EP-1325 EVALUATION OF ARCCHECK DIODE ARRAY FOR QA IN IMRT AND VMAT

ESTRO 31

S503

accommodate a rectangular phantom in order to perform film dosimetry in the device's central plane. This work has been carried out with the measurements corresponding to seven dynamic IMRT clinical treatments, one of each type of class solution that has been employed at our centre. The dose distributions were analysed according to the 3%/3mm Gamma index criterion, always considering relative doses. Results: The table shows the percentage of points fulfilling the Gamma index criterion (3%/3mm) that correspond to both the analysis performed with the ArcCHECK's diodes and the one carried out with the film, for each treatment type and film orientation. Film in coronal plane through the Film in sagittal plane through the isocentre isocentre Treatment

ArcCHECK γ(3%/3mm)≤1

Film γ(3%/3mm)≤1

ArcCHECK γ(3%/3mm)≤1

Film γ(3%/3mm)≤1

Prostate

99.5%

98.6%

99.8%

98.6%

97.3%

97.8%

96.8%

97.4%

Head neck

and

Head neck

and

98.3%

96.5%

99.3%

98.0%

Brain

97.7%

95.7%

94.6%

96.9%

Brain

98.8%

97.4%

98.3%

94.6%

Hemangioma 98.7%

99.3%

97.9%

94.8%

Rectum

96.2%

99.6%

96.1%

99.5%

The figure shows the number of cases in which both measurement systems pass the Gamma index tolerance criterion established at our centre (percentage of passing points equal to or over 96), the number of cases in which none of them pass it, and the ambiguous cases in which only one of them satisfies the tolerance criterion.

Conclusions: Considering the obtained results it seems that, globally, it is true that when relative dose distributions measured with the helical diode array comply with the tolerance for the Gamma index analysis, the relative dose distributions are also correct in the coronal and sagittal planes containing the isocentre. In order to explain the differences encountered in the ambiguous cases, it is important to bear in mind that comparisons have been made between systems with very different geometrical arrangements, dosimetric performance and sources of uncertainty in the measurement and analysis processes, and therefore, could not even be expected to fulfil the same tolerance criterion in percentage of points passing the Gamma criterion. EP-1325 EVALUATION OF ARCCHECK DIODE ARRAY FOR QA IN IMRT AND VMAT G. Reynés Llompart1, J. Puxeu Vaqué2, I. Sancho Kolster2, I. Modolell Farré2 1 Hospital de Bellvitge Princeps d'Espana, Física i protecció radiologica, Barcelona, Spain 2 Institut Català d'Oncologia, Física i protecció radiologica, Barcelona, Spain Purpose/Objective: With the most common use of complex techniques in radiotherapy many centers have opted to use a 3D detection system. ArcCHECK (Sun Nuclear) presents a possible solution through a cylindrical array of diodes. The aim of this study is to present the characterization of the dosimetric parameters significant for IMRT and VMAT verifications.

Materials and Methods: ArcCHECK is an isotropical cylindrical detector array for arc delivery QA and dosimetry, which consists of 1386 detectors of 0.8x0.8 mm in a helical geometry with a diameter of 21.0 cm and the detectors arranged in a grid of adjacent rings spaced 1 cm apart; the system multiplies the resolution measuring entrance and exit dose. The tests were performed in a TrueBeam (Varian), with energies of 6 MV and 10 MV, the calculation algorithm was AAA V10.27 in a Varian Eclipse. All measurements were analyzed using the Sun Nuclear software SNC Patient. The hollow phantom configuration of the device was used implying a large air-density inhomogeneity. Reproducibility and repeatability was analyzed using a 360º isocentric arc of 10x10 cm; spatial resolution was determined applying displacements on the positive cranio-caudal axis with the submillimetric precision of the TrueBeam couch. Moreover, dosimetric resolution was determined using square fields. Dependence on the field dose and dose rate were also performed. Additionally an arc with the isocenter near the surface of the grid was analyzed and similar tests with single and multiple square fields was done. Results: For systematic error a value of 0.5% (k=1) was obtained; for the reproducibility and set up errors a similar value was achieved in a non clinical practice. Spatial resolution, in 3D mode, is of 0.2 ± 0.1 mm; a 1.5% absolute dose resolution was found in measurements between 100-400 UM and 1.8% between 400-600 UM. The analysis of single fields shows that the reliability of the system increases with the number of fields. Conclusions: ArcCHECK is a sensitive and fast QA device with good reliability and precision for IMRT and VMAT. Further study should be done for other applications such as single field dosimetry. EP-1326 PHOTON BEAM QUALITY CORRECTION FACTORS UNDER NONREFERENCE CONDITIONS N. Chofor1, D. Harder2, B. Poppe1 1 Pius - Hospital, Clinic of Radiotherapy and Oncology, Oldenburg, Germany 2 Georg-August University, Medical Physics and Biophysics, Göttingen, Germany Purpose/Objective: Radiation quality correction factor kQ accounts for changes of detector response under reference conditions (photons of quality index Q, at a point on the beam axis at 10 cm water depth for a 10 x 10 cm² field) compared with calibration conditions (60Co gamma radiation, for the same geometry). Clinical dosimetry is usually performed under non-reference conditions, i.e. at varying depths, field sizes and off-axis distances including out-of-field points. The non-reference condition correction factor kNR accounts for the associated changes of detector response, while kNR= 1 under reference conditions. In this study, Monte Carlo methods were used to evaluate kNR for an air-filled ion chamber, two TLD detector types and two types of Si diode. Materials and Methods: Using Monte Carlo codes BEAMnrc and FLURZnrc, complete photon fluence spectra ΦE(E) for 6 and 15 MV photon beams from a Siemens Primus linac were scored in a large water phantom. The energy-dependent responses rt(E) of detector type t for monoenergetic photons, valid for a Farmer ion chamber (type NE2571), for TLD types LiF:Mg:Ti and LiF:Mg:CuP, and for the shielded EDP-10 and unshielded EDD-5 Si diodes were taken from the literature. By combining spectra ΦE(E), response functions rt(E) and mass energy absorption coefficients of water, μen(E)/ρ, the mean detector response Yt(x) under general conditions x was calculated and compared to its value Yt(xref) under reference conditions. Factor kNR was obtained as the ratio Yt(xref) / Yt(x). As a specialty for small field dosimetry, a 4 × 4 cm² field was used as the reference field size. Results: Within the field limits, kNR deviated less than 0.5% from unity for the Farmer chamber but was largest for the EDD-5 diode, by up to 16% at 6 MV due to overresponse for field sizes > 10 × 10 cm². Results for the EDP-10 diode were similar to those of the Farmer chamber, with slight underresponse for small fields. For the TLDs, kNR for the LiF:Mg:CuP detector varied by up to about 2% due to underresponse at larger fields, whereas the values were lowered by less than 0.5 % for LiF:Mg:Ti. For out-of-field regions, kNR values for the Farmer chamber were lowered by up to 2%, and reduced by up to 60% respectively 20% for the EDD-5 and EDP-10 diodes due to overresponse. kNR values for LiF:Mg:CuP and LiF:Mg:Ti were lowered by at most 15% and 5% respectively. - For small fields, it is advantageous to use a 4 × 4 cm² reference field (non-reference condition correction factor kNRSF) as illustrated for a 10 × 10 cm² field in Table 1.