Analysis of Intrafractional Organ Motion for Patients with Prostate Cancer Using Soft Tissue Matching Image-guided Intensity-modulated Radiation Therapy by Helical Tomotherapy

Aim: To analyze an intrafractional organ motion for patients with prostate cancer using soft tissue matching by megavolt computed tomography (MVCT) images during the course of image-guided intensity-modulated radiotherapy (IGRT-IMRT) using helical tomotherapy. Patients and Methods: Data from a total of 10 patients with prostate cancer who received IGRT-IMRT were analyzed, and MVCT images were acquired before and after radiation therapy. Intra- fractional movement and PTV margins for soft tissue matching were calculated by comparing treatment planning images with 740 MVCT images for right-left (RL), superior-inferior (SI), and anteroposterior (AP) dimensions. A total of 74 Gy/37 fractions were administered. A margin to compensate for these variations was calculated using the van Herk's equation. Results: The intrafractional motion was 0.03 (−1.3 to 1.4) ±0.39 mm in the RL dimension, 0.08 (−1.8 to 0.28) ±0.73 mm in the SI dimension, and 0.52 (−1.8 to 1.8) ±0.63 mm in the AP dimension. The required PTV margin was 0.60 mm, 1.10 mm, and 0.78 mm in the RL, SI, and AP dimensions, respectively. Only one patient exhibited a deviation greater than 5 mm only once in 37 fractions (1/370=0.2%) caused by anal contraction. Conclusion: The PTV margin in soft tissue matching IGRT-IMRT by helical tomotherapy for a patient with prostate cancer was 3 mm or less, and our tentative PTV margin of 3-5 mm is sufficient for most patients, if adequate