Phase I Feasibility Study of Prostate Cancer Radiotherapy Gating Using kV Intrafraction Monitoring (KIM)

Prostate cancer now accounts for one third of all new cancer diagnoses in men and approximately 30% of men will have external beam radiotherapy as their primary local therapy. Prostate motion during radiotherapy can be divided into interfraction and intrafraction motion. Interfraction motion has been well established and has been largely overcome by daily online image verification with either ultrasound, online CT or implanted fiducial markers, however motion during the radiation beam on time (intrafraction motion) is not corrected and can be the cause of significant errors in radiation dose delivery.

Intrafraction motion: Movement of the prostate after initial treatment setup has been termed intrafraction motion. Estimates of the magnitude and frequency of this motion were initially made using continuous Magnetic Resonance (MR) imaging. Padhani et al reported 16% of patients had > 5mm anterior:posterior motion when imaged for 7 minutes with similar results reported in subsequent MR cine studies, Intrafraction motion can be secondary to organ motion such as bladder filling, respiration or moving rectal gas, or can be due to physical patient motion. With the availability of real-time prostate tracking, clinical data is available to quantify the magnitude and frequency of motion. An early report from Kupelian et al using continuous radiotransponder positioning7 described 41% of fractions with >3mm of motion and 15% > 5mm of motion for > 30 seconds. The risk of motion was noted to increase with longer treatment time.

Our own data using offline autosegmentation of the fiducial marker position of 10 patients showed 38% of fractions >1mm, 4.7% of fractions >3mm and 1.7% of fractions >5mm instantaneously during treatment delivery of approximately 2.5 min 8.

Significance of correcting for motion:

If radiation dose is recalculated for each individual fraction and adjusted for intrafraction motion it is possible to estimate the real dose delivered to the target and compare this to the desired dose. This comparison gives a robust model of the potential benefit for real- time tracking and adjustment for the motion. Overview of kilovoltage intrafraction monitoring (KIM): Kilovoltage intrafraction monitoring is a novel real-time tumour localisation modality. It involves a single gantry- mounted kV x-ray imager (which is widely available on most linacs) acquiring 2D projections of implanted fiducial markers. As the treatment gantry rotates around the patient during treatment , the kV imager acquires 2D projections of the prostate . The fiducial markers are segmented using an in-house developed software package. 3D positions are determined via maximum likelihood estimation (MLE) of a 3D probability density.