A study of respiration-correlated cone-beam CT scans to correct target positioning errors in radiotherapy of thoracic cancer

Abstract

Purpose: There is increasingly widespread usage of cone-beam CT (CBCT) for guiding radiation treatment in advanced-stage lung tumors, but difficulties associated with daily CBCT in conventionally fractionated treatments include imaging dose to the patient, increased workload and longer treatment times. Respiration-correlated cone-beam CT (RC-CBCT) can improve localization accuracy in mobile lung tumors, but further increases the time and workload for conventionally fractionated treatments. This study investigates whether RC-CBCT-guided correction of systematic tumor deviations in standard fractionated lung tumor radiation treatments is more effective than 2D image-based correction of skeletal deviations alone. A second study goal compares respiration-correlated vs respiration-averaged images for determining tumor deviations.

Methods: Eleven stage II-IV nonsmall cell lung cancer patients are enrolled in an IRB-approved prospective off-line protocol using RC-CBCT guidance to correct for systematic errors in GTV position. Patients receive a respiration-correlated planning CT (RCCT) at simulation, daily kilovoltage RC-CBCT scans during the first week of treatment and weekly scans thereafter. Four types of correction methods are compared: (1) systematic error in gross tumor volume (GTV) position, (2) systematic error in skeletal anatomy, (3) daily skeletal corrections, and (4) weekly skeletal corrections. The comparison is in terms of weighted average of the residual GTV deviations measured from the RC-CBCT scans and representing the estimated residual deviation over the treatment course. In the second study goal, GTV deviations computed from matching RCCT and RC-CBCT are compared to deviations computed from matching respiration-averaged images consisting of a CBCT reconstructed using all projections and an average-intensity-projection CT computed from the RCCT.

Results: Of the eleven patients in the GTV-based systematic correction protocol, two required no correction, seven required a single correction, one required two corrections, and one required three corrections. Mean residual GTV deviation (3D distance) following GTV-based systematic correction (mean ± 1 standard deviation 4.8 ± 1.5 mm) is significantly lower than for systematic skeletal-based (6.5 ± 2.9 mm, p = 0.015), and weekly skeletal-based correction (7.2 ± 3.0 mm, p = 0.001), but is not significantly lower than daily skeletal-based correction (5.4 ± 2.6 mm, p = 0.34). In two cases, first-day CBCT images reveal tumor changes-one showing tumor growth, the other showing large tumor displacement-that are not readily observed in radiographs. Differences in computed GTV deviations between respiration-correlated and respiration-averaged images are 0.2 ± 1.8 mm in the superior-inferior direction and are of similar magnitude in the other directions.

Conclusions: An off-line protocol to correct GTV-based systematic error in locally advanced lung tumor cases can be effective at reducing tumor deviations, although the findings need confirmation with larger patient statistics. In some cases, a single cone-beam CT can be useful for assessing tumor changes early in treatment, if more than a few days elapse between simulation and the start of treatment. Tumor deviations measured with respiration-averaged CT and CBCT images are consistent with those measured with respiration-correlated images; the respiration-averaged method is more easily implemented in the clinic.

Figure 1

Coronal section from six phase-binned images of a respiration-correlated cone-beam CT (RC-CBCT) scan of Patient 4. Tumor is shown at cross hairs in each phase.

Figure 2

Example tumor trajectories in the anterior-posterior (A/P) and superior-inferior (S/I) directions, obtained from the respiration-correlated CT (RCCT) and one of the respiration-correlated cone-beam CT (RC-CBCT) image sets of Patient 11. Each point indicates displacement relative to the 50% phase reference point (approximately end expiration, EE) in the RCCT. Star symbol indicates the respiration average position in both image sets.

Figure 3

Graphical representation of procedure used to determine the displacement of the respiration-averaged gross tumor volume (GTV) position (“Resp avg position”) in the respiration-correlated cone-beam CT (RC-CBCT) relative to that in the respiration-correlated CT (RCCT). Circle and diamond symbols denote the GTV trajectory along anterior-posterior (AP) and superior-inferior (SI) directions in the RCCT and RC-CBCT image sets, respectively. Displacement of the respiration-averaged GTV (solid arrow) is derived from the sum of three vectors (dashed arrows): the displacement of the respiration-averaged GTV from the end-expiration (“Ref EE”) in the reference RCCT image; the corresponding displacement in the RC-CBCT; and displacement of the GTV at EE in the RC-CBCT relative to its EE position in the RCCT.

Figure 4

Flow chart of the clinical RC-CBCT guided (Type 1) correction process.

Figure 5

Example (Patient 1) of 3D deviations in GTV position on treatment fractions in which an RC-CBCT scan was acquired. Data labeled “GTV systematic” are the actual measurements resulting from the RC-CBCT guided (Type 1) correction shown in Fig. 4, whereas data for the other correction methods are retrospectively simulated as described in the text.

Figure 6

Mean 3D residual GTV position errors of GTV-based systematic (Type 1) correction, bony-based systematic (Type 2) correction, bony-based daily (Type 3) correction, and bony-based weekly (Type 4) correction versus patient. Error bars indicate 1-standard-deviation residual error.

Figure 7

Difference in mean GTV residual error between GTV-based systematic (Type 1) and bony-based systematic (Type 2) corrections, vs GTV motion extent in RCCT.

Figure 8

Difference in GTV superior-inferior displacement D_S/I between respiration-correlated (RCCT-to-RC-CBCT) and respiration-averaged (AVE-IP-to-CBCT using all projections) image registration (circle symbols). Square symbols indicate maximum S/I displacement of the GTV from its end-expiration position in the RCCT scan.

Figure 9

Coronal section of Patient 1 in (a) the end-expiration respiration-correlated CT (RCCT) at simulation; (b) the end-expiration respiration-correlated cone-beam CT (RC-CBCT) at treatment 19 days later. Contours indicate the outline of the GTV drawn on the RCCT. Arrows in (b) indicate regions where the GTV in the cone-beam CT has grown outside the RCCT-defined GTV.

Figure 10

Overlay of coronal sections from the respiration-correlated CT (RCCT, blue enhanced) and respiration-correlated cone-beam CT (CBCT, red) of Patient 7. The images are aligned to the vertebral column. Yellow curve indicates outline of the GTV drawn on the RCCT; blue curve indicates location in the cone-beam CT of the GTV, which has shifted 14 mm posteriorly relative to the RCCT (arrow).