Comparison of electromagnetic transmitter and ultrasound imaging for intrafraction monitoring of prostate radiotherapy.

Background And Purpose: To compare two in-beam monitoring devices for prostate radiotherapy: intra-prostatic electromagnetic transmitters (EM-T) (RayPilot®, Micropos Medical) and ultrasound imaging using transperineal probe (TP-US) (Clarity®, Elekta) used concomitantly on phantom and on patients.

Materials And Methods: The phantom study evaluated accuracy in presence of known translations and rotations. Then intra-fraction motions were analyzed for 10 prostate cancer patients implanted with the EM-T 8 days before the simulation CT (171 sessions). The percent time in which the differences between the systems were 1-5 mm were scored for each direction.

Results: Experiments on phantom confirmed no interference between the systems and showed deviations of less than 0.5 mm when translations were applied progressively. In presence of rotations (5-15°), both systems displayed systematic shifts up to 6.9 and 3.8 mm for the TP-US and the EM-T, respectively. Absolute mean differences between displacements observed on patients with EM-T and TP-US were ≤0.55 mm in all directions except for one patient (≤1.77 mm). With an exception for this patient, a strong correlation was found in left-right direction: differences >2 mm were monitored less than 0.22% of the time (mean acquisition time:164 minutes) and never exceeded 5 s. Maximum differences were observed in supero-inferior direction with differences >2 mm monitored more than 6.5% of the time for 3 patients. Large prostate rotations, the presence of gas and EM-T location in the prostate may explain important differences.

Conclusion: Apart from the systematic shifts induced by the rotations, the two systems were correlated and represent feasible solutions for monitoring prostate cancer treatment.