Realistic respiratory motion margins for external beam partial breast irradiation.
OBJECTIVE
Respiratory margins for partial breast irradiation (PBI) have been largely based on geometric observations, which may overestimate the margin required for dosimetric coverage. In this study, dosimetric population-based respiratory margins and margin formulas for external beam partial breast irradiation are determined.


METHODS
Volunteer respiratory data and anterior-posterior (AP) dose profiles from clinical treatment plans of 28 3D conformal radiotherapy (3DCRT) PBI patient plans were used to determine population-based respiratory margins. The peak-to-peak amplitudes (A) of realistic respiratory motion data from healthy volunteers were scaled from A = 1 to 10 mm to create respiratory motion probability density functions. Dose profiles were convolved with the respiratory probability density functions to produce blurred dose profiles accounting for respiratory motion. The required margins were found by measuring the distance between the simulated treatment and original dose profiles at the 95% isodose level.


RESULTS
The symmetric dosimetric respiratory margins to cover 90%, 95%, and 100% of the simulated treatment population were 1.5, 2, and 4 mm, respectively. With patient set up at end exhale, the required margins were larger in the anterior direction than the posterior. For respiratory amplitudes less than 5 mm, the population-based margins can be expressed as a fraction of the extent of respiratory motion. The derived formulas in the anterior/posterior directions for 90%, 95%, and 100% simulated population coverage were 0.45A/0.25A, 0.50A/0.30A, and 0.70A/0.40A. The differences in formulas for different population coverage criteria demonstrate that respiratory trace shape and baseline drift characteristics affect individual respiratory margins even for the same average peak-to-peak amplitude.


CONCLUSIONS
A methodology for determining population-based respiratory margins using real respiratory motion patterns and dose profiles in the AP direction was described. It was found that the currently used respiratory margin of 5 mm in partial breast irradiation may be overly conservative for many 3DCRT PBI patients. Amplitude alone was found to be insufficient to determine patient-specific margins: individual respiratory trace shape and baseline drift both contributed to the dosimetric target coverage. With respiratory coaching, individualized respiratory margins smaller than the full extent of motion could reduce planning target volumes while ensuring adequate coverage under respiratory motion.
Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta T2N 4N2, Canada and Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
Leigh
L
Conroy
Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta T2N 4N2, Canada and Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
Sarah
S
Quirk
Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta T2N 4N2, Canada; Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada; and Department of Oncology, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
Wendy L
WL
Smith
D001940:Breast / Q000528:radiation effects*; D005260:Female; D006801:Humans; D009068:Movement; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012119:Respiration