ESTRO 38 Abstract book

S561 ESTRO 38

The purpose of end-to-end testing (E2E) is to confirm that the entire logistic chain of a radiation treatment starting from CT imaging, treatment planning, patient positioning and verification and beam delivery is adequately implemented resulting in sufficient accuracy of planned dose delivery. A novel methodology for dosimetric E2E based on customized anthropomorphic phantoms using alanine dosimetry, ionization chambers and radiochromic films was established at a scanned proton therapy facility (called A here). Based on this methodology an independent dosimetry audit was developed and applied, for the first time, to a starting proton therapy facility (called B here) equipped with a scanned beam delivery system. We present results of both proton facilities including overall 4 different beam lines. Material and Methods A homogeneous polystyrene phantom and two anthropomorphic phantoms (pelvis and head phantom) have been customized to allocate different detectors such as radiochromic films, Farmer chamber and alanine pellets. During testing, the phantoms were moving through the workflow as real patients to simulate the entire clinical procedure. The CT scans were acquired with pre-defined scan protocols used at the A and B proton therapy facility for cranial and pelvic treatments. All treatment planning steps were performed with RayStation (RS) v6.1 and v7.0 TPS available respectively at A and B institute. A physical dose of 10 Gy was planned to clinically shaped target volumes in order to achieve reproducibility better than 0.5% on the dose delivered to the alanine pellets. In the treatment rooms the plans were delivered to the phantoms loaded either with alanine pellets and radiochromic EBT3 films or a Farmer chamber (figure 1). The alanine pellets (5.0 mm diameter and 2.4 mm thickness) and their read-out were provided by NPL. Corrections for the alanine “quenching” were derived by a Monte Carlo dose calculation platform implemented in a non-clinical version of RayStation.

Conclusion Our experience shows that alanine pellets are suitable detectors for dosimetric E2E test based audits and the developed procedures can be used to support implementation of upcoming new proton beam therapy facilities in Europe and may also serve as dosimetric credentialing for clinical trials in the future. PO-1015 Design of 2.5 MeV electron beam applicator for < 5 mm thick superficial lesions A. Valve 1 , H. Nurmi 1 , A. Kulmala 1 , S. Koskenmies 2 , M. Tenhunen 1 1 Helsinki University Hospital, Cancer Center, Helsinki, Finland ; 2 Helsinki University Hospital, Department of Dermatology, Helsinki, Finland Purpose or Objective Skin cancers are the most common of all malignant diseases. Many types of small cancerous skin lesions require only ≤ 5 mm treatment depth. Treating that kind of targets with any deeper penetration is unnecessary and may be harmful. We have modified a standard electron applicator to produce a set of well collimated circular fields, aiming nearly constant depth dose between 0 – 5 mm and a steep fall-off at depths > 6 mm. Material and Methods The standard electron 6x6 cm square applicator of the Elekta Versa HD linear accelerator has been modified to reduce nominal 4 MeV electron beam suitable for the purpose. With a 6 mm thick polymethyl-metacrylate (plexiglas) filter slab bottom edge located 18.0 cm from the isocenter the energy was reduced from R50 = 1.8 cm of the standard 4 MeV to R50 = 1.0 cm. To achieve sharp edges the fields has been collimated with the plexiglas tubes from 2.5 to 5 cm inner diameters. In addition, lateral radiation through open sides of the applicator outside the field edges has been reduced with plexiglas plates. The final design of the applicator is presented in Fig 1.

Results The measured absolute dose to water obtained with the Farmer chamber in all delivered plans was within 2% of the TPS calculated dose. A maximum lateral homogeneity index of 3.5% inside the treatment field was measured with EBT3 films. Doses derived with the alanine pellets after correction for the quenching effect showed a mean deviation within 2% and a maximum deviation below 5% in the homogeneous and anthropomorphic phantoms (figure 2). Several audits are planned to be performed in the near future and more results coming from other proton therapy facilities may be available at the time of the presentation.