Prostate SABR delivers large doses using a fast dose rate. This amplifies the effect geometric uncertainties have on normal tissue dose. The aim of this study was to determine whether the treatment DVH agrees with the planned dose to OAR.
Forty-one low- intermediate risk prostate cancer patients were treated with SABR using a linac based technique. Dose prescribed was 35 Gy in five fractions delivered on alternate days, planned using VMAT with 10XFFF. On treatment, prostate was matched to fiducial markers on CBCT. OAR were retrospectively delineated on 205 pre-treatment CBCT images. Daily CBCT contours were overlaid on the planning CT for dosimetric analysis. Verification plan used to evaluate the daily DVH for each structure. The daily doses received by OAR were recorded using the D%.
The median rectum and bladder volumes at planning were 67.1 cm (IQR 56.4-78.2) and 164.4 cm (IQR120.3-213.4) respectively. There was no statistically significant difference in median rectal volume at each of the five treatment scans compared to the planning scan ( = 0.99). This was also the case for median bladder volume ( = 0.79). The median dose received by rectum and bladder at each fraction was higher than planned, at the majority of dose levels. For rectum the increase ranged from 0.78 Gy-1.64Gy and for bladder 0.14 Gy-1.07Gy. The percentage of patients failing for rectum D35% < 18 Gy ( = 0.016), D10% < 28 Gy ( = 0.004), D5% < 32 Gy ( = 0.0001), D1% < 35 Gy ( = 0.0001) and bladder D1% < 35 Gy ( = 0.001) at treatment were all statistically significant.
In this cohort of prostate SABR patients we estimate the OAR treatment DVH was higher than planned. This was due to rectal and bladder organ variation.
OAR variation in prostate SABR using a FFF technique, may cause the treatment DVH to be higher than planned.

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