Prostate intrafraction motion assessed by simultaneous kilovoltage fluoroscopy at megavoltage delivery I: clinical...

To describe prostate intrafraction motion using kilovoltage fluoroscopy at treatment delivery for a hypofractionated radiotherapy protocol.

Kilovoltage images were acquired during treatment delivery, as well as pre- and posttreatment cone-beam computed tomography (CBCT) for each fraction of 30 patients, totaling 571 fractions for analysis. We calculated population statistics, evaluated correlation between interfraction and intrafraction motion, evaluated effect of treatment duration, classified whether motion resolved by posttreatment CBCT, and compared motion magnitude on a per-patient basis.

The elapsed time between pre- and post-CBCTs was (18.6 ± 4.5) min. The population mean of motion measured by kilovoltage fluoroscopy was (-0.1, 0.5, -0.6) mm, the systematic was (0.5, 1.3, 1.2) mm, and random was (0.9, 1.9, 2.0) mm in the right-left, anterior-posterior, and superior-inferior axes, respectively. The probability of motion increased with treatment duration, with the mean increasing to (0.0, 1.0, -0.9) mm and the systematic to (0.6, 1.7, 1.5) mm when measured using posttreatment CBCT. For any motion ≥2 mm, approximately 75% did not resolve by posttreatment CBCT. Motion magnitude varied considerably among patients, with the probability of a 5-mm displacement ranging from 0.0% to 58.8%.

Time dependency of intrafraction motion should be considered to avoid bias in margin assessment, with posttreatment CBCT slightly exaggerating the true motion. The patient-specific nature of the intrafraction motion suggests that a patient-specific management approach may be beneficial.