Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_10_3213 - Comparison of Two Mono-Isocenter Techniques with Multiple Non-Coplanar Arcs for Linac Based SRS of Multiple Brain Lesions

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Comparison of Two Mono-Isocenter Techniques with Multiple Non-Coplanar Arcs for Linac Based SRS of Multiple Brain Lesions
R. Ruggieri1, S. Naccarato1, F. Ricchetti1, R. Mazzola1, F. Gregucci1, V. Figlia1, A. Fiorentino1, S. Corradini2, and F. Alongi3; 1Radiation Oncology Dept, Sacro Cuore - don Calabria Hospital, Negrar (VR), Italy, 2Department of Radiation Oncology, University Hospital LMU, Munich, Germany, 3Radiation Oncology Dept, Sacro Cuore - don Calabria Hospital and University of Brescia, Negrar (VR), Italy

Purpose/Objective(s): As a result of the increased risk of detriment in neurocognitive functions for patients undergoing whole-brain radiotherapy, and thanks to the introduction of mono-isocenter rotational techniques, linac based SRS of multiple brain lesions is getting increased diffusion. Here we present a plan comparison study between the only two integrated planning packages for mono-isocenter SRS of multiple brain lesions with multiple non-coplanar arcs which are available nowadays: HyperArc (HA) and Multiple Brain Mets (MBM).

Materials/Methods: 20 patients treated by HA for multiple (mean 5, range 2-10) brain lesions (PTV(cc): mean 9.6, range 0.5-27.9), at prescribed doses (Dp) of 18-25 Gy, for single-fraction, and 21-27 Gy, for three-fractions SRS, were replanned by MBM. While HA uses a class-solution of mono-isocenter VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°), MBM is a conformal-arc technique where, for an user selected number of couch rotations, the number of arcs and their extension together with the couch rotations are optimised. A dose normalization of 100%Dp at 98%PTV was adopted, where a 2 mm isotropic margin from the union of all the lesions gave PTV, while D2%(PTV)<150%Dp was accepted. Plans had to respect the constraints for 1(3) fractions on maximum dose (D0.5cc) to the brainstem, 10(18)Gy, and to the optical nerves/chiasm, eyes and lenses, 8(15)Gy. By hypothesis testing, HA and MBM plans were then compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V12 and mean dose to the brain-minus-PTV, and in terms of efficiency metric by MU (per fraction).

Results: The only significant variation in favour of HA plans resulted for CI (p<.0001): (mean±sd) were (0.95±0.03), for HA, and (0.76±0.08), for MBM plans. No significant difference was computed for GI (p=.713), nor for the mean dose to the brain-minus-PTV (p=.871). Neither for V12 to the brain-minus-PTV the difference was significant (p=.123), although not negligible: (23.7±23.8)cc for HA vs. (36.3±36.4)cc for MBM plans. Further, no significant difference in terms of computed MU (p=.185) resulted.

Conclusion: HA plans assured a higher CI than MBM plans. This likely resulted from the higher degree of modulation in the leaf sequences of the inversely optimised HA plans, which might impact on the dosimetric accuracy of treatment delivery: a further study will try to clarify this point. The absence of any significant difference for all of the other here conceived dosimetric and efficiency metrics supports a judgement of near equal plan quality from the two planning techniques.

Author Disclosure: R. Ruggieri: Consultant; Varian Medical Systems. S. Naccarato: None. R. Mazzola: None. F. Gregucci: None. V. Figlia: None. A. Fiorentino: None. F. Alongi: Honoraria; Varian Medical Systems. Consultant; Varian Medical Systems.

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