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2D Water Equivalent Path Length Imaging Technique For Pre-Treatment Range Verification In Proton Therapy

M Testa

M Testa1*, H Bentefour2, M Rose3, H Paganetti4, H Lu5, (1) Massachussetts General Hospital , Boston, MA, (2) Ion Beam Applications IBA, Louvain La Neuve, Belgium ,(3) Sun Nuclear Inc., Melbourne, FL, (4) Massachusetts General Hospital, Boston, MA, (5) Massachusetts General Hospital, Boston, MA

MO-A-213AB-8 Monday 8:00:00 AM - 9:55:00 AM Room: 213AB

To assess the potentials of a novel detector for providing a transmission image of the patient in terms of the Water Equivalent Path Length (WEPL) values and to evaluate the detector potential for real-time imaging of moving targets.

The method is based on the principle that for passively scattered proton beams the WEPL of any point located in the dose plateau of a spread-out Bragg-Peak can be derived from the time dependence of the dose rate function measured at this point. A flat-panel 2D detector array (Sun Nuclear Corporation, 249 diodes, pitch ~6 mm, 2 ms time resolution) was placed distal to a range of phantoms with varying complexities. The dose rate received by all diodes were measured as functions of time and analyzed to obtain the WEPL values. To assess the potential real-time features of this imaging technique, a Lucite cube was imaged while moving with a sinusoidal pattern with the amplitude and period comparable to a typical mobile tumor.

In water tanks, millimeter accuracy in the determination of the WEPL could be achieved and the geometrical shape of wedge and sphere phantoms could be reproduced. In more complex phantoms such as a Lucite step-like compensator or Medulloblastoma patient compensators, multiple Coulomb scattering and range mixing cause a slight deterioration in the reconstruction of the WEPL. We found that tracking of a moving target in the coronal plane is potentially feasible.

The technique gives less than 1 cGy of dose to patients and is therefore ideal for 'range-tuning' prior to treatment. For clinical applications, the beam for 'tuning' will need a deeper range than the prescription and once the radiological path length to the dosimeter is determined and compared with that from planning, the proton range for the actual treatment can be adjusted.

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