Comparing target volumes used in radiotherapy planning based on CT and PET/CT lung scans with and without respiratory gating applied

Abstract

A study was done at Steve Biko Academic hospital to determine the influence that respiratory gating will have on target volumes used in radiotherapy treatment planning. The primary objective was to compare target volumes of respiratory gated scans to ungated scans and to determine whether it will be meaningful to permanently implement a 4D respiratory gating system on CT scanners in the South African public health sector and to use these images for target volume delineation in radiotherapy planning. The study consisted of three sections. In the first section, 4D respiratory gated CT images were obtained and delineated with 4D software. The full-inspiration and full-expiration phases of the gated scans were then fused to obtain ungated images which were also delineated. The gross tumor volumes (GTVs) of the gated phases were compared to the ungated GTVs, and found that on average the volumes decreased by 14.63% with a standard deviation of 7.96% when gating was applied. Yet another aim was to determine the influence that 4D imaging will have on radiotherapy treatment planning. One of the 4D study sets was imported to the XIO treatment planning system where IMRT treatment plans were created on both the gated and ungated scans. The plans conformed to the treatment aims and restrictions when clinical parameters such as DVHs were used to evaluate it. The planned target volume coverage differed by less than 1% between the gated and the ungated plans, but significant dose reductions to the OARs of up to 32.65% to the contralateral lung were recorded on the gated plan. In the second section of this study, respiratory gated CT scans were simulated by applying the breath-hold technique to lung cancer patients. The technique was applied during full-inspiration which fundamentally represents the maximum peak of the sinusoidal respiratory waveform. An ungated scan was also acquired during normal respiration. The clinical target volumes (CTVs) were identified on both scans by three oncologists and the average CTVs were compared. It was found that the CTVs decreased significantly by an average of 14.33%. Palliative patients receive parallel opposing field therapy which is planned from 2D films. It is very unlikely that these opposing field sizes will differ when gating is applied. It was therefore concluded that only radical lung patients, which was estimated to be a mere 0.03% of the total radiation therapy patient population, will benefit by implementing respiratory gating or any motion-reduction technique. For the third section of the study, respiratory gated PET scans were acquired on a PET/CT scanner to evaluate external, non-technical parameters that will influence respiratory gating. The results indicated that time and patient participation were not limiting factors. The biggest concerns however were the effectiveness of the gating system, software limitations and the gated results. These problems might be minimized with thorough training on the system. All three sections as well as the financial implications were considered to conclude that it will not be meaningful to implement 4D respiratory gating techniques in the South African public health sector Copyright