Intra-fractional set-up and organ motion errors in intensity-modulated radiation therapy for prostate cancer

The aim of this study is to investigate various intra-fractional errors and to determine the appropriate planning target volume (PTV) margins in intensity modulated radiation therapy (IMRT) for prostate cancer. Ten patients with prostate cancer treated with IMRT between July 2009 and March 2010 were analyzed. PTV was created by adding 4 mm posterior and 7 mm anterior and lateral margins to the clinical target volume (CTV) including prostate and proximal seminal vesicles. Intra-fractional set-up and organ motion errors were measured using cone beam computed tomography (CBCT) images before and after each irradiation. Systematic and random errors were calculated by van Herk and Stroom's models. Intra-fractional errors of set-up and organ motion were 0.70±0.84 mm and 0.88±0.95 mm in the left-right (L-R), 1.04±0.98 mm and 1.69±1.58 mm in the cranial-coudal (C-C), and 1.08±1.01 mm and 1.91±1.58 mm in the anterior-posterior (A-P) directions, respectively. The errors in the C-C and A-P were significantly larger than those in the L-R (p<0.01). The organ motion errors in the C-C and A-P were significantly larger than the set-up errors (p<0.01). The appropriate PTV margin estimated in this study was 4.73 mm. Intra-fractional errors in all directions were less than 2 mm and required PTV margin in the study was similar to actual posterior margin in our routine practice. It is important to determine intra-fractional errors as well as inter-fractional errors to deliver successful IMRT for prostate cancers. (author)