Set–up Error for Prostate Cancer Patients in Intensity–modulated Radiation Therapy with Pelvis Mask

Abstract

             The research aimed to study the set–up error in positioning for intensity–modulated radiation therapy (IMRT) with pelvic mask in prostate cancer patients to obtain an appropriate value of CTV to PTV margin for radiation therapy in Sakon Nakhon hospital. The set–up error values from patient positioning with pelvic mask for IMRT in prostate cancer patients were retrospectively recorded by comparing anteroposterior (AP) and lateral (LAT) images from an electronic portal imaging device (EPID) and digital reconstructed radiographs (DRRs). The recorded values were used to calculate CTV to PTV margin using Van Herk equation. The samples were 574 EPID and 82 DRRs images obtained from 41 prostate cancer patients.

               The results of the range of set–up error values in the craniocaudal, lateral, and anteroposterior axis were –0.6 to 0.9 cm, –0.6 to 0.6 cm, and –0.7 to 0.4 cm, respectively. The average set–up error and standard deviation in the craniocaudal, lateral, and anteroposterior axis were 0.08 ± 0.26, 0.00 ± 0.17, and –0.03 ± 0.16 cm, respectively. For all three axes, the median and quartile range was 0.0 ± 0.2 cm. The calculated CTV to PTV margins of the craniocaudal, lateral, and anteroposterior axis was 0.47, 0.24, and 0.21 cm, respectively. In conclusion, the set–up errors in positioning for IMRT with pelvic mask in prostate cancer patients were –0.7 to 0.9 cm in all three axes, and the CTV to PTV margins were within the recommended range from ICRU No.62.                             Keywords: Setup error, Intensity–modulated radiotherapy, Prostate cancer