HIGH-PRECISION PROSTATE-CANCER IRRADIATION BY CLINICAL-APPLICATION OFAN OFFLINE PATIENT SETUP VERIFICATION PROCEDURE, USING PORTAL IMAGING

Purpose: To investigate in three institutions, The Netherlands Cancer Institute (Antoni van Leeuwenhoek Huis [AvL]), Dr. Daniel den Heed Can cer Center (DDHC), and Dr, Bernard Verbeeten Institute (BVI), how much the patient setup accuracy for irradiation of prostate cancer can be improved by an offline setup verification and correction procedure, us ing portal imaging. Methods and Materials: The verification procedure consisted of two stages. During the first stage, setup deviations were measured during a number (N-max) of consecutive initial treatment ses sions. The length of the average three dimensional (3D) setup deviatio n vector was compared with an action level for corrections, which shru nk with the number of setup measurements. After a correction was appli ed, N-max measurements had to be performed again. Each institution cho se different values for the initial action level (6, 9, and 10 mm) and N-max (2 and 4). The choice of these parameters was based on a simula tion of the procedure, using as input preestimated values of random an d systematic deviations in each institution. During the second stage o f the procedure, with weekly setup measurements, the AvL used a differ ent criterion (''outlier detection'') for corrective actions than the DDHC and the BVI (''sliding average''). After each correction the firs t stage of the procedure was restarted. The procedure was tested for 1 51 patients (62 in AvL, 47 in DDHC, and 42 in BVI) treated for prostat e carcinoma. Treatment techniques and portal image acquisition and ana lysis were different in each institution. Results: The actual distribu tions of random and systematic deviations without corrections were est imated by eliminating the effect of the corrections. The percentage of mean (systematic) 3D deviations larger than 5 mm was 26% for the AvL and the DDHC, and 36% for the BVI. The setup accuracy after applicatio n of the procedure was considerably improved (percentage of mean 3D de viations larger than 5 mm was 1.6% in the AvL and 0% in the DDHC and B VI), in agreement with the results of the simulation. The number of co rrections (about 0.7 on the average per patient) was not larger than p redicted. Conclusion: The verification procedure appeared to be feasib le in the three institutions and enabled a significant reduction of me an 3D setup deviations. The computer simulation of the procedure prove d to be a useful tool, because it enabled an accurate prediction of th e setup accuracy and the required number of corrections.