Prostate Intrafraction Translation Margins for Real-Time Monitoring and Correction Strategies

The purpose of this work is to determine appropriate radiation therapy beam margins to account for intrafraction prostate translations for use with real-time electromagnetic position monitoring and correction strategies. Motion was measured continuously in 35 patients over 1157 fractions at 5 institutions. This data was studied using van Herk's formula of ( ) for situations ranging from no electromagnetic guidance to automated real-time corrections. Without electromagnetic guidance, margins of over 10？mm are necessary to ensure 95% dosimetric coverage while automated electromagnetic guidance allows the margins necessary for intrafraction translations to be reduced to submillimeter levels. Factors such as prostate deformation and rotation, which are not included in this analysis, will become the dominant concerns as margins are reduced. Continuous electromagnetic monitoring and automated correction have the potential to reduce prostate margins to 2-3？mm, while ensuring that a higher percentage of patients (99% versus 90%) receive a greater percentage (99% versus 95%) of the prescription dose. 1. Introduction The goal of conformal radiation therapy is to shape the dose distribution to the prescribed target volume as closely as possible without sacrificing target coverage. This technique results in the sparing of neighboring healthy tissues and often leads to fewer complications and higher quality of life. It may also allow higher doses to target volumes that are limited by toxicity of normal tissues, potentially resulting in better local tumor control. In the last five years, real-time electromagnetic tracking of the prostate has become commercially available and has been adopted as the preferred localization technique in many clinics where it is available. The technology makes it possible to essentially eliminate interfraction variations, greatly reduces systematic uncertainties, and allows intra-fraction target volume motion to be monitored continuously throughout treatment so that corrective adaptive action may be taken. A description of the Calypso System has been previously reported [1, 2]. Briefly, the system consists of a tracking station (placed in the control room) to display real-time deviations for the target volume from isocenter. Ceiling-mounted infrared cameras localize an electromagnetic array which is placed over the patient before and during treatment. The array excites and localizes transponders which are implanted transrectally into the prostate. The transponders are 8？mm long by 1.85？mm in diameter and are implanted transrectally with