Abstract
<h3>Purpose/Objective(s)</h3> Stereotactic ablative radiotherapy (SAbR) is emerging as a non-invasive ventricular tachycardia (VT) ablation treatment regimen (SAbR-VT) and has attained initial success of reducing VT that is refractory to drugs and standard catheter ablation. However, a single-fraction biological potent dose (25 Gy) ablating a large planning target volume (PTV, median 98.9cc, range 60.9–298.8cc in ENCORE-VT trial) has high risk of radiation toxicity. We propose a multi-stage SAbR-VT treatment planning strategy to mitigate radiation risk. <h3>Materials/Methods</h3> The principle of multi-staged SAbR-VT is to divide a large treatment volume into multiple stages with each stage irradiating a sub-target while minimizing the overall organ at risk (OARs) biological effective dose (BED)and toxicity. The N-staged planning problem is stated: given a set of sub-targets {<b>T<sub>1</sub>, ..., T<sub>N</sub></b>}, and their respective prescription doses {<b>p<sub>1</sub>, ..., p<sub>N</sub></b>}, and treatment sequence {<b>t<sub>1</sub>, ..., t<sub>N</sub></b>}, finding treatment plans {<b>x<sub>1</sub>, ..., x<sub>N</sub></b>} such that each plan <b>x<sub>n</sub></b> delivers the prescription dose <b>p<sub>n</sub></b> to target <b>T<sub>n</sub></b> in a single treatment session, and the total dose of all plans satisfies the OAR dose constraints C<i><sub>k</sub></i> of the <b>k</b>th OAR for all OARs {O<b><sub>1</sub>, ..., O<sub>K</sub></b>}. We formulate this problem as a constrained optimization. Let <b>d<sub>Tn</sub></b>(<b>x<sub>n</sub></b>) denote the dose function of target <b>T<sub>n</sub></b> of plan <b>x<sub>n</sub></b> and <b>d<sub>Ok</sub></b>(<b>x<sub>n</sub></b>)denote the OAR dose function of OAR <b>O<sub>k</sub></b> of plan, the constrained optimization problem can be written as min <b>f =</b> ∑w<sub>k</sub>·BED({d<sub>Ok</sub>},{t<sub>n</sub>}) s.t. <b>d<sub>Tn</sub></b>(<b>x<sub>n</sub></b>)≥ <b>p<sub>n</sub>,</b> ∀<sub>n</sub>∈{1,..., N} and ∑d<sub>Ok</sub>(<b>x<sub>n</sub></b>)≤ <b>C<sub>k</sub>,</b> ∀<sub>k</sub>∈{1,..., k} The objective <b>f</b> is the weighted sum of OARs' BED, where is the weighting factor <b>w<sub>k</sub></b> for the <b>k</b>th OAR. BED of OAR is the function of both physical dose distribution {<b>d<sub>Ok</sub></b>} and treatment sequence {<b>t<sub>n</sub>}</b>. <h3>Results</h3> One of the VT cases we treated has PTV of 188.2cc. In a single-stage treatment plan, the maximum doses to stomach and esophagus are 20.46Gy and 21. 57Gy. Under a 3-stage treatment strategy, the single-stage-equivalent-doses to stomach and esophagus are estimated as 14.85Gy and 15.63Gy, respectively, which is a decrease of 28 percent. <h3>Conclusion</h3> A multi-staged SAbR-VT planning framework has the potential to significantly reduce OAR BED and consequently radiation toxicity.
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