Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_8_3191 - Comparison of Spine SRS VMAT plans with Flattening Filter Free 6 MV or 10 MV beams

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

Comparison of Spine SRS VMAT plans with Flattening Filter Free 6 MV or 10 MV beams
S. Balik1, P. Qi1, A. Magnelli1, S. T. Chao2, J. H. Suh2, and T. Zhuang1; 1Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, 2Department of Radiation Oncology, Taussig Cancer Center, Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): Spine stereotactic radiosurgery (SRS) with VMAT plans are usually complex and use highly modulated beams to spare the spinal cord and to generate a conformal plan. The flattening filter free (FFF) beams are ideal because the high dose rates reduce delivery time. The dedicated SRS/stereotactic body radiation therapy (SBRT) linacs commonly have 6 MV FFF (1400 MU/min) and 10 MV FFF (2400 MU/min) energies available. In this study, we investigated the differences in the quality of VMAT plans made with both FFF modes for the spine SRS.

Materials/Methods: Twenty previously treated spine SRS patients (10 cervical (C) spine and 10 thoracic and lumbar spines(T,L) were planned using three VMAT arcs and treatment planning system’s auto-planning feature. Two plans were made for each patient with 18 Gy in one fraction using 6 MV FFF and 10 MV FFF beams. The same auto-planning template (objectives and settings) was used for both plans to reduce planner bias. 6 MV FFF plan (6FFF) was optimized using the auto-planning objectives and settings that were used for 10 MV FFF plan (10FFF). The template utilized varied across patients and determined iteratively by the planner until the spinal cord (or cauda equina) and plan quality objectives (a conformal plan with minimum of 90% coverage) were achieved. All 6FFF plans met planning constraints using 10FFF template except for 2 cases in which cord constraint was violated. These cases were replanned using a different template until a clinically acceptable plan was achieved.

Results: Mean values of maximum cord/cauda dose (Dmax), volume of cord/cauda equina receiving 10 Gy (V10Gy), minimum dose to target (Dmin), Paddick conformity index (CI), gradient index (GI), total monitor unit (MU) and total beam on time are shown in the table below. Values shown in bold were found to be statistically significant.
C Spine T,L Spine
6FFF 10FFF 6FFF 10FFF
Cord Dmax (Gy) 11.2 ± 1.0 11.9 ± 0.8 11.5 ± 1.1 11.6 ± 1.2
V10Gy (%) 3.3 ± 2.2 4.9 ± 1.6 4.2 ± 2.5 4.4 ± 1.8
Tumor Dmin (Gy) 11.3 ± 2.4 11.7 ± 2.4 10.6 ± 1.3 10.8 ± 1.3
Conformity Index 0.71 ± 0.10 0.70 ± 0.11 0.82 ± 0.06 0.82 ± 0.04
Gradient Index 3.6 ± 0.3 3.7 ± 0.4 3.3 ± 0.3 3.2 ± 0.2
MU 8283 ± 1870 7997 ± 1600 10368 ± 2462 9554 ± 1565
Time (min) 5.9 ± 1.3 3.3 ± 0.7 7.4 ± 1.8 4.0 ± 0.7
The differences between 6 FFF and 10FFF plans were not statistically different except for cord/cauda equina Dmax and V10Gy for C-spine cases (p <0.05) and target Dmin for all cases (p<0.05). Total beam on time for 10FFF cases were significantly lower for all cases.

Conclusion: Both energies provided similar plan qualities for all spine SRS cases. In general, 10 MV FFF would be better for spine SRS due to faster treatment delivery. 6 MV FFF would be preferable to achieve lower cord dose for C-Spine.

Author Disclosure: S. Balik: None. P. Qi: None. A. Magnelli: None. S.T. Chao: Honoraria; Varian Medical Systems, Zeiss, Abbvie. Consultant; Abbvie. J.H. Suh: Consultant; ACMUI. Board member; Korean American Society for Therapeutic Radiation. T. Zhuang: None.

Salim Balik, PhD

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