Assessment of setup uncertainties for various tumor sites when using daily CBCT for more than 2200 VMAT treatments

Abstract

The aim of this study was assess the patient setup errors for various tumor sites based on clinical data from a sufficient number of treatments with volumetric-modulated arc therapy (VMAT) using daily pretreatment CBCT imaging guidance. In addition, we calculated and compared the planning target volume (PTV) margins for all disease sites based on an analysis of specific systematic and random errors in our institution. All patients underwent pretreatment kV-CBCT imaging. The various tumor sites were divided into four categories; 21 brain (438 fractions), 35 head-and-neck tumors (H&N, 933 fractions), 19 thorax and abdomen tumors (T&A, 313 fractions), and 17 prostate cancer tumors (546 fractions). Overall distributions of setup corrections in all directions, frequencies of 3D vector lengths, institution-specific setup error, and PTV margins were analyzed. The longitudinal distribution for the T&A site represented an asymmetric offset in the negative direction. Rotational distributions were comparable for all treatment sites, and the prostate site had the narrowest distribution of ≤ ± 2°. The cumulative frequencies of 3D vector length of ≥ 7 mm were rare for brain lesions and H&N, but more common for T&A and prostate lesions at 25.6% and 12.1%, respectively. The overall mean error for all treatment sites were within ± 1 mm and ± 0.1°, with the exception of the T&A site, which had overall mean error of 2 mm in the negative longitudinal direction. The largest magnitude of systematic error and random error for the brain lesions and H&N was 1.4 mm in the translational directions, and 3.3 mm for T&A and prostate lesions. The PTV margins required in this analysis are ≤ 4 mm for the brain lesions and H&N in all translational directions, but ranged from 4 to 10 mm for T&A and prostate lesions. Analysis of each institution's specific setup errors using daily CBCT is essential for determining PTV margins and reducing setup uncertainties, because setup errors vary according to each immobilization system and patient.

Figure 1

The immobilization systems: brain (a), H&N (b), T&A (c), prostate (d). Thermoplastic fixation mask and personal designed headrest are utilized for brain and H&N sites; double vacuum device is applied to T&A sites; knee pillow is used for prostate site. The dashed circles indicate additional straightening markers to assist alignment in all directions. H&N = head and neck, T&A = thorax and abdomen.

Figure 2

Overall distributions of translational shift in each treatment site. The distributions are narrow for brain, H&N, and prostate, whereas the longitudinal distribution for T&A represents asymmetric offset toward the negative direction. H&N = head and neck, T&A = thorax and abdomen.

Figure 3

Overall distributions of rotational shift in each treatment sites (°). All distributions do not exceed $\pm 3^{\circ }$; the prostate has the narrowest distribution. H&N = head and neck, T&A = thorax and abdomen.

Figure 4

Cumulative frequencies of 3D vector lengths for translational setup errors. The frequencies of ≥ 7 mm are not observed for brain and H&N, whereas the values of ≥ 10 mm represent T&A and prostate. H&N = head and neck, T&A = thorax and abdomen.

Figure 5

Cumulative frequencies of rotational setup errors. All frequencies are comparable regardless of treatment site; brain has the highest value of ≥ 1°. H&N = head and neck, T&A = thorax and abdomen.

Figure 6

$\text{Mean}\pm \text{SD}$ of individual patient setup corrections for brain and H&N sites in all directions. The $\text{mean}\pm \text{SD}$ setup corrections indicate intertreatment variations of 21 brain patients and 35 H&N patients. H&N = head and neck, SD = standard deviation.

Figure 7

$\text{Mean}\pm \text{SD}$ of individual patient setup corrections for T&A and prostate sites in all directions. The $\text{mean}\pm \text{SD}$ setup corrections indicate intertreatment variations of 19 T&A patients and 17 prostate patients. T&A = thorax and abdomen, SD = standard deviation.

Figure 8

A possible longitudinal setup error for T&A patient using vacuum‐mold system. Patient' skin and external fiducials tend to be pulled in a negative longitudinal direction during the creation of a whole‐body vacuum mold; thus, setup error in the negative longitudinal direction for T&A sites may occur. T&A = thorax and abdomen.