Abstract
BackgroundBreathing motion should be considered for stereotactic body radiotherapy (SBRT) of lung tumors. Four-dimensional computer tomography (4D-CT) offers detailed information of tumor motion. The aim of this work is to evaluate the influence of inhomogeneous dose distributions in the presence of breathing induced target motion and to calculate margins for motion compensation.MethodsBased on 4D-CT examinations, the probability density function of pulmonary tumors was generated for ten patients. The time-accumulated dose to the tumor was calculated using one-dimensional (1D) convolution simulations of a 'static' dose distribution and target probability density function (PDF). In analogy to stereotactic body radiotherapy (SBRT), different degrees of dose inhomogeneity were allowed in the target volume: minimum doses of 100% were prescribed to the edge of the target and maximum doses varied between 102% (P102) and 150% (P150). The dose loss due to breathing motion was quantified and margins were added until this loss was completely compensated.ResultsWith the time-weighted mean tumor position as the isocentre, a close correlation with a quadratic relationship between the standard deviation of the PDF and the margin size was observed. Increased dose inhomogeneity in the target volume required smaller margins for motion compensation: margins of 2.5 mm, 2.4 mm and 1.3 mm were sufficient for compensation of 11.5 mm motion range and standard deviation of 3.9 mm in P105, P125 and P150, respectively. This effect of smaller margins for increased dose inhomogeneity was observed for all patients. Optimal sparing of the organ-at-risk surrounding the target was achieved for dose prescriptions P105 to P118. The internal target volume concept over-compensated breathing motion with higher than planned doses to the target and increased doses to the surrounding normal tissue.ConclusionTreatment planning with inhomogeneous dose distributions in the target volume required smaller margins for compensation of breathing induced target motion with the consequence of lower doses to the surrounding organs-at-risk.
Highlights
Breathing motion should be considered for stereotactic body radiotherapy (SBRT) of lung tumors
For P105, a margin of 2 mm was needed for standard deviation of the probability density function (PDF) σPDF (AMax) = 10 mm and σPDF = 3.4 mm
If the motion (AMax and σPDF) was twice as large, the margin size increased to 6 mm
Summary
Breathing motion should be considered for stereotactic body radiotherapy (SBRT) of lung tumors. The aim of this work is to evaluate the influence of inhomogeneous dose distributions in the presence of breathing induced target motion and to calculate margins for motion compensation. The process of treatment planning and dose calculation in radiation therapy is currently based on a three-dimensional (3D) patient model. Different strategies have been developed for quantification of breathing motion [3,4,5] and for integration of the motion information into the planning and treatment workflow [6,7,8,9,10,11,12]. Precise assessment of breathing induced target motion and calculation of adequate safety margins are essential for this technique
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