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Internal Organ Motion Effect On Radiation Dose to a Point Under Half-Beam Block Match Line

S Zhou

S Zhou*, X Zhu , M Zhang , D Zheng , Y Lei , Q Zhang , S Li , J Driewer , S Wang , C Enke , University of Nebraska Medical Center, Omaha, NE


SU-E-T-508 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall


Half-beam block is a field matching technique frequently used in radiotherapy. With no setup error, a well calibrated linac, and no internal organ motion, two photon fields can be matched seamlessly dosimetry-wise with their central axes passing the match line. However, in actual clinical situations, internal organ motion is often inevitable. This study was conducted to investigate its influence on radiation dose to patient internal points directly under the matching line.


A clinical setting is modeled as two half-space (x<0 and x>0) radiation fields that are turned on sequentially with a time gap of integer times of the patient internal organ motion period (T₀). Our point of interest moves with patient internal organs periodically and evenly in and out of the radiation fields, resulting in an average location at x=0. When the fields are delivered without any motion management, the initial phase of the point's movement is unknown. Statistical methods are used to compute the expected value () and variance (σ) of the point dose given the uncertainty.


Analytical solutions are obtained for and σ of dose received by a point directly under the match line. is proportional to the total beam-on time (T₁), and σ demonstrates previously unknown periodic behavior. /σ has local maximums (minimums) when T₁ is 2n*T₀ ((2n+1)*T₀) (n is an integer) with an upper envelope scales as 1/T₁. Worst case scenario the point dose can be zero or double of the expected value.


We have analytically analyzed the internal organ motion effect on radiation dose received by a patient internal point directly under a half-beam block match line. Our results help us to better understand this phenomenon and facilitate the reduction of point dosimetric uncertainties in our clinical practice.

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