Purpose: Stereotactic ablative radiotherapy (SAbR) is emerging as a non-invasive ventricular tachycardia (VT) ablation regimen and has attained initial success of reducing refractory VT. 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 consideration of an optimized, multi-stage treatment planning strategy to mitigate radiation risk to adjacent organs at risk (OARs).
Methods: The rationale for multi-staged SAbR-VT is to divide an otherwise very large single-fraction treatment volume into sub-volumes allowing adjacent normal tissues to experience more tolerable fractionated radiation thereby minimizing the overall OARs biological effective dose (BED) and toxicity. The N-staged planning problem is stated: given a set of sub-targets and their respective prescription doses, and treatment sequence, find treatment plans such that each plan delivers the prescription dose p to target T in a single treatment session, and the total dose of all plans satisfies the OAR dose constraints. We formulate this problem as a constrained optimization. The objective of the constrained optimization is to ensure PTV coverage at prescription dose and minimize the weighted sum of OARs’ BED. We use alpha beta ratio as 2 to estimate OAR BED reduction.
Results: One of the VT cases we treated has PTV of 188.2cc. In a 1-stage treatment plan, the maximum doses to stomach and esophagus are 20.46Gy and 21. 57Gy, respectively. Under a 3-stage treatment strategy, the biologically-equivalent 1-stage doses to stomach and esophagus are estimated as 14.85Gy and 15.63Gy constituting 28 percent reduction.
Conclusion: Clinical investigation is required to confirm our modelled predictions regarding toxicity reduction with multi-stage SAbR-VT and characterize the success of the approach in controlling arrhythmia.