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Experimental Study of Acceptable Movement Conditions for SBRT Lung Treatments

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M Ribas Morales

P Carrasco de Fez , A Ruiz-Martinez , N Jornet , T Eudaldo , A Latorre-Musoll , M Ribas Morales*, Hospital de la Santa Creu i Sant Pau, Barcelona, Barcelona


SU-E-T-178 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To experimentally study the acceptable movement conditions for SBRT lung treatments we quantified with film dosimetry the change in dose distributions due to periodic movements of 5 different amplitudes and 4 respiratory gating duty cycles on a SBRT treatment plan.

We planned a SBRT treatment plan for the QUASARâ„¢ (Modus Medical) phantom equipped with the respiratory motion device. We placed a 3 mm water-equivalent sphere simulating a tumour inside the lung-equivalent insert. This sphere is divided in two hemispheres that allow placing films in between. We used radiochromic EBT2â„¢ (Ashland) films. We oriented the lung insert in such a way that sagittal dose distributions could be measured. We applied a sinusoidal movement with 3 s period for 5 different amplitudes of 0(static), 5, 7, 10, 15 and 20 mm without gating. For the 20 mm amplitude we studied the gating technique with 4 duty cycles of 20, 40, 60 & 80% of the respiratory cycle. Each situation was irradiated in a Clinac 2100 linac (Varian) equipped with the RPMâ„¢ system. FilmQA Proâ„¢ (Ashland) software together with an Expression 10000XL scanner (EPSON) were used to analyze and compare the measured dose distributions with those planned by the Eclipseâ„¢ TPS v. 8.9 (Varian) by means of gamma analysis with 6 criteria: 5%/3mm, 5%/2mm, 5%/1mm, 3%/3mm, 3%/2mm and 2%/2mm (threshold of 10%).

Movements with amplitude of less than 7mm do not significantly modified the dosimetry. Gating duty cycles of less than 40% yielded also acceptable results for a 2 cm amplitude movement.

To safely perform daily accurate SBRT treatments, movements have to be restricted to 7 mm amplitude (±3.5 mm). Otherwise, a gating strategy should be considered.
This work was partially financed by grants FIS PI12-00521 and Barcelona board of the Spanish Association Against Cancer (AECC) 2012.

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