Accounting for respiratory motion in partial breast intensity modulated radiotherapy during treatment planning: a new patient selection metric.
OBJECTIVE
External beam partial breast irradiation intensity modulated radiotherapy (PBI IMRT) plans experience degradation in coverage and dose homogeneity when delivered during respiration. We examine which characteristics of the breast and seroma result in unacceptable plan degradation due to respiration.


METHODS
Thirty-six patient datasets were planned with inverse-optimised PBI IMRT. Population respiratory data were used to create a probability density function. This probability density function (PDF) was convolved with the static plan fluences to calculate the delivered dose with respiration. To quantify the difference between static and respiratory plan quality, we analysed the mean dose shift of the target dose volume histogram (DVH), the dose shift at 95% of the volume and the dose shift at the hotspot to 2 cm(3)of the volume. We explore which patient characteristics indicate a clinically significant degradation in delivered plan quality due to respiration.


RESULTS
Dose homogeneity constraints, rather than dosimetric coverage, were the limiting factors for all patient plans. We propose the dose evaluation volume-to-planning target volume (DEV-to-PTV) ratio as a delineating metric for identifying patient plans that will be more degraded by respiratory motion. The DEV-to-PTV ratio may be a more robust metric than ipsilateral breast volume because the seroma volume is contoured more consistently between physicians and clinics.


CONCLUSIONS
For patients with a DEV-to-PTV ratio less than 55% we recommend either not using PBI IMRT or employing motion management. Small DEV-to-PTV ratios occur when the seroma is close to inhomogeneities (i.e. air/lung), which exacerbates the dosimetric effect of respiratory motion. For small breast sizes it is unlikely that the DEV-to-PTV ratio will meet these criteria.
Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada; Department of Medical Physics, Tom Baker Cancer Centre, Calgary, AB, Canada. Electronic address: sarah.quirk@gmail.com.
Sarah
S
Quirk
Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada; Department of Medical Physics, Tom Baker Cancer Centre, Calgary, AB, Canada.
Leigh
L
Conroy
Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada; Department of Medical Physics, Tom Baker Cancer Centre, Calgary, AB, Canada; Department of Oncology, University of Calgary, Calgary, AB, Canada.
Wendy L
WL
Smith
D001943:Breast Neoplasms / Q000503:physiopathology* / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal; D050397:Radiotherapy, Intensity-Modulated; D015656:Respiratory Mechanics / Q000502:physiology* / Q000528:radiation effects
Breast neoplasms (C04.588.180 and C17.800.090.500); Computer assisted (E02.950.825 and L01.700.508.100.710.600.608); Conformal (E02.815.635.700 and L01.700.500.100.710.600.550); Human (B01.050.150.900.649.801.400.112.400.400); Radiotherapy; Radiotherapy planning; Respiratory mechanics (G09.772.770.155.700)