Dosimetric impact of interplay effect on RapidArc lung stereotactic treatment delivery

Volumetric modulated arc therapy (RapidArc; Varian Medical Systems, Palo Alto, CA) allows fast delivery of stereotactic radiotherapy for Stage I lung tumors. We investigated discrepancies between the calculated and delivered dose distributions, as well as the dosimetric impact of leaf interplay with breathing-induced tumor motion.

In 20 consecutive patients with Stage I lung cancer who completed RapidArc delivery, 15 had tumor motion exceeding 5 mm on four-dimensional computed tomography scan. Static and dynamic measurements were performed with Gafchromic EBT film (International Specialty Products Inc., Wayne, NJ) in a Quasar motion phantom (Modus Medical Devices, London, Ontario, Canada). Static measurements were compared with calculated dose distributions, and dynamic measurements were compared with the convolution of static measurements with sinusoidal motion patterns. Besides clinical treatment plans, additional cases were optimized to create excessive multileaf collimator modulation and delivered on the phantom with peak-to-peak motions of up to 25 mm. γ Analysis with a 3% dose difference and 2- or 1-mm distance to agreement was used to evaluate the accuracy of delivery and the dosimetric impact of the interplay effect.

In static mode film dosimetry of the two-arc delivery in the phantom showed that, on average, fewer than 3% of measurements had γ greater than 1. Dynamic measurements of clinical plans showed a high degree of agreement with the convolutions: for double-arc plans, 99.5% met the γ criterion. The degree of agreement was 98.5% for the plans with excessive multileaf collimator modulations and 25 mm of motion.

Film dosimetry shows that RapidArc accurately delivers the calculated dose distribution and that interplay between leaves and tumor motion is not significant for single-fraction treatments when RapidArc is delivered with two different arcs.