Abstract
BackgroundTo investigate deviations between planned and applied treatment doses for hypofractionated prostate radiotherapy and to quantify dosimetric accuracy in dependence of the image guidance frequency.MethodsDaily diagnostic in-room CTs were carried out in 10 patients in treatment position as image guidance for hypofractionated prostate radiotherapy. Fraction doses were mapped to the planning CTs and recalculated, and applied doses were accumulated voxel-wise using deformable registration. Non-daily imaging schedules were simulated by deriving position correction vectors from individual scans and used to rigidly register the following scans until the next repositioning before dose recalculation and accumulation. Planned and applied doses were compared regarding dose-volume indices and TCP and NTCP values in dependence of the imaging and repositioning frequency.ResultsDaily image-guided repositioning was associated with only negligible deviations of analyzed dose-volume parameters and conformity/homogeneity indices for the prostate, bladder and rectum. Average CTV T did not significantly deviate from the plan values, and rectum NTCPs were highly comparable, while bladder NTCPs were reduced. For non-daily image-guided repositioning, there were significant deviations in the high-dose range from the planned values. Similarly, CTV dose conformity and homogeneity were reduced. While TCPs and rectal NTCPs did not significantly deteriorate for non-daily repositioning, bladder NTCPs appeared falsely diminished in dependence of the imaging frequency.ConclusionUsing voxel-by-voxel dose accumulation, we showed for the first time that daily image-guided repositioning resulted in only negligible dosimetric deviations for hypofractionated prostate radiotherapy. Regarding dosimetric aberrations for non-daily imaging, daily imaging is required to adequately deliver treatment.
Highlights
Prostate cancer is the most common malignancy in men with an estimated incidence ranging between 100 and 170 per 100 000 people per year [1]
Due to its well-known changes in volume during radiotherapy, the bladder exhibited the highest variability between the applied and prescribed doses, and the dosimetric deviations were most pronounced in the higher-dose range for both the bladder and the rectum (Figure 2 and Supplementary Figure 1)
No significant deviations were observed for the dose conformity indices of the clinical target volume (CTV) (p = 0.160 for conformity index (CI), p = 0.084 for conformal index (COIN)), and a small decrease was only noted for dose homogeneity (p = 0.002 for homogeneity index (HI), p = 0.846 for gEUD)
Summary
Prostate cancer is the most common malignancy in men with an estimated incidence ranging between 100 and 170 per 100 000 people per year [1]. Modern treatment techniques including image-guided and intensity-modulated radiotherapy (IMRT) have contributed to reducing late therapy-related toxicities to the bladder and rectum and allowed application of higher fractional and overall treatment doses [5, 6]. The increasing utilization of highprecision radiation techniques for the treatment of prostate cancer has rendered the application more susceptible to dosimetric deviations from the treatment plan due to interand intrafractional alterations in the pelvic anatomy, and the application of higher single doses in a reduced number of treatment fractions may increase the potential for therapeutic misses [10, 11]. To investigate deviations between planned and applied treatment doses for hypofractionated prostate radiotherapy and to quantify dosimetric accuracy in dependence of the image guidance frequency
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