Anatomic and pathologic variability during radiotherapy for a hybrid active breath-hold gating technique

Abstract

Purpose: To evaluate intra- and interfraction variability of tumor and lung volume and position using a hybrid active breath-hold gating technique.

Methods and materials: A total of 159 repeat normal inspiration active breath-hold CTs were acquired weekly during radiotherapy for 9 lung cancer patients (12-21 scans per patient). A physician delineated the gross tumor volume (GTV), lungs, and spinal cord on the first breath-hold CT, and contours were propagated semiautomatically. Intra- and interfraction variability of tumor and lung position and volume were evaluated. Tumor centroid and border variability were quantified.

Results: On average, intrafraction variability of lung and GTV centroid position was <2.0 mm. Interfraction population variability was 3.6-6.7 mm (systematic) and 3.1-3.9 mm (random) for the GTV centroid and 1.0-3.3 mm (systematic) and 1.5-2.6 mm (random) for the lungs. Tumor volume regressed 44.6% +/- 23.2%. Gross tumor volume border variability was patient specific and demonstrated anisotropic shape change in some subjects. Interfraction GTV positional variability was associated with tumor volume regression and contralateral lung volume (p < 0.05). Inter-breath-hold reproducibility was unaffected by time point in the treatment course (p > 0.1). Increases in free-breathing tidal volume were associated with increases in breath-hold ipsilateral lung volume (p < 0.05).

Conclusions: The breath-hold technique was reproducible within 2 mm during each fraction. Interfraction variability of GTV position and shape was substantial because of tumor volume and breath-hold lung volume change during therapy. These results support the feasibility of a hybrid breath-hold gating technique and suggest that online image guidance would be beneficial.

Figure 1

First (a) and last (b) week breath-hold CT images for Patient 4. Substantial tumor volume regression is evident. GTV from the first week is shown in red. (c) Tumor centroid position relative to bone compared with the tumor volume. As the tumor volume changed, the position of the tumor centroid trended superiorly.

Figure 1

First (a) and last (b) week breath-hold CT images for Patient 4. Substantial tumor volume regression is evident. GTV from the first week is shown in red. (c) Tumor centroid position relative to bone compared with the tumor volume. As the tumor volume changed, the position of the tumor centroid trended superiorly.

Figure 1

First (a) and last (b) week breath-hold CT images for Patient 4. Substantial tumor volume regression is evident. GTV from the first week is shown in red. (c) Tumor centroid position relative to bone compared with the tumor volume. As the tumor volume changed, the position of the tumor centroid trended superiorly.

Figure 2

Gross tumor volume, expressed as a percentage of the first week of treatment tumor volume, observed over the treatment course (n = 9). The individual logarithmic fits shown provided the best fit for the data on average.

Figure 3

Stability of the left lung volume under breath-hold observed over the treatment course for 9 patients. Each data point indicates the daily lung volume (mean ± standard deviation).

Figure 4

(a) Patient 7 had increased fluid in his affected lung (pleural effusion), which impacted inter-fraction stability of the tumor throughout treatment. This patient also had the largest systematic error in the study population, emphasizing the importance of image-guided adaptive radiotherapy. (b) For Patient 7, change in daily tidal volume with respect to change in gross tumor volume size over treatment.

Figure 4

(a) Patient 7 had increased fluid in his affected lung (pleural effusion), which impacted inter-fraction stability of the tumor throughout treatment. This patient also had the largest systematic error in the study population, emphasizing the importance of image-guided adaptive radiotherapy. (b) For Patient 7, change in daily tidal volume with respect to change in gross tumor volume size over treatment.