Isotoxic stereotactic reirradiation for recurrent pelvic cancers

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Reirradiation of pelvic tumors is complex, balancing tumor dose with organ at risk (OAR) constraints.
• Standard radiotherapy prescriptions can be conservative, failing to account for individual patient OAR dose variability.

Purpose of the Study:

• To assess the technical feasibility of isotoxic pelvic reirradiation.
• To individualize radiotherapy by delivering the highest equieffective dose in 2 Gy per fraction (EQD2Gy) while respecting OAR constraints.

Main Methods:

• A technical feasibility study involving 30 patients previously treated with pelvic stereotactic body radiotherapy (SBRT).
• Deformable registration of previous and reirradiation image sets to map prior dose.
• Voxel-by-voxel EQD2Gy optimization to achieve isotoxic plans, escalating dose up to 50 Gy.

Main Results:

• Clinically acceptable isotoxic plans were achieved for 25 out of 30 patients.
• 23 patients received doses exceeding the standard UK reirradiation prescription of 30 Gy/5 fractions.
• The median isotoxic prescription dose was 42 Gy/5 fractions, with vessels and sacral plexus being common dose-limiting structures.

Conclusions:

• Isotoxict pelvic reirradiation is technically feasible.
• The approach allows for higher, personalized doses compared to standard prescriptions.
• Further research into automation and predictive models is recommended for clinical implementation.