PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): In this prospective phase II study, we investigated how the change in the bladder and the rectal volumes could account for the difference in shifts between CBCT based IGRT and 2D KV based IGRT in post radical prostatectomy setting. We also assessed whether CBCT scan would be a better method of IGRT than kV imaging in the post-RP setting by comparing the shifts required to align the patient at the treatment.
Materials/Methods: A total of 419 treatment fractions from 26 patients were included in this analysis. The bladder and rectum volumes were measured on the CBCT images in offline review for each fraction. These dimensions were then compared with the baseline bladder and rectum sizes measured on the CT simulation scan. The surrogate volume of the bladder was obtained by multiplying the vertical, lateral, and longitudinal bladder dimensions. The surrogate rectal volume was obtained by multiplying the vertical dimension and the most lateral extents. The correlation between the bladder and rectum sizes with the shifts was analyzed with Spearman’s correlation test. We also recorded the shifts required to align the patient during treatment fractions by performing a daily kV imaging and CBCT scans, the shift values were recorded in the vertical , lateral, and longitudinal directions. The absolute shift values from kV images and CBCT scans were then compared with Wilcoxon signed-rank test. Finally we investigated the proportion of treatment fractions in which the absolute value of the difference between the kV and CBCT shifts exceeded 0.7cm (our PTV margin).
Results: Our results showed that the vertical and longitudinal CBCT shifts were highly correlated with changes in the bladder (p < 0.001 and p = 0.001, respectively) and rectal volumes (p < 0.001 for both), but not for kV shifts. The p values for the correlation between the vertical and longitudinal shifts with the change in bladder size were 0.166 and 0.077 respectively, and 0.278 and 0.058 for the change in rectum size. There was no correlation between changes in the lateral kV and CBCT shifts with changes in bladder or rectum sizes . The mean anterior-posterior, lateral, and superior-inferior shifts in absolute value with kV imaging were 0.359 (interquartile range (IQR) 0.1-0.5), 0.348 (IQR 0.1-0.5), and 0.312 (IQR 0.1-0.5) cm, respectively, compared to 0.389 (IQR 0.1-0.5), 0.345 (IQR 0.1-0.5), and 0.300 (IQR 0.1-0.4) cm with CBCT scan. There was a significant difference for the anterior-posterior shifts (p = 0.046). Further analysis showed that the proportion of treatment fractions in which the absolute value of the difference between the kV and CBCT shifts exceeded 0.7cm were 21% in the anterior-posterior, 12% in the lateral, and 13% in the superior-inferior directions.
Conclusion: CBCT improves localization of the prostate bed by addressing the interfractional changes of the bladder and rectal volumes and should be considered as the IGRT modality of choice in the daily monitoring of treatment for prostate cancer post RP.
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