Interfraction and intrafraction setup variability for prone breast radiation therapy.

PURPOSE

To report the interfraction and intrafraction setup variation for prone breast radiotherapy and to determine an appropriate clinical tumor volume (CTV) to planning target volume (PTV)_ margin to account for motion and positional uncertainties.

METHODS AND MATERIALS

Ten consecutive patients were prospectively enrolled in a protocol of accelerated, hypofractionated prone breast irradiation. Portal images were acquired using an electronic portal imaging device in cine mode. Interfraction setup error was determined by comparing the first image from each fraction with the digitally reconstructed radiograph. The intrafraction motion was determined by evaluating every image acquired during each fraction and measuring the maximum displacement of an external fiducial and the breast surface. Mean values and 95% confidence intervals (CI) were calculated. Based on these results, a CTV to PTV expansion was derived using the equation M = 2.5Sigma(tot) + 0.7sigma(tot.)

RESULTS

The mean interfraction setup variability for the fiducial was 0.08 cm (CI: 0.02-0.14) in the anterior to posterior (AP) direction and -0.04 cm (CI: -0.07-0.00) in the superior to inferior (SI) direction. The mean interfraction variability of the breast surface was -0.14 cm (CI: -0.24 to -0.04) in the AP direction. The mean intrafraction displacements of the fiducial and the breast surface were 0.13 cm (CI: 0.12-0.15) and 0.15 cm (CI: 0.14-0.17), respectively. Using the systematic and random errors for the external fiducial, the calculated CTV to PTV expansion was 1.4 cm.

CONCLUSIONS

Acceptable interfraction and intrafraction variability were demonstrated. The findings resulted in a CTV to PTV expansion of 1.4 cm.