Normal brain sparing approach for static field intensity modulated radiosurgery of single brain metastasis using a...

Area of Science:

• Radiation Oncology
• Medical Physics
• Neurosurgery

Background:

• Stereotactic radiosurgery (SRS) for single brain metastases (SBM) requires precise radiation delivery to maximize tumor control while minimizing dose to surrounding normal brain tissue.
• Intensity-modulated radiation therapy (IMRT) planning involves complex optimization of radiation beams to achieve steep dose gradients.
• Normal tissue objective (NTO) functions guide the inverse planning process, influencing dose distribution and sparing of organs at risk.

Purpose of the Study:

• To investigate the impact of a distance-driven normal tissue dose gradient function on sparing normal brain tissue during static field intensity-modulated SRS planning for SBM.
• To determine the cause and effects of dose gradients on various planning parameters.

Main Methods:

• Twenty-seven SBM cases were planned using 6 MV beams with a prescription dose of 20 Gy.
• Six normal tissue objective (NTO) combinations were evaluated, varying fall-off parameters (0.2, 0.6, 1.0 mm-1) and final dose priorities (10%+100, 30%+200).
• Planning metrics including field size (FS), high-to-low brain dose volumes (V14, V12, V8), gradient index (GI), conformity index (CI), prescription isodose level (PIDL), and monitor units per Gray (MU/Gy) were assessed.

Main Results:

• NTO combinations with fall-off ≥0.6 mm-1 and priority = 200 significantly reduced FS, V14, V12, V8, GI, and PIDL (P < 0.0001).
• Increased fluence conformity and field size regression led to substantial decreases in high-to-low brain dose volumes.
• Gradient index (GI) improved significantly, and PIDL decreased, with a corresponding increase in MU/Gy (P < 0.0001).
• A strong positive correlation was found between FS and V12 (r > 0.9), with an inverse correlation between GI and V12 (r < -0.7).

Conclusions:

• Stricter normal tissue objective (NTO) parameters lead to fluence contraction, effectively sparing normal brain tissue.
• Optimized NTO functions yield improved dose metrics, achieving a better balance in treatment plan goals for SBM SRS.
• This approach enhances the ability to spare normal brain tissue in SRS planning.