Background and purpose Deformation and correlated target motion remain challenges for margin recipes in radiotherapy (RT). This study presents a statistical deformable motion model for multiple targets and applies it to margin evaluations for locally advanced prostate cancer i.e. RT of the prostate (CTV-p), seminal vesicles (CTV-sv) and pelvic lymph nodes (CTV-ln). Material and methods The 19 patients included in this study, all had 7-10 repeat CT-scans available that were rigidly aligned with the planning CT-scan using intra-prostatic implanted markers, followed by deformable registrations. The displacement vectors from the deformable registrations were used to create patient-specific statistical motion models. The models were applied in treatment simulations to determine probabilities for adequate target coverage, e.g. by establishing distributions of the accumulated dose to 99% of the target volumes (D99) for various CTV-PTV expansions in the planning-CTs. Results The method allowed for estimation of the expected accumulated dose and its variance of different DVH parameters for each patient. Simulations of inter-fractional motion resulted in 7, 10, and 18 patients with an average D99 >95% of the prescribed dose for CTV-p expansions of 3 mm, 4 mm and 5 mm, respectively. For CTV-sv and CTV-ln, expansions of 3 mm, 5 mm and 7 mm resulted in 1, 11 and 15 vs. 8, 18 and 18 patients respectively with an average D 99 >95% of the prescription. Conclusions Treatment simulations of target motion revealed large individual differences in accumulated dose mainly for CTV-sv, demanding the largest margins whereas those required for CTV-p and CTV-ln were comparable.

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Radiotherapy & Oncology
Department of Radiation Oncology

Thörnqvist, S, Hysing, L.B, Zolnay, A.G, Söhn, M, Hoogeman, M.S, Muren, L.P, … Heijmen, B.J.M. (2013). Treatment simulations with a statistical deformable motion model to evaluate margins for multiple targets in radiotherapy for high-risk prostate cancer. Radiotherapy & Oncology, 109(3), 344–349. doi:10.1016/j.radonc.2013.09.012