Abstract Book

S125

ESTRO 37

Abstract text To move away from the PTV concept, the probabilistic approaches use a-priori available population or patient- specific information on target location uncertainties to provide a level of robustness ensuring tumor coverage and OAR sparing. Today the image information on inter- fraction and intra-fraction anatomical/geometrical difference acquired using devices integrated in treatment units has become of high quality, both in volumetric information (CT, CBCT, MRI) and time-resolved respiratory related motion. Consequently real-time online adaptive radiotherapy could be considered as another route to retire the PTV concept. An overview will be given on the feasibility of that idea and the challenges still to be conquered. SP-0244 IGRT from 2D to 4D, changing the verification paradigm M. Rossi 1 1 Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands Abstract text Image guidance has been an integral part of radiation oncology for many decades. Various kV imaging systems were already integrated on cobalt units in the 1950s and 60s. These systems, however, were not commercially developed for a large market. Alternatively if treatment fields had to be verified the patient was re-sent to the simulator. The mainstay of IGRT in the previous century was based on portal imaging, initially using analogue films and cassettes which had a high work load and were limited to visual comparison with simulator images. In the 80’s electronic portal imaging systems started to be developed. Initially a large number of MU were needed to provide an image of the bony anatomy and treatment field. Later the current amorphous silicon portal image devices were introduced resulting in a large reduction in MU. These 2D digital images now allowed (automatic) registration with digitally reconstructed radiographs (DRR’s) improving the accuracy and efficiency of patient setup. The MV beam only enables the visualization of high contrast objects. Verification was therefore limited to the bony anatomy leaving tumor motion relative to the bony anatomy uncorrected unless fiducial markers were implanted. The introduction of IMRT providing increased conformity, required further improvements in the accuracy of treatment delivery. In the late 90s, the linac integrated CBCT idea was introduced mounting a kV tube and detector orthogonal to the treatment beam. These 3D images provided soft tissue contrast while simultaneously reducing the imaging dose compared to portal imaging. This provided the radiation oncologists with the option of reducing treatment margins and reducing toxicity. 3D MV volume imaging became available around the same time but soft tissue remained poorer than kV imaging. The availability of daily soft- tissue contrast revealed the complexity of the day-to-day changes both in terms of organ motion and anatomical changes as well as treatment response. The next logical step was to develop 4D imaging for tumours and organs at risk moving with respiration such as the lung, liver and adrenals. 3D imaging of moving tumours results in a blurred image. 4D CBCT provides sharper images, improves registration accuracy of targets moving with large amplitudes and allows verification of both the position and amplitude of the target on a daily basis. Similarly, time resolved planar imaging enables kilo- voltage intrafraction monitoring (KIM) for verification of active motion management such as gating or tracking. Symposium: IGRT, IGART and SGRT

imaging. These error distributions are essential for shaping the dose gradients around targets correctly. One particularly well suited situation for the clinical application of CovP is that of simultaneous integrated boost (SIB) in pelvis or head-and-neck, especially to lymph nodes. Here, classic PTV-margin formulae result in a gross over-estimation of required margin around the boost volume, since they were derived for a 100-0% dose drop around the CTV, while in the SIB situation it is more likely a 100-85% dose drop. Further, the PTV-to-CTV ratio is particularly bad for small tagets, resulting in a huge amount of dose dumped in the patient for no other reason than to follow the dogma.First clinical and planning results with CovP in SIB for cervix and prostate patients indicate that substantial sparing of normal tissues, predominantly the bowel, can be achieved. Validation studies based on repeated CBCT images suggest that the loss in dose coverage of SIB volumes is marginal and occurs rarely. Treatment plans were obtained from research software or using gradually increasing dose prescriptions on a set of nested volumes to shape the dose gradients in a controlled, CovP-like fashion with conventional software. Thus, in practice CovP treatment plans boil down to a set of dose prescriptions for the PTV, that are more relaxed than usual, but are precisely adjusted to the geometrical uncertainties that are likely to be encountered. This makes CovP-based dose optimization arguably the smallest step away from strict PTV planning. SP-0242 Analysis and reporting of plan robustness F. Albertini 1 1 Paul Scherrer Institute PSI, Department of Radiotherapy, Villigen PSI, Switzerland Abstract text In conventional radiotherapy uncertainties due to motion and patient positioning are dealt with by defining adequate margins around the treated volume. For proton therapy, although for single field uniform dose (SFUD) plan, the concept pf planning target volume (PTV) can still be used, this is not valid anymore in case of highly modulated IMPT plans. This is mainly because the static dose cloud approximation (i.e. the assumption that the dose distribution in space does not change due to uncertainties) is violated (example shown in the picture). The effect of uncertainties on the (deterioration of) dose distribution has to be evaluated performing a robustness analysis. Examples on how to perform the robustness analysis will be presented (e.g. error bar distributions, dose volume histograms (DVHs) bands), also taking into account the impact of fractionation. For IMPT plans the robustness of a plan to uncertainties has to be guaranteed differently: the uncertainties should be explicitly accounted for during the optimization process. The robust optimization approach will guarantee that a plan is clinically acceptable even under the effect of specified uncertainties. However plan robustness comes often at the price of plan quality. Practically the challenge when planning is to find the optimal tradeoff between plan quality and robustness. The concept of a site specific robustness protocol (summarized by comparing the robustness of a given plan with that of a previously determined database) will be shown to be a potential tool to help the user finding the appropriate tradeoff. SP-0243 A bright future for the PTV?: gating, tracking, on-line re-planning T. Depuydt 1 1 University Hospital Gasthuisberg, Radiation Oncology, Leuven, Belgium