TU-H-CAMPUS-JeP2-04: Deriving Delivered Doses to Assess the Viability of 2.5 Mm Margins in Head and Neck SBRT

Authors


Abstract

Purpose:

To calculate the delivered dose for head and neck SBRT patients using pre-treatment images. This delivered dose was then used to determine the viability of 2.5 mm margins.

Methods:

Daily cone beam CTs (CBCTs) were collected for 20 patients along with a planning CT, planned dose, and planning structures. The day 1 CBCT was aligned to the planning CT using the treatment shifts (six degrees of freedom) and then the dose and contours were transferred to the CBCT. The day 1 CBCT becomes the reference image for days 2–5. The day 2–5 CBCTs were also aligned to the planning CT using the treatment shifts given and the dose transferred. The day 2–5 CBCTs were then deformably registered to the day 1 CBCT. The doses delivered on days 2–5 were then deformed to the day 1 CBCT where they could be accumulated. This was achieved with MIM 6.5.1 (MIM Software, Cleveland OH). The accumulated doses for the 20 patients were evaluated against the planned doses using the initial planning criteria as points of comparison.

Results:

The delivered CTV dose conformed to the planned 98.6% coverage, with an average decrease of 2.6% between planned and delivered coverage. This implies the 2.5 mm margin was sufficient. Larger CTVs correlated to smaller differences between planned and delivered coverage. Delivered dose to critical structures including the spinal cord, mandible, brain, brainstem, and larynx was acceptable, with differences between planned and delivered max dose <5% on average. Similarly for the parotid glands, globes, cochlear, optic nerve, lens, and submandibular glands, differences between planned and delivered doses were generally <5%.

Conclusion:

The 2.5 mm margin provided acceptable CTV coverage, adequately accounting for setup errors. Organ at risk sparing was also satisfactory. Small tumor volumes (<20 cc) may require a larger margin to treat effectively.

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