ESTRO 37 Abstract book

S924

ESTRO 37

For the electrons, all commercial algorithms save one can report dose-to-tissue and do not require a correction. PB is rarely a single option any more and despite the use of some tissue-specific parameters essentially produces dose-to-water, thus requiring a correction. Conclusion • Linac reference calibration should be reported in water and never converted to muscle per se.

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If necessary, a 0.99 times the dose-to-water correction should be applied in the TPS reference dose specification. This should be done on an algorithm-by-algorithm basis, bearing in mind that in a general family of algorithms specific implementation may change the approach. A qualified medical physicist should ascertain if the specific TPS algorithm reports dose-to- water or dose-to-tissue and accordingly set the TPS reference dose for that algorithm TPS vendors are encouraged to evolve their algorithms to calculate and report dose-to- tissue, so that manual corrections to the reference dose, with their associated extra uncertainty, are no longer necessary.

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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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 in-house developed phantom is a versatile 4D antropomorphic lung phantom, well suited for (routine) QA measurements of all techniques involved in lung stereotactic treatments, and also for end-to-end tests of such treatment. It can be applied for the QA of 4D-CT, 3D- and 4D-CBCT as well as dosimetry. With material costs of less than 200 euros, the phantom has an unparalleled price/quality ratio. EP-1728 Small field dosimetry formalism implemented using various detectors on several linear accelerators M. Ghazal 1 , A. Carlsson-Tedgren 1 , H. Benmakhlouf 1 1 Karolinska University Hospital, Department of Radiation Physics and Nuclear Medicine, Stockholm, Sweden Purpose or Objective The objective of the study was to implement the new small photon field dosimetry formalism by Alfonso et al 2008 to linear accelerators (LINAC) at Karolinska University Hospital. The detector response in small photon fields was analysed. Monte Carlo (MC) calculated output correction factors (OCF) were applied to