Obtaining information on intra-fraction prostate displacement in radiotherapy determined from pre- and post-treatment kV imaging

摘要

Since 2007 all patients treated with radical intent for prostate cancer at Peter MacCallum Cancer Centre have three implanted fiducial markers to facilitate reproducible daily set-up using kV on-board imaging on a linear accelerator. We collected all set-up information prospectively through a record and verify system. In order to assess the appropriateness of patient movements and determine prostate displacement during treatment a second set of two orthogonal kV images was acquired after the radiotherapy delivery. In 5778 image pairs of 184 patients the initial displacement between planning and patient set-up was found to be 4.9 +/-3.0mm as a 3D vector (mean +/- 1SD). All identified set-up differences were corrected. The post-treatment images showed a displacement of 2.2 +/- 2.0mm between pre- and post-treatment image. There was a trend towards larger displacements of the prostate during treatment with increasing time between pre-and post-treatment imaging. However, even in relatively short times there was a 5% probability that the prostate has moved more than 3.5mm. There was a large variation in average displacement between patients (range 1 +/- lmm to 6 +/- 2mm) and no apparent trends throughout the treatment course were identified. Intra-fraction displacement of the prostate gland appears to be a limiting factor when considering internal margins for radiotherapy. Given the variation between patients, a uniform set of margins for all patients may not be satisfactory when doses above 75Gy are to be delivered.