Favorable IMRT Experience Treating Obese Prostate Cancer Patients in the Prone Position using Electromagnetic Tracking and a “Belly Board”

Abstract

We sought to extend the use of electromagnetic localization and tracking technology (Calypso Medical Technologies, Seattle, WA) to obese men with large anterior-posterior (AP) separations who would otherwise not be eligible to have their prostate localized and/or tracked if they were treated in a supine position. Four patients of large girth (weights 189-326 lbs) were treated. The A-P dimensions of these patients precluded electromagnetic localization and tracking when they were in a supine position. This is due to the specifications of the tracking system, which limits the maximum distance from the array to the transponders to prevent gantry collision. These patients were placed in the prone position for both the treatment planning CT as well as daily radiation treatments. The first patient was treated on a solid slab of Styrofoam. In an attempt to minimize prostate motion caused by respiration, the remaining three patients were treated on a conventional “belly board.” PTV prescription doses ranged from 77.4-79.2 Gy. Tracking limits were set to 5 mm for all dimensions except for 4 mm posterior. Localization and tracking were accomplished successfully for all 173 fractions. The tracking logs for all patients demonstrated patterns of prostate motion attributable to breathing, predominantly in the AP axis with lesser effect along the superior-inferior (SI) axis and no effect on the lateral axis. For the first patient, the excursions due to respiratory motion were typically ± 1 mm in the SI axis and ± 2 mm in the AP axis. When a “belly board” was used for the subsequent 3 patients, the excursions were decreased and were typically ± 0.5 mm in the SI axis and ± 1 mm in the AP axis. Beam pauses and/or interruptions for patient re-positioning were no more frequent than for more slender patients treated in the supine position. In our experience, prone positioning on a “belly board” allows electromagnetic localization and tracking technology to be used on all obese patients. Prostate motion from breathing is small and acceptable. As a result, high dose IMRT for prostate cancer can be delivered accurately and reliably to obese men.