QUALITY ASSURANCE USING PORTAL IMAGING - THE ACCURACY OF PATIENT POSITIONING IN IRRADIATION OF BREAST-CANCER

Purpose: To study the accuracy of patient positioning in irradiation of breast cancer. Methods and Materials: Megavolt portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 17 breast cancer patients immobilized with plastic fixation masks on a flat board with arm support and in 14 patients positioned without a mask on either a flat or a wedge-shaped board. Quantitative analysis of 510 megavolt portal images and comparison to 66 digitized simulation films was performed. Differences between the positioning techniques were evaluated. Results: For the position of the patient in the field, standard deviations of the difference between simulation and treatment images were 3.2 mm and 4.6 mm for irradiation with and without masks, respectively. Larger standard deviations were found for the field width and length (5-7 mm), for collimator rotation (1.5-2-degrees), and for the position of the lung shielding block for patients positioned on the flat board (10-16 mm). The changes in field size and collimator rotation appeared to be largely due to the inclination of the technologists to slightly adapt fields in order to obtain a seemingly better congruity of the field with the skin or mask markings. Comparison of the accuracy of patient positioning with and without masks yielded similar error rates; standard deviations and extremes tended to be somewhat larger in positioning without a mask. The wedge-shaped board was preferred because of the ease of patient set-up and because the use of a lung block is avoided. The transition from simulation to treatment setup yielded larger deviations than repeated treatment set-ups. Conclusion: These results emphasize again the continuous need for focusing attention on the accuracy of patient positioning in order to achieve maximal precision in radiotherapy. The electronic portal imaging system is very suitable for both quick on-line treatment verification and off-line analyses.