Residual Motion and Duty Time in Respiratory Gating Radiotherapy Using Individualized or Population-Based Windows

Abstract

The efficiency and precision of respiratory gated radiation therapy for tumors is affected by variations in respiration-induced tumor motion. We evaluated the use of individualized and population-based parameters for such treatment. External respiratory signal records and images of respiration-induced tumor motion were obtained from 42 patients undergoing respiratory gated radiation therapy for liver tumors. Gating window widths were calculated for each patient, with 2, 4, and 10 mm of residual motion, and the mean was defined as the population-based window width. Residual motions based on population-based and predefined window widths were compared. Duty times based on whole treatment sessions, at various window levels, were calculated. The window level giving the longest duty time was defined as the individualized most efficient level (MEL). MELs were also calculated based on the first 10 breathing cycles. The duty times for population-based MELs (defined as mean MELs) and individualized MELs were compared. Tracks of respiration-induced tumor motion ranged from 3 to 50 mm. Half of the patients had larger actual residual motions than the assigned residual motions. Duty times were greater when based on individualized, rather than population-based, window widths. The MELs established during whole treatment sessions for 2 mm and 4 mm of residual motion gave significantly increased duty times, whereas those calculated using the first 10 breathing cycles showed only marginal increases. Using individualized window widths and levels provided more precise and efficient respiratory gated radiation therapy. However, methods for predicting individualized window levels before treatment remain to be explored.