Pacemaker Dosimetry in Head-and-Neck IMRT: Treatment Plan Versus In Vivo Dosimetry

Purpose/Objective(s): To compare pacemaker maximum doses calculated by TPS versus measurements by in vivo dosimeters in head and neck cancer patients with pacemakers located outside of the treatment volume. Materials/Methods: From July 2011 to July 2013, 6 patients with squamous cell carcinoma of the head and neck were treated with IMRT with a pacemaker device located in their left superior chest. All pacemakers were located near but outside of the planned radiation treatment volume. Each patient was treated to 70 Gy in 35 fractions to the primary tumor with bilateral cervical neck nodal volumes, including left level four. The pacemaker volume was contoured in the TPS covering the device but not including the wires from the device into the atrium or ventricles. Five out of the six patients’ treatment plans were developed for both Tomotherapy and linear accelerator based IMRT; one patient had a linear accelerator IMRT plan only. Eclipse, Tomotherapy, and Pinnacle TPS were used. Dose constraints, for treatment planning purposes, on pacemakers were set to attempt to keep the total pacemaker maximum dose less than 5 Gy. All patients with a Tomotherapy plan were treated with that plan. On the first day of treatment, optically stimulated luminescence dosimeters (OSLDs) were placed on the skin over the pacemaker with or without a lead shield with 5mm bolus underneath, as a means of in vivo dosimetry. Analysis of the maximum dose measured by in vivo dosimetry was compared to the maximum dose calculated on the TPS. OSLDs measurements included dose from image guidance: cone-beam computed tomography or megavoltage computed tomography. Results: Six patients’ plans and OSLD results were reviewed. The maximum pacemaker dose in our tomotherapy plans ranged from 1.54 3.58 Gy(mean: 2.28 Gy), the linear accelerator plans ranged from 1.76 7.1 Gy (mean: 3.89 Gy), and the OSLD doses ranged from 2.2-10.4 Gy (mean: 4.67 Gy). OSLD maximum doses were consistently measured higher than those calculated by the TPS with doses ranging from 127 468% higher. Conclusions: Using in vivo OSLD measurements as a surrogate for pacemaker dose showed TPS calculations consistently underestimate the maximum pacemaker dose. Based on these preliminary results, it is recommended that in vivo measurements be performed and compared to the TPS calculated maximum pacemaker dose in order to ensure manufacturer recommendations are not exceeded. Author Disclosure: D.A. Elliott: None. N. Nabavizadeh: None. B. Merz: None. E. Severson: None. P. Cooper: None. J.M. Holland: None. R. Crilly: None.