Background: Image guided radiotherapy (IGRT) is an essential pre-requisite for delivering high precision\nradiotherapy. We compared daily variation detected by two non-ionizing imaging modalities (surface imaging and\ntrans-abdominal ultrasound, US) to verify prostate patient setup and internal organ variations.\nMethods: Forty patients with organ confined prostate cancer and candidates to curative radiotherapy were\nenrolled in this prospective study. At each treatment session, after laser alignment, all patients received imaging by\na 3D-surface and a 3D-US system. The shifts along the three directions (anterior-posterior AP, cranial-caudal CC, and\nlater-lateral LL) were measured in terms of systematic and random errors. Then, we performed statistical analysis on\nthe differences and the possible correlations between the two modalities.\nResults: For both IGRT modalities, surface imaging and US, 1318 acquisitions were collected. According with Shapiro\nWilk test, the positioning error distributions were not Gaussian for both modalities.\nThe differences between the systematic errors detected by the two modalities were statistically significant only in LL\ndirection (p < 0.05), while the differences between the random errors were not statistically significant in any directions.\nThe 95% confidence interval of the residual errors obtained by subtracting the random errors detected with surface\nimages to those detected with US was included in the range from âË?â??7 mm to 7 mm corresponding to the minimum\nPTV margin adopted in AP direction in our clinical routine.\nConclusions: From our data, it emerges that setup misalignments measured by surface imaging can be predictive of\nUS displacements after the adjustment for systematic errors. Moreover, surface imaging can detect setup errors\npredictive of registration errors measured by US. This data suggest that the two IGRT modalities could be considered\nas complementary to each other and could represent a daily ââ?¬Å?low-costââ?¬Â and non-invasive IGRT modality in prostate\ncancer patients.
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