Abstracts / Physica Medica 30 (2014) e45ee74 e63

other things,with respect to the radiation applied, thematerials used and themanner of dose detection. The result of the experimentwas amean absorbeddose per incident particle which could be compared to the result of an MCcalculation on an absolute basis. A special radiation source is necessary toevaluate the mean absorbed dose per incident particle in an experiment,since the number of source particles is usually not known for radiationsources. An accurate characterisation of the source is also important to definea realistic radiation source for the calculation.A radiation sourcewhichmeetsthese requirements is operated by PTB. The experimental part of the beam-line at PTB‘s research linear accelerator is equipped with several devices forthe characterisation of the electron beam. There are beam current monitorsto evaluate the number of electrons, a magnetic spectrometer to obtain in-formation on the spectral fluence of the electron beam and beam profilemonitors to measure geometric characteristics of the beam.The benchmark experiment was modelled in a widely used MC softwaretool for radiation transport calculations, too. A good agreement was foundfor the result of the benchmark experiment and the corresponding resultof the MC calculation. The related uncertainties were less than 1 % in bothcases. The results of the work exemplify evidence of the absolute cor-rectness of dosimetric quantities calculated with the MC method in theapplication field of radiation therapy.

IMPACT TO TOTAL SCATTER FACTORS OF THE DOSIMETER ON THECALCULATED DOSE DISTRIBUTION IN STEREOTACTIC RADIOSURGERY

O.A. García-Gardu~no a, J.M. L�arraga-Guti�errez a. a Laboratorio de FísicaMedica, Instituto Nacional de Neurología y Neurocirugía, Insurgentes sur3877, C.P. 14269, M�exico, D.F., M�exico

Physical and technical challenges exist in the dosimetry of small-fieldswhich are wide highlighted in the literature. However, the literature didnot report the impact of the dosimeter in the calculated dose distribution.The goal of this workwas to assess the impact of relative measurements of:off axis ratios, tissue maximum ratios and especially total scatter factor(TSF) on the calculated dose distribution. Six detectors were employed tocharacterize circular collimated beams generated with a dedicated linearaccelerator: three diodes, one ionization chamber and two radiochromicfilms. The relative measurements were incorporated in the treatmentplanning system (TPS) in order to compare and analyze the calculated dosedistributions (DD). The major discrepancies were in the TSF that showedthat all the radiation detectors had a good agreement within their statis-tical uncertainties with the exception of the CC01 and EBT2 film. Thecalculated DD were normalized with respect to the TSF and analyzed bygamma index test by using a different calculation grid size. The DD did notshow any important difference when a gamma criteria of 3/3% was used.According with the results presented in this work, we consider that alldetectors were excellent for the relative dosimetry of small photon beamsfor plan evaluation. But it is necessary to consider the TSF, because the lateris involved in the calculation of the monitor units which is not reflected onthe calculated dose distribution by TPS and may impact the delivered doseto the patient.

NONCOPLANAR BEAM ANGLE OPTIMIZATION IN IMRT TREATMENTPLANNING USING DERIVATIVE-FREE ALGORITHMS

H. Rocha a, J. Dias b, B. Ferreira c, M.C. Lopes d. a INESC-Coimbra, Coimbra,Portugal; b FEUC, Coimbra, Portugal; c I3N, Aveiro, Portugal;; d IPOC-FG EPE,Coimbra, Portugal

Purpose/Objective: The planning of radiation therapy treatments requiresdecisions regarding the angles used for radiation incidence. The beamangle optimization (BAO) problem consists in finding the optimal numberand incidence directions of the irradiation beams. The selection ofappropriate radiation incidence directions is important for the quality ofthe treatment. However, the possibility of improving the quality of treat-ment plans by an optimized selection of the beam incidences is seldomdone in clinical practice. Adding the possibility for noncoplanar incidencesis even more rarely used. Nevertheless, the advantage of noncoplanarbeams is well known. We present the benefits of using pattern searchmethods (PSM) for the optimization of the highly non-convex noncoplanarBAO problem.

