Abstract
The aim is to compute all sources of geometrical uncertainty in prostate radiotherapy using fiducial markers and determine the safety treatment margins. Based on the markers position, correlations between prostate rotation/deformation and rectal and bladder fillings as well as changes in prostate volume during the treatment course are analyzed. The study includes 375 pre-treatment CBCT images from 15 prostate cancer patients treated with hypofractionated radiotherapy. The position coordinates of the markers were obtained from each image acquisition. In addition, rectum and bladder were outlined on CBCTs. The intrafractional error was estimated by an additional post-treatment CBCT acquired on alternate days. Tau-Kendall analysis was performed to correlate organ fillings with prostate rotation/deformation. Delineation uncertainty was assessed from contours of 10 patients performed by two radiation oncologists and repeated twice. The CT contouring was assisted by a multiparametric MR approach combining a T2-weighted with diffusion-weighted imaging, and a gradient recalled echo for fiducial marker identification. Uncertainty associated to treatment unit was estimated from phantom measurements. The obtained clinical margins were 4.4, 7.3, 5.1 mm in the Left-Right, Superior-Inferior, and Anterior-Posterior directions, respectively, being the contouring the most important contribution. The mechanical limitations of the beam delivery system and the associated imaging device entailed errors of the same order as prostate motion, rotation or deformation. Weak correlations between variation of the rectal volume and the presence of rotations/deformations were found (correlation coefficient 0.182, p = 0.001 for rotations around lateral axis; correlation coefficients 0.1, p < 0.05 for deformations). The distance between markers decreased with session number, becoming more pronounced from fraction 13 and reaching 1 - 1.8 mm at the end of the treatment. In summary we have determined the optimal treatment margins based on geometrical uncertainty assessment using van Herk formalism. An appropriate preparation of rectum and bladder involves minimizing the effect of prostate rotations/deformations. The prostate tends to decrease in size during the treatment which could influence treatment re-planning strategies.
Highlights
Radiotherapy (RT) is currently a routine modality in the treatment of prostate cancer
The aim is to compute all sources of geometrical uncertainty in prostate radiotherapy using fiducial markers and determine the safety treatment margins
The study includes 375 pre-treatment Cone Beam CT (CBCT) images from 15 prostate cancer patients treated with hypofractionated radiotherapy
Summary
Radiotherapy (RT) is currently a routine modality in the treatment of prostate cancer. Randomized trials evidence the benefit of higher radiation doses in clinical outcomes improving local control and reducing the risk of biochemical failure for prostate cancer patients. This increment in the delivered dose may increase the incidence of urinary and rectal complications. Throughout the radiotherapy process the patient’s geometry is subject to numerous sources of uncertainty that make this objective difficult to achieve. In order to ensure the effectiveness of treatment, it is necessary to add a margin to the initial volume CTV to take into account these uncertainties. Different methods have been described to correlate the geometrical uncertainty and the treatment margin that are usually synthesized in margin recipes [3] [4]
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More From: International Journal of Medical Physics, Clinical Engineering and Radiation Oncology
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