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 15 mm in LR (left-right) and SI (superior-inferior) and 5-10 mm 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 1.48 mm in AP) than intra systematic errors (0.44 in LR, 0.69 in SI and 0.80 mm in AP). Same findings for the random error in SI direction with 3.19 (inter) and 2.30 mm (intra), whereas in LR and AP results were alike with 1.89 (inter) and 1.91 mm (intra) and 2.10 (inter) and 2.27 mm (intra), respectively. The calculated margin revealed dimensions of 4-5 mm in LR, 8-9 mm in SI and 6-7 mm in AP direction. Treatment plans which had to be adapted showed smaller variations with 1.12 (LR) and 1.72 mm (SI) for Σ and 4.17 (LR) and 3.75 mm (SI) for σ compared to initial plans with 1.77 and 2.62 mm for Σ and 4.46 and 5.39 mm for σ in LR and SI, respectively. CONCLUSION: The currently clinically used margin of 15 mm in LR and SI and 5-10 mm 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.

Investigation of Elekta AQUA software for kilovoltage to megavoltage radiation isocenter coincidence.

Elekta AQUA v2.02 software (Gantry Runout isocenter test) was investigated as a tool for verification of kilovoltage to megavoltage gantry radiation isocenter coincidence. AQUA reported megavoltage (6 MV) isocenter was independent of field size over the range 5 cm × 5 cm to 20 cm × 20 cm. For the 10 cm × 10 cm field size, standard deviation in AQUA reported 3D megavoltage (6MV) isocenter over ten consecutive deliveries was less than 0.04 mm for any axis. Compared to the full AQUA test delivery (Gantry Runout), the shorter AQUA test version (Gantry Runout short) gave a root mean square MV isocenter (± 1 SD) difference of 0.18 mm ± 0.08. Across 7 machines, root mean square differences between AQUA and PIPS PRO reported MV isocenter (for 6 MV and 6 MV FFF beams) was 0.1 mm ± 0.1 mm, with most of the difference observed in the gun-target (Y-axis). AQUA 6 MV isocentre position was offset to gantry relative to Elekta XVI customer acceptance test (CAT) workflow by 0.25 mm to 0.52 mm. For 6 MV FFF beams, AQUA reported an MV isocenter of between 0.27 and 0.39 mm offset to target relative to Elekta CAT.