ESTRO 36 Abstract Book

S425 ESTRO 36 _______________________________________________________________________________________________

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QUASAR phantom (Modus Medical) MATLAB R2015b software (MathWorks)

Methods 4 treatment plans with various prescription doses were selected: liver SBRT [3x20Gy], lung SBRT [3x18 Gy], breast IMRT [27x1.85Gy@breast; 27x2.44Gy@boost] and head- and-neck IMRT [33x2.12Gy@high-risk; 33x1.8Gy@medim- risk; 33x1.6Gy@low-risk]. Plans were copied onto the QUASAR phantom. All treatments were planned with 6 MV RX but the liver SBRT that was planned with 15 MV. All plans had 7-11 fields. Two 6 cmx25.4 EBT3 film strips were attached to the QUASAR phantom along the axial plane. Plans were delivered on two twin linacs with different table tops. All experiments were repeated thrice. Films were read with FilmQA software. Dose maps were exported and then imported by MATLAB to apply the equivalent depth correction factors (EDCF) to EBT3 films (on areas not having any direct contact with the table top) to get dose at the ICRU skin depth (70µm). EDCFs have been determined from measurements made in a previous work [1] using a PTW23392 Extrapolation ion Chamber and measurements made now with EBT3 films. The application of other correction factors was not required as a result of other study presented by the authors at this meeting where EDCFs for EBT3 are also reported. We compared dose profiles along the middle of the strips for the two table tops. Results Figure 1 shows one example of the dose maps corresponding to the two strips for one session for each plan and table top. The effect of the Exact C ouch grid is visible, and it can increase the dose in contact to the grid up to 180%, but doses in contact to the IGRT Couch are much larger (see for example the two bottom subplots scale).

Results The first round of in-vitro tests in clinical ambient allowed to demonstrate that fiber optical based dosimeters are suitable for dosimetry in regimes such the ones in HDR prostate brachytherapy. The versatility of this kind of device and easiness of use allows application in other radiotherapy modalities. The dosimeter response was evaluated under irradiation with a 10.07 Ci Ir-192 HDR- brachytherapy The dosimeter shows a linear response with dose and is capable of detecting µGy dose variations like an ionization chamber.

Conclusion The first round of in-vitro tests in clinical ambient allowed to demonstrate that fiber optical based dosimeters are suitable for dosimetry in regimes such the ones in HDR prostate brachytherapy. The versatility of this kind of device and easiness of use allows application in other radiotherapy modalities. The dosimeter shows a linear response with dose and is capable of detecting µGy dose variations like an ionization chamber. PO-0798 Identification of areas of high-risk skin toxicity in SBRT and IMRT treatments P. Carrasco de Fez 1 , M.A. Duch 2 , N. Jornet 1 , M. Lizondo 1 , T. Eudaldo 1 , A. Latorre-Musoll 1 , C. Cases 1 , A. Ruiz 1 , M. Ribas 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain 2 Universitat Politècnica de Catalunya, Institut de Ciències Energètiques, Barcelona, Spain Purpose or Objective The AAPM TG176 shows examples of dramatic unexpected skin effects. This work is aimed: 1. To develop an in-phantom method to identify what areas of skin could be exposed to high doses and are of potential risk in order to decide where to perform skin in vivo dosimetry on SBRT (3DCRT) and IMRT treatments (sliding windows). To assess the impact of 2 table tops for these techniques. 2.

Table I shows the maximum total doses measured on dose profiles along the centre of the strips for the two table tops and the relative increase in dose due to the IGRT couch. For the IGRT couch top there are areas larger than 2 cm with a dose larger than twice the dose measured for the Exact Couch top.

Material and Methods •

Two Clinac 2100 CD (Varian) with the Exact Couch, one with the Exact Couch top (carbon fiber grid) and one with the IGRT Couch top EBT3 films+Film QA Pro 2014 Software (Ashland)+EPSON EXPRESSION 10000XL scanner

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