Dose prescription with spatial uncertainty for peripheral lung SBRT

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Current lung SBRT practice often prescribes to the planning target volume (PTV), leading to potentially excessive doses in the internal target volume (ITV).
• This occurs because PTV margins frequently have low physical density, resulting in dose inhomogeneity.

Purpose of the Study:

• To investigate an alternative SBRT planning strategy for lung cancer by prescribing to the ITV, incorporating positional uncertainties.
• To compare dose distributions achieved with PTV-based prescription versus ITV-based prescription using probabilistic methods.

Main Methods:

• Retrospective analysis of five lung cancer patients treated with volumetric modulated arc therapy.
• Generation of five treatment plans per patient: static PTV D95% prescription, three probabilistic ITV D95% prescriptions with varying uncertainty distributions (uniform, delta function, Gaussian), and a re-planned static ITV D95% plan.

Main Results:

• Prescribing to the ITV resulted in a median ITV D50% of 10% higher than the prescription, compared to 23% higher for static PTV planning.
• The specific uncertainty distribution (uniform, delta, Gaussian) had a minimal impact (<2%) on the median ITV dose.
• Probabilistic planning to the ITV yielded a median ITV D95% only 1.5% higher than re-planning a static plan and evaluating probabilistically.

Conclusions:

• A robust probabilistic approach to lung SBRT planning, prescribing to the ITV, achieves a dose distribution closer to the intended prescription.
• The precise modeling of spatial uncertainty distributions is not critical for achieving robust ITV dose coverage.