Evaluation of safety margins for cone beam CT-based adaptive prostate radiotherapy

Details

Supervisors

Dechambre, David ; Sterpin, Edmond

Degree label

Master [120] en sciences physiques, à finalité spécialisée: physique médicale

Abstract

Purpose/objective Adaptive radiotherapy is characterized by the use of a daily imaging system, such as CBCT (Cone-Beam Computed Tomography) images to re-optimize the treatment plan before the delivery of each fraction based on the daily anatomy and position of the patient. By systematically re-delineating the Clinical Target Volume (CTV) and adapting the treatment at each fraction, target delineation uncertainty features a random component instead of a pure systematic one in a non-adaptive workflow. The goal of this work is to identify and characterize all sources of errors in CBCT-based adaptive workflow and compute a new relevant PTV (Planning Target Volume) safety margin. Material/methods A total of 169 radiotherapy sessions from 10 prostate cancer patients treated on the Varian ETHOS treatment system have been analyzed. For each of them, the CBCT images and the RTSTRUCT file containing the CTV contours were used to compute the systematic and random contributions of, respectively, the intrafraction motion and the delineation errors. For the later, intra-patient and inter-patient variabilities were computed in six directions, by considering the prostate as a rigid, non-rotating volume. By doing so, we were able to directly compare the delineations done by the physicians on daily CBCT images with the initial delineation done on the CT-sim with the help of an MRI, and sort the directions using the polar coordinates of every point of each contour. The computed variabilities were then corrected for the prostate shape variations based on literature data (Deurloo et al. [2005]) and quadratically added to the random and systematic margin contributions, respectively. For machine-specific errors, the daily machine performance checks reports were used to evaluate the setup error and the convolution product of a parabola and a Gaussian function was used to fit the measured dose profile of the beam at 10 cm in water in order to determine the penumbra contribution. Results A penumbra of 2.8mm and a systematic setup error of 0.3mm have been measured. The intrafraction motion error had a systematic contribution of 0.8±0.3, 1.7±0.6 and 0.8±0.4mm for the left-right, anteroposterior and cranocaudal directions, respectively, and a random contribution of 1.2, 2.5 and 1.3mm for the same directions. These results were obtained with time intervals ranging from 10 to 40 minutes. Due to an actual treatment time of only less than 5 minutes, a relatively small population of patients and intrafraction errors significantly greater than what can be found in literature (McPartlin et al. [2016]), we made the choice to use literature data for the intrafraction motion error to compute the total margin. Finally, the addition of the systematic and random contributions of the delineation uncertainty gives a total margin of 3.8, 3.7, 3.9, 3.7, 6.4 and 4.8mm in the left, right, anterior, posterior, cranial and caudal directions, respectively, which represents a reduction of 11 to 40% compared to the margins of 4.8, 6.1 and 7.3mm computed with a fully systematic delineation uncertainty of 1.7, 2 and 2.5mm in the LR, AP and CC directions, respectively. Conclusion According to our results, the gain offered by the separation of the delineation uncertainty into systematic and random contributions thanks to the adaptive delineation process justify a reduction of the isotropic PTV margin from 7mm to 5mm. It could even be safely lowered down to 4mm in the left, right, anterior and posterior directions.