Intra-fraction displacement of the prostate bed during post-prostatectomy radiotherapy

Abstract

Background: To measure intra-fraction displacement (IFD) in post-prostatectomy patients treated with anisotropic margins and daily soft tissue matching.

Methods: Pre-treatment cone beam computed tomography (CBCT) scans were acquired daily and post-treatment CBCTs for the first week then weekly on 46 patients. The displacement between the scans was calculated retrospectively to measure IFD of the prostate bed (PB). The marginal miss (MM) rate, and the effect of time between imaging was assessed.

Results: A total of 392 post-treatment CBCT's were reviewed from 46 patients. The absolute mean (95% CI) IFD was 1.5 mm (1.3-1.7 mm) in the AP direction, 1.0 mm (0.9-1.2 mm) SI, 0.8 mm (0.7-0.9 mm) LR, and 2.4 mm (2.2-2.5 mm) 3D displacement. IFD ≥ ± 3 mm and ≥ ± 5 mm was 24.7% and 5.4% respectively. MM of the PB was detected in 33 of 392 post-treatment CBCT (8.4%) and lymph nodes in 6 of 211 post-treatment CBCT images (2.8%). Causes of MM due to IFD included changes in the bladder (87.9%), rectum (66.7%) and buttock muscles (6%). A time ≥ 9 min between the pre and post-treatment CBCT demonstrated that movement ≥ 3 mm and 5 mm increased from 19.2 to 40.5% and 5 to 8.1% respectively.

Conclusions: IFD during PB irradiation was typically small, but was a major contributor to an 8.4% MM rate when using daily soft tissue match and tight anisotropic margins.

Keywords: IGRT; Intra-fraction motion; Post-prostatectomy; Prostate bed; Radiotherapy.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Schematic diagram of the division of the prostate bed. Diagram of the location descriptions of the regions of the a prostate bed and b the area adjacent to the superior and inferior aspects of the prostate bed

Fig. 2

Frequency and magnitude of intra-fraction displacement. The intra-fraction displacement frequency and magnitude is displayed for the AP (blue), SI (red), LR (green), and 3D displacement (orange) directions

Fig. 3

Location and amount of prostate bed marginal miss detected on the post-treatment images. The location and number of marginal misses of the prostate bed along with the amount of prostate bed not covered by the PTV is displayed. Each measure is displayed in the a upper superior, b upper posterior, c upper anterior, d upper lateral, e lower inferior, f lower posterior, g lower anterior, and h lower lateral location in the prostate bed

Fig. 4

Causes of marginal miss detected on post-treatment images. The causes of marginal miss are displayed as a percentage of all post-treatment images that displayed a marginal miss of the prostate bed

Fig. 5

Correlation between time between pre and post-treatment imaging and IFD. The correlation between the time between the pre and post-treatment CBCT images is correlated against the IFD detected

Fig. 6

Images showing intra-fraction displacement. The pre (a, c, e) and post-treatment (b, d, f) CBCT scans for 3 patients are displayed. In patient 1′s pre-treatment a the superior clip in within the prostate bed PTV (red volume) but on the post-treatment b the bladder had continued to fill by a small amount and caused the prostate bed and superior clip to move posterior and superior causing a marginal miss. Patient 2’s pre-treatment c had little rectal gas present but on the post-treatment d a large amount of gas appeared in the prostate bed region which caused the prostate bed to move anteriorly resulting in a marginal miss. The third patient’s pre-treatment e showed relaxed buttock check and in the post-treatment f the left buttocks cheek had tensed after the bladder continued to fill and caused the prostate bed to move laterally