220. Evaluation of robustness to setup and range uncertainties of a proton radiosurgery plan

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Abstracts / Physica Medica 56 (2018) 133–278

220. Evaluation of robustness to setup and range uncertainties of a proton radiosurgery plan R. Righetto, D. Amelio, D. Scartoni, M. Schwarz APSS, Protonterapia, Trento, Italy Purpose . To evaluate plan robustness against set up and range uncertainties on a radiosurgery plan for a small volume meningioma.

Methods and materials. A single proton therapy fraction of 14GyRBE was prescribed. 3 posterior beams were planned and 3 different scenarios were generated with single field optimization (SFO): 1. uniform dose prescribed to the CTV; 2. uniform dose prescribed to the PTV;

Abstracts / Physica Medica 56 (2018) 133–278 Table 1

CTV, V1400cGy (%) Brain Tissue, dose to 5cc (cGyRBE)

scenario 1

scenario 2

scenario 3

scenario 4

Nominal Worst Case Nominal

96,5 54,4 955

98,8 81,9 1223

99,7 83,1 1100

99,8 90,6 1103

Worst Case

1036

1271

1169

1175

197

Conclusion. The use of appropriate margins and independent beam directions contribute to plan robustness and robustness analysis may be a part of proton therapy planning even in case of SFO for treatments delivered in one session where a set-up error is systematic. https://doi.org/10.1016/j.ejmp.2018.04.231

3. uniform dose prescribed to the CTV with robust optimization assuming 2 mm as maximum patient position uncertainty and 3% as range uncertainty. Another beam arrangement (scenario 4) including one posterior and 2 lateral field was planned using the robust optimization options of scenario 3. Each nominal plan was recomputed moving the isocenter on vertices of a cube of side length 3 mm, centered at the planning isocenter and applying a density perturbation of +3%. The DVH for the CTV and for the brain tissue were collected. The volume of CTV receiving a dose greater than 1400cGyRBE (V1400) and the dose to 5 cc of brain tissue were compared for the nominal plan and the worst case scenario. Results. The DVH for CTV and brain tissue are shown in Fig. 1. V1400 and the dose to 5 cc of brain tissue are presented in Tab. 1. V1400 of the CTV is between 96 and 99% for all nominal plans, in the worst case V1400 drops to 54% for scenario 1 and remains greater than 80% for the other scenarios where a margin was used in planning. In scenario n.4 beams arrangement contributes to the robustness of V1400.

221. A pre-absorber optimization technique for pencil beam scanning (PBS) proton therapy treatments F. Fracchiolla a, F. Fellin a, M. Innocenzi b, M. Lipparini a, M. Cianchetti a, F. Dionisi a, S. Vennarini a, M. Schwarz a,c a

Ospedale Santa Chiara di Trento, Centro di Protonterapia, Trento, Italy Università degli studi di Roma ‘‘La Sapienza”, Italy c TIFPA, Trento Institute for Fundamentals Physics Applications, Trento, Italy b

Purpose. To design and implement a new protontherapy planning method for the treatment of shallow lesions with PBS and to compare it to the standard method. Methods. In order to treat tumors located at depths lower than our minimum range available (4.1 cm), a pre-absorber is needed (Range Shifter – RS). The RS also causes scatter, so it is associated to some dose degradation, which should be minimized. We studied three tumor localizations (pelvic, H&N and Cranio-Spinal-Irradiation) requiring RS and we created three plans for each case: one is the standard method with the RS always used as close as possible to the patient, one with the RS used only for the shallow part of the