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Computing Proton Dose to Irregularly Moving Targets

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J Phillips

J Phillips1*, G Gueorguiev2, J Shackleford3, C Grassberger4, S Dowdell5, H Paganetti6, G Sharp7, (1) Massachusetts General Hospital, Boston, MA, (2) Massachusetts General Hospital, Boston, MA, (3) Drexel University, Philadelphia, PA, (4) Francis H. Burr Proton Therapy Center, Boston, MA, (5) Massachusetts General Hospital, Boston, MA, (6) Massachusetts General Hospital, Boston, MA, (7) Massachusetts General Hospital, Boston, MA

TU-A-108-6 Tuesday 8:00AM - 9:55AM Room: 108

Purpose: While 4DCT and deformable registration can be used to assess the dose delivered to
regularly moving targets, there are currently few methods available for irregularly moving targets.
This study describes a method for computing the dose delivered to irregularly moving targets based
on 1D or 3D waveforms captured at the time of delivery.

Methods: The procedure requires 4DCT images for dose calculation, and 1D or 3D respiratory
waveforms to estimate target position at time of delivery. Dose volumes are converted from their
Cartesian geometry into a beam-specific radiological depth space, parameterized in 2D by the beam
aperture, and longitudinally by the radiological depth. In this water-equivalent depth space, the
proton doses are translated according to the motion found in the 1D or 3D trajectory. These
translated dose volumes are weighted and summed, then transformed back into Cartesian space,
yielding an estimate of the dose that includes the effect of the measured breathing motion.

Results: The method was validated using a synthetic lung CT phantom with a mobile tumor.
Simulated 4DCT was generated for the phantom with 2 cm peak-to-peak motion. A passively-
scattered proton treatment plan was generated using a 7 mm smearing radius. The method was
tested without motion, and with two simulated breathing signals: a 2 cm amplitude sinusoid, and
a 2 cm amplitude sinusoid with 3 cm linear drift. Motion-corrected dose was computed based on
the mid-ventilation CT image. Gamma evaluation (3 mm, 3%) was 98.7% without motion, 92.9%
for 2 cm sinusoidal motion, and 98.0% with 3 cm drift.

Conclusion: We have demonstrated a method for accurately reproducing proton dose to an
irregularly moving target from a single CT image. We believe this algorithm could prove a useful
tool to study the dosimetric impact of baseline shifts either before or during treatment.

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