Megavoltage computed tomography image-guided helical tomotherapy for multiple metastases

Abstract

Objective To investigate an appropriate megavoltage computed tomography (MVCT) protocol to guide helical tomotherapy (HT) for multiple metastases. Methods According to the location of target volume, 48 patients with multiple metastases undergoing HT were divided into head/chest group (n=15), head/pelvis group (n=15), and chest/pelvis group (n=18). Each target volume received MVCT scans during the treatment. The obtained MVCT images were registered to CT images and the setup errors were recorded. The CTV-PTV margins were calculated. Comparison was made by paired t test. Results In the head/chest group, there was no significant difference in the setup error in x-axis between the head and chest (-0.15±1.25 vs. -0.21±2.34, P=0.71), while the head had significantly smaller setup errors in y-and z-axis than the chest (0.73±1.22 vs. 1.56±2.54, P=0.00; 0.93±1.44 vs. 2.65±1.88, P=0.00). In the head/pelvis group, the head had significantly smaller setup errors in x-, y-, and z-axis than the pelvis (-0.16±1.31 vs. -1.29±3.72, P=0.00; 0.81±1.34 vs. 3.20±3.90, P=0.00; 1.24±1.75 vs. 5.49±2.80, P=0.00). In the chest/pelvis group, there were no significant differences in setup errors in x-or y-axis between the chest and pelvis (-0.25±2.90 vs. -0.22±3.65, P=0.06; 0.35±3.60 vs. 0.38±3.78, P=0.87), while the chest had a significantly smaller setup error in z-axis than the pelvis (1.95±2.81 vs. 3.35±3.05, P=0.00). In the three groups, the CTV-PTV margins of lower target volume were reduced in three dimensions after the correction of upper target volume, in which y-axis showed the largest reduction of CTV-PTV margins (5.13 vs. 4.01; 9.17 vs. 8.30; 8.52 vs. 7.13). Conclusions The setup error of individual target volume should not be used for correction of the overall setup error in HT for multiple metastases with isolated target volume. An MVCT protocol that provides image-based guidance for multiple target volumes is recommended. Key words: Metastases neoplasms/tomotherapy; Image-guided program; Setup errors; Margin