Adaptive radiotherapy and the dosimetric impact of inter- and intrafractional motion on the planning target volume for prostate cancer patients

Purpose To investigate the dosimetric influence of daily interfractional (inter) setup errors and intrafractional ( intra) target motion on the planning target volume (PTV) and the possibility of an offline adaptive radiotherapy (ART) method to correct larger patient positioning uncertainties in image-guided radiotherapy for prostate cancer (PCa). Materials and methods A CTV (clinical target volume)-to-PTV margin ranging from 15mm in LR (left-right) and SI (superior-inferior) and 5-10mm in AP (anterior-posterior) direction was applied to all patients. The dosimetric influence of this margin was retrospectively calculated by analysing systematic and random components of inter and intra errors of 31 consecutive intermediate- and high-risk localized PCa patients using daily cone beam computed tomography and kV/kV (kilo-Voltage) imaging. For each patient inter variation was assessed by observing the first 4 treatment days, which led to an offline ART-based treatment plan in case of larger variations. Results: Systematic inter uncertainties were larger (1.12 in LR, 2.28 in SI and 1.48mm in AP) than intra systematic errors (0.44 in LR, 0.69 in SI and 0.80mm in AP). Same findings for the random error in SI direction with 3.19 (inter) and 2.30mm (intra), whereas in LR and AP results were alike with 1.89 (inter) and 1.91mm (intra) and 2.10 (inter) and 2.27mm (intra), respectively. The calculated margin revealed dimensions of 4-5mm in LR, 8-9mm in SI and 6-7mm in AP direction. Treatment plans which had to be adapted showed smaller variations with 1.12 (LR) and 1.72mm (SI) for Sigma and 4.17 (LR) and 3.75mm (SI) for sigma compared to initial plans with 1.77 and 2.62mm for Sigma and 4.46 and 5.39mm for sigma in LR and SI, respectively. Conclusion The currently clinically used margin of 15mm in LR and SI and 5-10mm in AP direction includes inter and intra uncertainties. The results show that offline ART is feasible which becomes a necessity with further reductions in PTV margins.