Dosimetric consequences of prostate-based couch shifts on the precision of dose delivery during simultaneous IMRT irradiation of the prostate, seminal vesicles and pelvic lymph nodes

Abstract

Introduction: To evaluate the impact interfraction prostate (CTV1) motion corrections on doses delivered to seminal vesicles (CTV2) and lymph nodes (CTV3), and to determine ideal planning target volume (PTV) margins for these targets with prostate-based position verification.

Material and methods: Retrospective analysis based on 253 cone beam computed tomography (CBCT) studies of 28 patients. The isocenter-shift method was used to estimate the interfraction prostate and bony shift effects on the original plan coverage. Friedman's test was used to assess statistical significance between dose-volume histogram (DVH) parameters which were calculated for prostate-based sum plans, bony-based sum plans and original treatment plans. The van Herk formula was used to determine the set-up margin size for prostate-based verification.

Results: The tracked shifts influenced the minimum, maximum and mean CTV2 and CTV3 doses, with a range differential of 0.17%-2.63% (prostate shifts) and 0.13%-1.92% (bony shifts) compared to the corresponding original parameters. Friedman's test revealed significant differences in the minimum doses to the CTV3 and maximum doses to both the CTV2 and CTV3. The calculated set-up margins of 1.22 cm (vertical), 0.19 cm (longitudinal) and 0.39 cm (lateral) should be added to CTV3 while performing prostate-based positioning.

Conclusion: To avoid geographical miss during simultaneous irradiation of independently moving targets (CTV1-3) appropriate margins should be used in accordance with the position verification method used. Based on our findings the following margin sizes should be used: 0.7 cm for the CTV1, 0.8-0.9 cm for the CTV2 , and asymmetric 1.0 cm (vertically) and 0.5 cm (other axes) for the CTV3.

Keywords: IGRT; IMRT; PTV expansion; Prostate cancer; Set-up error.