Abstract Book

S923

ESTRO 37

EP-1727 Low-cost versatile 4D lung phantom for QA of 4D-CT, 4D-CBCT, planning and dosimetry E. Franken 1 , E. Lefeber 1 1 Haga Ziekenhuis Locatie Leyenburg, Radiotherapy, Den Haag, The Netherlands Purpose or Objective For stereotactic treatment of lung tumors complex treatment techniques are needed to accurately deliver the high fraction doses to the moving tumor, while sparing the surrounding organs at risk. To be able to do QA of the techniques involved, such as 4D-CT, IGRT with 3D- and 4D-CBCT imaging and dosimetry, or for a complete end-to-end test, a dynamic (“4D”) antropomorphic phantom is needed. Such phantoms are commercially available (Quasar Respiratory Motion Phantom (ModusQA), Dynamic Thorax Phantom (CIRS), …) but cost over 10-15.000 Euro. The in-house developed phantom has similar performance capabilities, but at a material cost of less than 200 Euro. Material and Methods The phantom consists of a plastic bottle, filled with cork- powder with lung tissue like density, with a 2-cm diameter plastic ball as “tumor” (Figure 1). For accurate dosimetry, a measurement probe with a diameter of < 7 mm can be placed in the "tumor" center via a plastic tube from the bottom of the bottle. The bottle tightly fits in a double walled, water filled cylinder, constructed from PVC pipes (inner/outer diameter 10/20 cm). Hence, the “lung” (with "tumor") is surrounded by a 5-cm layer of “thorax tissue”. In the water-filled compartment, a 4- cm-diameter PVC pipe serves as a “spinal cord”, with a series of holes to facilitate longitudinal positioning of the phantom using CBCT. An electromotor moves the bottle in a sinusoidal pattern in the cylinder, with adjustable amplitude (peak-peak 0-5 cm) and frequency (5-25 rpm), resulting in a realistic breathing cycle motion-trajectory of the “tumor”. Via a rubber band, a pressure cycle is created for an Anzai belt respiratory gating system sensor, in order to facilitate 4D-CT registration. This phantom element can be adjusted for other gating systems. The construction of the phantom is straightforward, without complex manufacturing techniques.

Conclusion The performance of an angular independent silicon detector (edgeless) developed by CMRP was investigated in two different Cyberknife machines (G4 and M6) with both collimator types (fixed cones and Iris). It has been found that the results obtained by edgeless detectors were in close agreement with the data obtained by common commercial detectors employed in Cyberknife QA and with TPS calculations, proving its suitability to be the candidate for dosimetric verification in non-isocentric SRS modalities. EP-1726 Implementation of AAPM recommendations on medium for TPS reference dose specification

Results The phantom is used for a series of tests. The tumor volumes and breathing artifacts were verified in the phases of a 4D-CT. From this scan also the breathing amplitude was obtained, which was then used for dose planning. On the linac, the same IGRT positioning procedure was performed as for actual lung patients, applying dual matching with both 3D-CBCT (clipbox around the “spinal cord”) and 4D-CBCT imaging (mask around the “tumor”). The VMAT plan delivery was verified dosimetrically for a CC-profile with a PTW diode in the “tumor” center at a series of stationary positions (Figure 2). Also for a moving “tumor” (10 rpm, amplitude 5 cm), the results corresponded to within 0.3% with the treatment planning system.

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