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Normal brain sparing approach for static field intensity modulated radiosurgery of single brain metastasis using a
Shabbir Ahamed1, R Padma Suvarna2, Navin Singh3
1Department of Radiation Physics, MNJ Institute of Oncology and Regional Cancer Center, Hyderabad, Telangana, India.
Purpose:
To determine the cause and effects of sparing normal brain using a distance driven normal tissue dose gradient function for static field intensity modulated stereotactic radiosurgery planning of single brain metastasis (SBM).
Materials And Methods:
Twenty seven SBM cases were planned for a dose of 20 Gy using 6 MV beams, to examine the cause and effects of dose gradients due to six NTO combinations formed from fall-off = 0.2, 0.6, and 1.0 mm-1, final-dose + priority = 10% +100 and 30% + 200. For comparative analysis, NTO combination of fall-off = 1.0 mm-1, final-dose + priority = 10% +100 was chosen as the control. Each combination was assessed for changes in field size (FS), high-to-low brain dose volumes (V14, V12, and V8), gradient index (GI), conformity index (CI), prescription isodose level (PIDL), and MU/Gy. Correlational analysis was performed to find meaningful relationships between FS, V12 and GI for all NTO combinations.
Results:
Groups with fall-off ≥0.6 mm-1 plus priority = 200 showed considerable reductions for FS, V14, V12, V8, GI and PIDL (P < 0.0001). Substantial high-to-low brain dose volume decrease was observed because of increasing fluence conformity and consequent FS regression. CI was constant (±0.01), while GI improved significantly and PIDL decreased with associated MU/Gy increase, (P < 0.0001). Strong positive correlation between FS and V12 (r > 0.9) was observed meaning GI correlated inversely (r < -0.7).
Conclusion:
NTO strictness causes fluence contraction that spares normal brain, yielding better dose metrics to attain balanced plan goals.
