ESTRO 38 Abstract book

S1155 ESTRO 38

exposed map: calculated and measured values compare well. This is also reflected in the Gamma analysis: for each of the 3 films irradiated inside the Rando head phantom a passing rate higher than 97% (3%/2 mm, local) between planned and delivered dose distributions were measured.

EP-2091 How to measure high dose in functional disorder treatment: an innovative silicon diode detector. P. Gallo 1 , E. De Martin 1 , M.L. Fumagalli 1 , F. Ghielmetti 1 , M. Carrara 2 , S. Alhujaili 3 , M. Lerch 3 , A.B. Rosenfeld 3 , M. Marchetti 1 , L. Fariselli 1 , M. Petasecca 3 1 Fondazione IRCCS Istituto Neurologico Carlo Besta, Health Department, Milan, Italy ; 2 Fondazione I.R.C.C.S. Istituto Nazionale dei Tumori di Milano, Health Department, milan, Italy ; 3 Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia Purpose or Objective Purpose: Stereotactic radiosurgery (SRS) of intra-cranial lesions is a treatment modality where a well defined target volume receives a high radiation dose in a short fractionation regimen. SRS is usually characterised by steep dose distributions achieved through the delivery of small radiation fields and therefore requires dosimetry performed with small active volume detectors . The purpose of this study is to validate the use of an innovative angular independent edgeless-diode detector, already characterised for standard Quality Assurance (QA) of Cyberknife, in stereotactic radiosurgery for brain functional disorders and trigeminal neuralgia. Material and Methods Methods and materials : An anthropomorphic head phantom (RANDO, The Phantom Laboratory, Salem, NY) was CT imaged with three edgeless diodes inserted in central, 8 mm cranial and 26 mm caudal positions (Figure1) targeting the trigeminal nerve close to its root entry in the brainstem. The image set was transferred to the treatment planning system and a treatment volume was contoured by simulating a typical trigeminal target in correspondence of the central diode. In the framework of a dose escalation protocol, a radiosurgery plan (Rando) was optimized and delivered with the central diode receiving prescription doses respectively of 75, 100, 140 and 200 Gy in single fraction. Simultaneously to the 75 Gy - Rando delivery an EBT3 film, properly prepared and calibrated, was also arranged in correspondence with the caudal diode position, where a prescription dose of 12 Gy was delivered. The EBT3 reading was used to validate the corresponding diode’s recorded value. Quality-assurance (QA) verifications were also performed for three patient planes treated at different prescription doses. In this case the central diode was in correspondence with the contoured trigeminal target. Finally, the doses measured by the three diodes were compared to the planned ones. Results The comparison between measured and planned doses is show in the following table. The recorded values of the dose to the central diode (target) were within 3.6% of the plan for all the seven delivered plans (4 Rando plans and 3 patients). The difference between the EBT3 reading and the caudal diode’s recorded value in the Rando-plan was smaller than 5%. Conclusion C onclusion: The edgeless-diode detector was employed to measure the dose delivered by a full trigeminal stereotactic treatment. The obtain results combined with its mechanical and electrical characteristics make it a suitable choice for QA dosimetry of trigeminal neuralgia

Conclusion These results give an in phantom demonstration that multiple brain lesions can be treated with the requested accuracy by HT. When treating real patients, contributions to the uncertainty in dose delivery such as intra-fraction motions have also to be taken into account.