A comprehensive dosimetric study on switching from a Type-B to a Type-C dose algorithm for modern lung SBRT

Area of Science:

• Radiation Oncology
• Medical Physics
• Cancer Treatment Planning

Background:

• Type-C dose algorithms offer improved accuracy in lung stereotactic body radiation therapy (SBRT) planning.
• Current dosimetric protocols, developed for Type-B algorithms, require validation for Type-C applicability.
• Previous studies had small sample sizes and conflicting results regarding Type-C algorithm implementation.

Purpose of the Study:

• To assess the dosimetric impact of switching from Type-B to Type-C dose algorithms in lung SBRT.
• To evaluate the applicability of current dosimetric protocols with Type-C algorithms in a large patient cohort.
• To determine necessary treatment planning adjustments for Type-C algorithm implementation in lung SBRT.

Main Methods:

• Compared four treatment plans for 52 lung SBRT patients: original Type-B, Type-C re-calculated, re-normalized, and re-optimized.
• Utilized coplanar VMAT arcs normalized to D95% = prescription dose for Type-B plans.
• Evaluated multiple dosimetric endpoints including RTOG criteria, PTV coverage, homogeneity index, and conformity index.

Main Results:

• Re-calculated Type-C plans showed compromised PTV coverage (mean D95% and V100% reduction) despite similar RTOG compliance.
• Re-normalized Type-C plans restored target coverage but degraded plan conformity and worsened RTOG criteria adherence in some cases.
• Re-optimized Type-C plans largely restored RTOG compliance and dosimetric values, with slightly increased target dose heterogeneity.

Conclusions:

• Type-B SBRT lung plans can overestimate target coverage, necessitating Type-C re-normalization or re-optimization.
• Current RTOG dosimetric criteria remain appropriate for evaluating lung SBRT plans with Type-C algorithms.
• Transitioning to Type-C algorithms requires careful re-optimization to ensure accurate dosimetry and treatment efficacy.