Background: Noninvasive cardiac radioablation has been used for treatment refractory VT. However, there are no guidelines on ideal treatment planning techniques, such as beam arrangement and use of arc delivery or fixed beam delivery. This can result in substantial variability in treatment plan quality and dose to normal heart tissue (NHT), which has been shown to affect outcomes in cancer radiotherapy.
Objective: In this study we developed a standardized approach to treatment planning to improve the plan quality by minimizing the dose to NHT.
Methods: Thirteen VT patients, who previously received a single dose of 25 Gy radioablation utilizing 3 volumetric-modulated arc therapy (VMAT) technique were retrospectively re-planned using ten arcs (Ten-arcs) with optimal collimator and couch angles. For the Ten-arcs plans, both 6 MV (6FFF) and 10 MV (10FFF) flattening filter free beam energies were used. Inverse planning optimization technique was used to maximize the NHT sparing while protecting surrounding stomach, esophagus and colon etc.
Results: Ten-arcs plans showed similar target coverage (V_95%Rx=>95%), better high dose conformity (CI=1.02 vs 1.14) and faster spatial dose drop-off. Improved NHT sparing was achieved by reducing the volumes receiving 20 Gy by 39 cc and 5 Gy by 271 cc. Average NHT mean dose reduction of 2.5 (range: 0.6 - 6) Gy was observed in Ten-arc plans. Figure 1A-F compares isodose lines from 3-arcs & Ten-arcs plans and 1G shows the average volume (cc) saved by Ten-arcs plans.
Conclusion: The standardized technique used in this study showed improved NHT sparing and better dose conformity. This result may be important for reducing potential long term cardiotoxicity.