Positioning of port films for radiation: variability is present

Pelvic radiation treatment demands precision and consistency in patient setup for efficacy of therapy and to limit radiation dosage to normal tissue. Despite the use of immobilization devices and positioning techniques, there is still concern for variation in daily setup. The purpose of this retrospective study was to determine the presence and degree of variation in sacral slope in 20 subjects receiving radiation therapy for pelvic malignancies.

Each of the 20 subjects received between 20 and 25 fractions of external beam radiation treatment to the pelvis. The sacral slope was measured on each of the daily port films taken prior to treatment and compared to the sacral slope angle measured on the initial treatment planning simulation digitally reconstructed radiographic imaging.

Compared to this initial imaging, the average sacral slope variation across all 20 subjects was 2.27° (± 1.43°), and the average variation among patients ranged from 1.22° to 5.09°. Variation in sacral slope across all 20 subjects from one treatment day to the next was 2.05° (± 1.47°), and ranged from 0.97° to 3.21°.

This study demonstrates that despite the rigorous use of immobilization devices, there still exists day-to-day variation in sacral slope angle between treatment days and compared to initial baseline imaging off which the treatment plan is developed. There is an on-going study at our institution with an attempt to reduce this variation by offering exercises prior to radiation.