf speeds in the high modulation plan. A high modulation plan has a slower gantry speed and therefore samples the breathing cycle at a coarser frequency leading to inaccuracies between the measured and planned doses. Conclusion: A simple program, including a novel method for increasing sampling frequency beyond the control point frequency, has been developed to simulate respiratory motion in VMAT plans by shifting the MLC leaf positions.

Poster - 21: Verification of Monitor Unit Calculations for Breast Field-In-Field Three-Dimensional Conformal Radiotherapy Plans

Purpose: To determine the source of systematic monitor unit (MU) calculation discrepancies between RadCalc and Eclipse treatment planning software for three-dimensional conformal radiotherapy field-in-field breast treatments. Methods: Data were reviewed for 28 patients treated with a field-in-field breast technique with MU calculations from RadCalc that were larger than MU calculations from Eclipse for at least one field. The distance of the calculation point from the jaws was measured in each fields beams-eye-view and compared with the percentage difference in MU (%ΔMU) between RadCalc and Eclipse. 10×10, 17×13 and 20×20 cm2 beam profiles were measured using the Profiler 2 diode array for 6-MV photon beams and compared with profiles calculated with Eclipse and RadCalc using a gamma analysis (3%, 3 mm). Results: The mean %ΔMU was 1.3%±0.3%. There was a statistically-significant correlation between %ΔMU and the distance of the calculation point from the Y jaw (r=−0.43, p<0.001). RadCalc profiles differed from measured profiles, especially near the jaws. The gamma pass rate for 6-MV fields of 17×13 cm2 field size was 95%±1% for Eclipse-generated profiles and 53%±20% for RadCalc-generated profiles (p=0.01). Conclusions: Calculations using RadCalc for field-in-field breast plans resulted in MUs that were larger than expected from previous clinical experience with wedged plans with calculation points far from the jaws due to the position of the calculation point near the jaws in the beams-eye-view of each field.

MO-F-CAMPUS-T-05: Correct Or Not to Correct for Rotational Patient Set-Up Errors in Stereotactic Radiosurgery

Purpose: To quantify the effect of patient rotation in stereotactic radiation therapy and establish a threshold where rotational patient set-up errors have a significant impact on target coverage. Methods: To simulate rotational patient set-up errors, a Matlab code was created to rotate the patient dose distribution around the treatment isocentre, located centrally in the lesion, while keeping the structure contours in the original locations on the CT and MRI. Rotations of 1°, 3°, and 5° for each of the pitch, roll, and yaw, as well as simultaneous rotations of 1°, 3°, and 5° around all three axes were applied to two types of brain lesions: brain metastasis and acoustic neuroma. In order to analyze multiple tumour shapes, these plans included small spherical (metastasis), elliptical (acoustic neuroma), and large irregular (metastasis) tumour structures. Dose-volume histograms and planning target volumes were compared between the planned patient positions and those with simulated rotational set-up errors. The RTOG conformity index for patient rotation was also investigated. Results: Examining the tumour volumes that received 80% of the prescription dose in the planned and rotated patient positions showed decreases in prescription dose coverage of up to 2.3%. Conformity indices for treatments with simulated rotational errors showed decreases of up to 3% compared to the original plan. For irregular lesions, degradation of 1% of the target coverage can be seen for rotations as low as 3°. Conclusions: This data shows that for elliptical or spherical targets, rotational patient set-up errors less than 3° around any or all axes do not have a significant impact on the dose delivered to the target volume or the conformity index of the plan. However the same rotational errors would have an impact on plans for irregular tumours.

Po-Thur Eve General-02: Set-up error simulations for head and neck IMRT

We have compared four computational methods for quantifying the effect of set‐up error and uncertainty on delivered doses to targets and organs at risk in the IMRT treatment of head and neck cancer. These four methods were direct simulation, simple convolution plus two modified convolution approaches: the corrected convolution and the truncated convolution proposed by our group. Discrepancies of up to 20% in the Equivalent Uniform Dose (EUD) between direct simulation and simple convolution were estimated for the relatively superficial parotid gland at a systematic set‐up error of 6mm standard deviation and a random uncertainty of 2mm standard deviation. Truncated convolution agreed with direct simulation to within 6% for all situations studied. However, of the four methods, only direct simulation can quantify the range of outcomes (EUD) associated with a finite number of courses and fractions. Our results are particularly relevant to the design of dose escalation studies in head and neck cancer.

