Author: Alanah M. Bergman, Marc W Deyell, Tania Karan, Jakob Marshall, Justin Poon, Devin Schellenberg, Steven Thomas, Richard Thompson π¨βπ¬
Affiliation: University of British Columbia, University of Alberta, BC Cancer π
Purpose: A conservative approach to account for random errors due to intra-fraction cardiac and respiratory motion during cardiac radioablation (CR) is to define a margin equal to the amplitude of cardiorespiratory motion. The validity of this approximation and the alternative of using a margin formula is evaluated.
Methods: 7350 cardiorespiratory motion traces were generated by combining (i) cardiac motions from 30 Hz cine magnetic resonance images of 50 patients and (ii) respiratory motion traces of the implantable cardioverter defibrillator leadβs tip from 7 patients imaged using 5 Hz bi-planar fluoroscopy. All patients had reduced ejection fractions. The impacts of cardiac, respiratory, and cardiorespiratory motion were assessed by blurring a dose profile (Gaussian dose fall-off with Οp = 3.2 mm) with the motions. Since CR is predominantly a single fraction treatment, margins accounting for random errors were computed as the shift between the planned and motion blurred profiles at the 95% dose level. Margins for cardiorespiratory motion were compared to (i) a linear sum of cardiac and average respiratory motion amplitudes (half the peak-to-peak motion) and (ii) the margin formula (1.64(Ο-Οp)) derived by van Herk et al.
Results: Linear addition of cardiac and respiratory amplitudes significantly (pβ€0.05) overestimated the margin needed to account for cardiorespiratory motion in CR by [2.2Β±0.7 right-left, 2.6Β±0.9 ant-post, 2.6Β±1.0 inf-sup] mm. Margins calculated with the blurring method were similar to using the margin formula with average differences of [0.0Β±0.1, 0.0Β±0.2, 0.0Β±0.6] mm with variations observed due to irregular patient breathing.
Conclusion: Linear addition of margins accounting for the amplitude of cardiac and respiratory motion overestimates the needed margins for CR leading to excessive healthy tissue irradiation. Further work is needed to quantify and include systematic errors, crucial in the single fraction CR treatment, into margin determination.