Intra- and interfractional reproducibility of tangential breast fields: a prospective on-line portal imaging study

A perception exists that weekly verification films accurately reflect the setup of the tangential breast portals. This prospective study was undertaken to assess patient movement during treatment and setup reproducibility of tangential breast fields using electronic on-line portal imaging.

Thirteen patients with carcinoma of the breast were treated on a linear accelerator equipped with an on-line portal imaging system. Patients were immobilized daily with an alpha cradle. The medial and lateral tangential fields were imaged and 139 fractions, 225 portal fields, and 4450 images were obtained. Images were then analyzed off line and 22,250 measurements were made from these images. Anatomical features recorded include the lung area (LA), central lung distance (CLD), central breast distance (CBD), central flash distance (CFD), and inferior central margin (ICM). Intrafractional variations were calculated for every portal field and fraction for each patient. Interfractional variations were determined by finding the variance of intrafractional means for each patient. A population standard deviation for each of the five parameters for intra- and interfractional variations were determined. The simulation to treatment setup errors were calculated for all five variables.

Lung area variation was 1.50 and 4.19 cm(2) [1 standard deviation (SD)] for intra- and interfractional movement. Intrafractional variation for the other four variables ranged from 0.85 mm for ICM to 2.1 mm (1 SD) for CBD, while interfractional variations ranged from 3.2 to 6.25 mm for CBD and ICM, respectively. The simulation-to-treatment setup variation was greater than the interfractional variation for three of the five variables and was similar for the other two.

On-line verification of intrafractional variation shows a moderate deviation from the treatment setup position for all five parameters studied, while interfractional variation showed even greater deviations for these five parameters. To cover the breast target in 95% of cases, margins of 7.70, 7.70, and 10.30 mm corresponding to the CLD, CFD, and ICM distances, respectively, are required.