Daily isocenter correction with electromagnetic-based localization improves target coverage and rectal sparing during prostate radiotherapy.

PURPOSE: To evaluate dosimetric consequences of daily isocenter correction during prostate cancer radiation therapy using the Calypso 4D localization system.

METHODS AND MATERIALS: Data were analyzed from 28 patients with electromagnetic transponders implanted in their prostates for daily target localization and tracking. Treatment planning isocenters were recorded based on the values of the vertical, longitudinal, and lateral axes. Isocenter location obtained via alignment with skin tattoos was compared with that obtained via the electromagnetic localization system. Daily isocenter shifts, based on the isocenter location differences between the two alignment methods in each spatial axis, were calculated for each patient over their entire course. The mean isocenter shifts were used to determine dosimetric consequences of treatment based on skin tattoo alignments alone.

RESULTS: The mean SD of the percentages of treatment days with shifts beyond 0.5 cm for vertical, longitudinal and lateral shifts were 62% 28%, 35% 26%, and 38% 21%, respectively. If daily electromagnetic localization was not used, the excess in prescribed dose delivered to 70% of the rectum was 10 Gy and the deficit in prescribed dose delivered to 95% of the planning target volume was 10 Gy. The mean isocenter shift was not associated with the volumes of the prostate, rectum, or bladder, or with patient body mass index.

CONCLUSIONS: Daily isocenter localization can reduce the treatment dose to the rectum. Correcting for this variability could lead to improved dose delivery, reduced side effects, and potentially improved treatment outcomes.