Materials and Methods: PSM are derivative-free optimization algorithmsthat require few function evaluations to progress and converge and havethe ability to better avoid local entrapment making them a suitableapproach for the resolution of the noncoplanar BAO problem. Locallyadvanced head and neck clinical cases were selected to test this approach.The planning target volumes included the primary tumor, the high and lowrisk lymphnodes. Organs-at-risk included the parotids, the brainstem, andthe spinal cord. For each case, a setup with five equidistant beams waschosen and the resulting treatment plan using a multicriteria optimizationapproach was then compared with the coplanar and noncoplanar plansusing the optimal beam setup obtained by PSM.Results: For the clinical cases retrospectively tested, the use of nonco-planar directions in our tailored approach showed a positive influence onthe quality of the solution found. For the same target coverage, thenoncoplanar treatment plans obtained an average reduction of the pa-rotid's mean dose irradiation in 4.2 Gy compared to the equidistanttreatment plans while the coplanar treatment plans only obtained a 2.7 Gyaverage decrease.Conclusions: PSM has shown ability to avoid local entrapment and effi-ciency on the number of function evaluations even using noncoplanardirections, which is a major advantage compared to other global heuristicswhere the continuous solution space is confined to a discrete subset, oftenonly considering coplanar irradiation angles to achieve clinically accept-able computation times.

DOSIMETRIC ANALYSIS OF PATIENTS WITH IMPLANTED CARDIACDEVICES UNDERGOING RADIOTHERAPY. ADDITIONAL PROPOSEDPRECAUTIONS TO THE LAST UPDATED GUIDELINES

Guadalupe Martín Martín a, Rocío Bermudez Luna a, Alfonso L�opezFern�andez a, C�esar Rodríguez Rodríguez a, Mª Victoria de TorresOlombrada b. aMedical Physics Department. Radiation Therapy Service.Hospital Universitario de Fuenlabrada, Madrid (Spain); bRadiationOncology Department. . Radiation Therapy Service. Hospital Universitariode Fuenlabrada, Madrid (Spain)

Aims: There is few literature available regarding radiotherapy in patientswith Cardiac Implantable Electronic Devices (CIEDs). Official guidelineshave not been updated since 1994 and CIEDs manufacturing technology, aswell as radiotherapy techniques, have considerably changed since then. Anupdated practical guideline was developed in The Netherlands to takethese changes and the patient’s perspective into account (1). The aims ofthis work are to describe the implementation of this new protocol in ourInstitution, and to propose additional precautions regarding some imagingtechniques not described in detail within this guideline.Materials and methods: Between 2013 and 2014, fifteen patients withCIEDs of different primary sites were treated in our institution. A treatmentplan optimally covering the target area and maximally sparing the CIEDwas generated for all patients. Dose to the CIED was assessed and patientswere classified according to their risk as recommended by the Dutchguideline. Patients were evaluated before, during and after radiotherapyby a cardiologist.Results: Our patient focus group consisted of 7, 7 and 1 patients cat-egorised into low, medium and high risk groups, respectively. The medianage was 71 (range 54-91) years. There were five men with head and neckprimaries, threemenwith lung primaries, 5 menwith pelvic primaries andtwo women with breast primaries. The prescribed dose ranged from 20 to78 Gy with a daily dose ranging from 1.8 to 4 Gy. The maximum doses tothe CEIDs were 179.1 cGy, 751 cGy and 1270 cGy, for the low, medium andhigh risk groups, respectively. Radiation therapy was safely delivered in allpatients, but in two of them we experienced a CIED malfunction (withoutany untoward effects for the patients' health), while Cone-Beam acquisi-tion previous to the treatment was taking place.Conclusion: Radiotherapy was safely delivered in a series of fifteen patientswith CIEDs following the recommendations of the Netherlands task group.However, we included some additional precautions related to 3D imagingacquisition techniques which are not described in detail in their guideline.References(1) Management of radiation oncology patients with a pacemaker or ICD: anew comprehensive practical guideline in The Netherlands. Dutch Societyof Radiotherapy and Oncology (NVRO). Radiat Oncol. 2012 Nov 24;7:198.