TH‐D‐ValA‐09: An Objective Approach to Establishing Tolerances On Photon Beam Modeling Using the Equivalent Uniform Dose

Purpose: To perturb a photon beam model in a controlled manner and to examine the consequences for the Equivalent Uniform Doses (EUD) of the target and organs at risk in external beam radiation therapy of the prostate. Method and Materials: We have developed seven similar but different therapy beam models in the Pinnacle® Treatment Planning System. One model generates beam data close to the golden data provided by Varian® and serves as the reference model for this study. The six other models are modifications of the reference model designed to result in controlled deviations of a particular region of the dose profiles (descending depth dose, build‐up, horns, tail, penumbra and field width). We have analyzed the consequences of planning with these perturbed models on the quality of 4 prostate treatment plans in terms of the EUDs of the PTV, rectum and bladder in comparison with the reference model. Monitor units were kept constant for all plans. Results: to maintain a change in the EUD to the prostate, bladder or rectum of less than 2%, tolerances on the various regions of the dose profiles are as follows: descending depth dose 2%; horns 3%; field width ±1mm. Deviations in the build‐up region and tail of ±10% and ±5% respectively did not change the EUDs of any structure by more than 2%. Conclusion: Currently accepted tolerances on photon beam modeling are broadly internally consistent in so far as they result in similar effects on plan dosimetry, at least for 4 field conformal prostate treatments.

Po‐Thur Eve General‐04: Squeezing a balloon: the co‐dependences of calculated photon beam characteristics on model parameters

Modeling photon beams in modern treatment planning systems is an iterative procedure as the model parameters generally influence the behaviour of more than one feature of the computed beam characteristics. In this work we explore the co‐dependence of the calculation of specified regions of beam profiles on the adjustable source parameters in Pinnacle®. The regions chosen are those used by Venselaar in recommending tolerances to be applied during the modeling process. A reference model, at 15MV, was modified in a controlled fashion so as to primarily influence one region of either the depth dose or cross beam profile at 10cm depth in a 20×20cm2 field. The effect on the five other profile regions was examined thus illuminating the degree of co‐dependence of the different regions on the model parameters. The regions of interest were: build‐up, descending depth dose, horns, tail, high and low penumbra. Our study suggests a sequence which could enhance efficiency in modeling measured photon beam data.

MO‐D‐T‐6E‐03: IMRT Vs. 3D‐CRT for Oropharyngeal Cancer: Relative Sensitivity to Set‐Up Uncertainty

Purpose: To compare the impact of set‐up uncertainty on compliance with the objectives and constraints of the RTOG H‐0022 protocol using an IMRT plan versus a conventional 3D‐CRT plan. Method and Materials: Two treatment plans (7 beam IMRT and 4 beam 3D‐CRT) were created using Pinnacle® for the same volumetric data set based on the objectives and constraints defined in the RTOG H0022 protocol. Dose volume constraints for the targets and organs at risk (OARs) were met and matched as closely as possible in both plans. Monte‐Carlo based simulations of set‐up uncertainty were performed in three orthogonal directions for “simulated courses” incorporating systematic and random uncertainties. A population based approach was used to compare the IMRT and 3D‐CRT plans in terms of Dose‐Volume Histograms (DVHs) and Equivalent Uniform Doses (EUDs) Results: Based on DVH and EUD data, the compliance of the delivered treatment with the objectives defined for the CTV66 and CTV54 shows considerably greater sensitivity to set‐up uncertainty for the IMRT plan than for the 3D‐CRT. Three of the OARs defined in this study (larynx, spinal cord, and brainstem) continue to meet the criteria in the presence of set‐up uncertainties for both plans. Dose constraints for the mandible were not met for the 3D‐CRT and neither was parotid sparing possible. The static IMRT plan was able to meet the criteria for parotid sparing. However, even at relatively low levels of set‐up uncertainty, parotid sparing was compromised in the IMRT protocol. Conclusion: The IMRT plan target doses are more sensitive to set‐up uncertainty than the 3D‐CRT when looking at both the DVHs and EUDs. In the presence of reported levels of set‐up uncertainty, parotid sparing is compromised in the IMRT plan. However, parotid doses always remain lower than those seen with the 3D‐CRT plan.