Image-guided IMRT for localized prostate cancer with daily repositioning: inferring the difference between planned dose and delivered dose distribution

Introduction: To investigate the dosimetric impact of daily on-line repositioning during a full course of IMRT for prostate cancer.

Materials and methods: Twenty patients were treated with image-guided IMRT. Each pre-treatment plan (Plan A) was compared with a post-treatment plan sum (Plan B) based on couch shifts measured. The delivered dose to the prostate without a daily repositioning was inferred by considering each daily couch shift during the whole course of image-guided IMRT (i.e., plan B). Dose metrics were compared for prostate CTV (P-CTV) and PTV (P-PTV) and for organs at risk. Ten patients were treated with a 5 mm margin and 10 patients with a 10 mm margin.

Results: For plan A vs. plan B: the average D95, D98, D50, D mean and EUD were: 76.4 Gy vs. 73.9 Gy (p = 0.0007), 75.4 Gy vs. 72.3 Gy (p = 0.001), 78.9 Gy vs. 78.4 Gy (p = 0.014), 78.7 Gy vs. 77.8 Gy (p = 0.003) and 78.1 Gy vs. 75.9 Gy (p = 0.002), respectively for P-CTV, and 73.2 Gy vs. 69.3 Gy (p = 0.0006), 70.7 Gy vs. 66.0 Gy (p = 0.0008), 78.3 Gy vs. 77.5 Gy (p = 0.001), 77.8 Gy vs. 76.4 Gy (p = 0.0002) and 74.4 Gy vs. 69.2 Gy (p = 0.003), respectively for P-PTV. Margin comparison showed no differences in dose metrics between the two plans except for D98 of the rectum in plan B which was significantly higher with a 10 mm margin.

Conclusions: The absence of daily image-guided IMRT resulted in a significantly less uniform and less homogeneous dose distribution to the prostate. A reduction in PTV margin showed neither a lower target coverage nor a better spare of OAR with and without daily image-guided IMRT.