Using Respiratory Motion to Guide Planning Target Volume Margins for External Beam Partial Breast Irradiation

Abstract

Although toxicity associated with external-beam accelerated partial breast irradiation (APBI) has been reported to be acceptable in some studies, others have shown higher rates of fibrosis and suboptimal cosmesis using the fractionation schedule of 385 cGy twice daily to 3850 cGy. Given that this fractionation schedule may be close to the limits of normal tissue toxicity, any reduction in irradiated normal tissue volume that can be achieved without compromising target coverage may have meaningful clinical significance. Recent studies have explored the use of image guidance to reduce the planning target volume (PTV)margin required to accommodate setup errors. There have also been many recent studies of respiratory motion in patients treated with APBI in the supine position (1–19), and it may be time to reevaluate the PTV margin required to accommodate respiratory motion as well. The 19 studies summarized in the following table report a wide range of target motion that, at its extremes, spans a whole order of magnitude. Harris et al. (19) reported the use of four-dimensional computed tomography (4D-CT) to track gold fiducial markers placed at the periphery of the surgical bed in 15 patients and found that ‘‘the average intrafraction respiration induced fiducial motion was 0.8 mm.’’ By contrast, Yue et al. reported an average CTV motion of 8.5 mm in 4D-CTs of 4 APBI patients (8). Of the 19 studies, the ‘‘take-home’’ motion magnitude was 5 mm in 4 studies. One early study on the larger end of the spectrum is that by Baglan et al. (1). Evaluating surgical clip positions in CTs of 16 patients acquired at normal inspiration and expiration breath-holds, they reported a mean motion of 6 mm (range, 3–9 mm). From this, they suggested that ‘‘a conservative margin of 5 mm around the CTV would completely account for breast motion during quiet respiration in all patients.’’ (Note that if a target is localized to its midventilation posi-