David Sjöström

The feasibility of using Pareto fronts for comparison of treatment planning systems and delivery techniques

Pareto optimality is a concept that formalises the trade-off between a given set of mutually contradicting objectives. A solution is said to be Pareto optimal when it is not possible to improve one objective without deteriorating at least one of the other. A set of Pareto optimal solutions constitute the Pareto front. The Pareto concept applies well to the inverse planning process, which involves inherently contradictory objectives, high and uniform target dose on one hand, and sparing of surrounding tissue and nearby organs at risk (OAR) on the other. Due to the specific characteristics of a treatment planning system (TPS), treatment strategy or delivery technique, Pareto fronts for a given case are likely to differ. The aim of this study was to investigate the feasibility of using Pareto fronts as a comparative tool for TPSs, treatment strategies and delivery techniques. In order to sample Pareto fronts, multiple treatment plans with varying target conformity and dose sparing of OAR were created for a number of prostate and head & neck IMRT cases. The DVHs of each plan were evaluated with respect to target coverage and dose to relevant OAR. Pareto fronts were successfully created for all studied cases. The results did indeed follow the definition of the Pareto concept, i.e. dose sparing of the OAR could not be improved without target coverage being impaired or vice versa. Furthermore, various treatment techniques resulted in distinguished and well separated Pareto fronts. Pareto fronts may be used to evaluate a number of parameters within radiotherapy. Examples are TPS optimization algorithms, the variation between accelerators or delivery techniques and the degradation of a plan during the treatment planning process. The issue of designing a model for unbiased comparison of parameters with such large inherent discrepancies, e.g. different TPSs, is problematic and should be carefully considered. fc

A beam-matching concept for medical linear accelerators

The flexibility in radiotherapy can be improved if a patient can be moved between any one of the departments medical linear accelerators without the need to change anything in the patients treatment plan. For this to be possible, the dosimetric characteristics of the various accelerators must be the same, or nearly the same i.e. the accelerators must be beam-matched. During a period of nine months, eight Varian iX accelerators with 6 and 15 MV photon beams and 6–18 MeV electron beams (only four of the eight) were installed at our clinic. All accelerators fulfilled the vendor-defined “fine beam-match” criteria, and a more extensive set of measurements was carried out during commissioning. The measured absorbed dose data for each accelerator were compared with the first accelerator, chosen as reference, and the TPS calculations. Two of the eight accelerators showed a larger discrepancy for the 15 MV beam not revealed by the vendor-defined acceptance criteria, whereas the other six accelerators were satisfactorily matched. The beam-matching acceptance criteria defined by the vendor are not strict enough to guarantee optimal beam-match. Deviations related to dose calculations and to beam-matched accelerators may add up. The safest and most practical way to ensure that all accelerators are within clinical acceptable accuracy is to include TPS calculations in the evaluation. Further, comparisons between measurements and calculations should be done in absolute dose terms.

Evaluation of setup accuracy for NSCLC patients; studying the impact of different types of cone-beam CT matches based on whole thorax, columna vertebralis, and GTV

Abstract Purpose. The aim of this study is to evaluate the patient setup accuracy by investigating the impact of different types of CBCT matches, performed with 3 (translations only) or 6 (including rotations) degrees-of-freedom (DOF). The purpose is also to calculate and compare CTV to PTV margins based on the various CBCT matches, setups using 2D kV planar imaging or setups using skin markers only (non-IGRT). Material and methods. Setup images from 16 NSCLC patients with weekly CBCT and daily 2D kV planar imaging were analyzed retrospectively. The CBCT matches were based on the columna vertebralis (CV), the whole thorax (WT) and the soft tissue (ST) delineated GTV, where the ST match was chosen as reference. Thus the translational and rotational shifts in three dimensions were assessed. Finally, setup margins were calculated using van Herks margin recipe. Results. For 80% of the investigated 3 DOF/2D kV CV setups, the translational shifts were within [−3, 2] mm for all three directions. Corresponding values for the 6 DOF/non-IGRT CV and the 6 DOF/non-IGRT ST matches were [−5, 8] mm. Furthermore, 80% of all setups were within ± 2° for pitch-, roll- and yaw-rotations, and none exceeded 5°. The calculated margins for non-IGRT, about 10 mm, were reduced to approximately 4 mm, regardless of using IGRT setup by CBCT or 2D kV imaging on CV. However, if using WT CBCT setup, the margin in LNG direction was slightly larger, approximately 6 mm. Conclusion. IGRT for NSCLC is an essential tool for margin reduction, since patient setups based on IGRT leads to approximately half the margin sizes compared to non-IGRT setups. Both CBCT and 2D kV planar imaging yields approximately the same margins for CV/ST matches. The magnitudes of the patient rotations were <5°.

