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In image guided radiation therapy (IGRT) using implanted fiducial marker by two-dimensional radiography for prostate cancer, temporal positional relationship during treatment between the isocenter and the prostate is changed by respiratory phase at the time of image acquisition. We examined influence of the respiratory phase in the IGRT on dose variation by interplay effect. Intra-fractional prostate motions of patients who were implanted fiducial marker were measured using fluoroscopy, then we reconstructed plans considering for the respiratory phase in IGRT and the respiratory motion during volumetric modulated arc therapy. Averages of the intra-fractional prostate motion in left-right, anterior-posterior and superior-inferior direction were 0.039, 0.49 and 1.6 mm respectively. There was a patient whose intra-fractional prostate motion was larger than 4 mm that was planning target volume margin. By changing the respiratory phase like inspiration, exhalation and dispersing respiratory phase in each fraction, dose variation from original plan became smaller in order of the inspiration, exhalation and dispersion. The largest variations of dose indices in clinical target volume, bladder and rectum were 8.0%, 4.5% and 9.1% respectively when IGRT was done in inspiration. When the IGRT is performed by the same respiratory phase in each fraction, systematic dose variations may occur even if the respiratory phase at the timing of irradiation is changed. By dispersing the respiratory phase in each fraction, the variations in all dose indices were&lt;1% from original plan. We realized that dispersing the respiratory phase in IGRT by each fraction is effective to reduce the dose variation caused by the respiratory phase in IGRT.

Influence of Respiratory Phase during Image Acquisition on Prescribed Dose in Image Guided Radiation Therapy Using Implant Marker for Prostate Cancer