Choosing the prescription isodose line
A robotic plan is prescribed far lower than a C-arm plan on purpose — the heterogeneity is where the gradient comes from.
Built in·updated 2026-08-03
On a conventional linac the optimal prescription isodose for SBRT sits around 59–69% for lung and 67–77% for liver. On a robotic linac the same study found the optimum at 40–48% for lung and 41–42% for liver — a completely different regime.
The reason is that a non-isocentric delivery builds its gradient out of a heterogeneous target dose. Prescribing to a lower line means the hot core sits well above prescription and the fall-off outside is correspondingly steeper.
The floor. The same authors advise not going below 50%, because past that the inhomogeneity inside the target stops being useful and starts being a dose distribution nobody can interpret. In practice spine series report a median prescription isodose of 70% (IQR 65–77%).
Why this matters for a scored plan. Homogeneity index and conformity index pull in opposite directions here. A challenge that scores HI heavily is implicitly scoring against the technique the machine is good at, and a planner who prescribes to 80% to protect their HI will lose more on gradient index than they gain. Read the point weights before you choose the line.
Where the numbers come from
- Optimization of normalized prescription isodose selection for stereotactic body radiation therapy: conventional vs robotic linac
Med Phys / PubMed · 2013
Robotic optimum 40–48% (lung), conventional 59–69%; below 50% not advised
- Fiducial-free CyberKnife SBRT for single vertebral body metastases
Front Oncol · 2012
Spine practice: median prescription isodose 70%, IQR 65–77%