Introducing multiple treatment plan-based comparison to investigate the performance of gantry angle optimisation (GAO) in IMRT for head and neck cancer

Abstract Background and purpose. The purpose of this study was to evaluate the performance of gantry angle optimisation (GAO) compared to equidistant beam geometry for two inverse treatment planning systems (TPSs) by utilising the information obtained from a range of treatment plans. Material and methods. The comparison was based on treatment plans generated for four different head and neck (H&N) cancer cases using two inverse treatment planning systems (TPSs); Varian Eclipse™ representing dynamic MLC intensity modulated radiotherapy (IMRT) and Oncentra® Masterplan representing segmented MLC-based IMRT. The patient cases were selected on the criterion of representing different degrees of overlap between the planning target volume (PTV) and the investigated organ at risk, the ipsilateral parotid gland. For each case, a number of ‘Pareto optimal’ plans were generated in order to investigate the trade-off between the under-dosage to the PTV (VPTV,D < 95%) or the decrease in dose homogeneity (D5-D95) to the PTV as a function of the mean absorbed dose to the ipsilateral parotid gland (parotid gland). Results. For the Eclipse system, GAO had a clear advantage for the cases with smallest overlap (Cases 1 and 2). The set of data points, representing the underlying trade-offs, generated with and without using GAO were, however, not as clearly separated for the cases with larger overlap (Cases 3 and 4). With the OMP system, the difference was less pronounced for all cases. The Eclipse GAO displays the most favourable trade-off for all H&N cases. Conclusions. We have found differences in the effectiveness of GAO as compared to equidistant beam geometry, in terms of handling conflicting trade-offs for two commercial inverse TPSs. A comparison, based on a range of treatment plans, as developed in this study, is likely to improve the understanding of conflicting trade-offs and might apply to other thorough comparison techniques.

Quantitative image quality evaluation of pelvic computed tomography-based imaging systems: A novel concept in radiotherapy

Different cone beam computed tomography (CBCT) systems are commercially available for radiotherapy (RT). The image quality of these systems differs as a consequence of different hardware and software. However, image quality assessment based on images of phantoms and related physical measures usually cannot tell us whether we should prefer one system over the other, in a specifi c clinical setting. On the other hand, subjective, nonquantitative evaluations based on the preferences of individual observers are often not suffi cient either. Therefore, a quantitative evaluation method based on clinically relevant features of images of actual patients is called for. A method called visual grading characteristics (VGC) analysis has been developed by B å th and colleagues [1,2]. The VGC is a non-parametric, rank-invariant statistical analysis which can be used for quantitative evaluation of the difference in image quality between two imaging systems. To our knowledge, VGC has never been applied in the fi eld of RT. The aim of this study was to test VGC analysis as a tool to evaluate clinically relevant image quality differences between CT-based imaging systems used in RT. Furthermore, the aim was to give a preliminary indication of whether one of two specifi c CBCT systems should be preferred when used for visualization of the bladder in RT [3].

POSTER: PHYSICS TRACK: IMAGING: FOCUS ON CLINICAL APPLICATIONSPO-0865: Quantitative clinical image quality comparison of pelvic CBCT for two imaging systems

Results: The DIR performs well visually and the sum of squares for the difference between the deformed CTs and the real CTs ranged between 4x10 and 5x10. For comparison, the difference between four volumes in which only noise generates differences between the images, gave a comparable SQD of around 3.7 x 10. For all the image pairs, the SQD of the deformed CTs is approximately half of that of the non-registered images (ranging from 0.45 to 0.59). Conclusions: A limitation of an US based deformation field is that the area of the CT on which one can perform the deformation field is limited to the area of which US data is available. However, this phantom experiment does show that the application of such an US based CT DIR in principle could work. Whether the US based CT DIR is also valid for patient cases, has to be studied further with patient examples.

EVALUATION OF GANTRY ANGLE OPTIMIZATION FOR IMRT TREATMENT PLANNING SYSTEMS USING THE PARETO FRONT APPRO1ACH

Introduction; The aim of this study was to examine the possibilities for MRI-based radiotherapy treatment planning. The work included evaluation of geometric image distortions in MR-images, investigations of how segmentation of bone could be performed from images acquired with the ultra short time of echo (UTE) pulse sequence and assessment of dosimetric differences between treatment planning based on these images as compared to the original CT-based treatment plans.Methods: A phantom with a well-defined rod geometry was used for the studying of geometric distortions in the imaging plane in MR-images. The ability of the UTE pulse sequence to visualize bone and other tissues with short T2-relaxation, was investigated by imaging the head of four healthy volunteers and bone, air and soft tissues were segmented from these images. The dosimetric accuracy expected from such segmented images, as well as from images where the head was treated as one homogenous volume, was compared to the gold standard of using CT-images. The dosimetric evaluation involved nine patients with existing conformal treatment plans and three patients with existing intensity modulated treatment plans.Results: The geometric distortions in the imaging plane were in most cases less than the 2 mm demand in radiotherapy within the investigated volume and with the vendor’s distortion correction applied. Segmentation was feasible using the UTE pulse sequence although some areas were misclassified. For conformal radiotherapy, dose calculations based on the segmented images showed a very close resemblance to the original CT-based plans in regard to dose volume histogram (DVH), dose to planning target volume (PTV) and dose to organ at risk (OAR) although approximating the head as one homogenous volume gave clinically acceptable deviations as well. This trend was also seen for the intensity modulated radiotherapy (IMRT) plans with the same optimization as in the original treatment plan however, making a new optimization based on the derived plans resulted in large deviations in DVH and dose to OAR, possibly a result from misclassifications in the segmentation.Conclusion: Adding the UTE pulse sequence to the T1W- and the T2W-pulse sequences used when imaging patients obviously result in a prolonged scan time (of about 10 min) but has the advantage of making a more accurate treatment planning possible and could lead to MRI being used even in more complicated cases however, more patients are needed for evaluation. To completely fulfill the aim of this study, scanning a patient with both modalities is necessary since it could reveal the performance of the segmentation method and enable a dosimetric evaluation of the MRI-images to be conducted.