ESTRO 37 Abstract book

S1243

ESTRO 37

of the tumor. The simulations were performed using the Monte Carlo PENELOPE code specifically adapted to simulate the beta-decay spectra and using the emitter maps previously measured as probability distributions. The irradiation time corresponded to the time needed to reach 700 Gy at the sclera, that is 132.83 h (93.83 h) for the CCA1364 (CCB1256) using the data given by the manufacturer in the plaque certificate. With these irradiation times at least 100Gy at the apex are guarantees.

with the CCA1256 eye plaque for the modeled anterior tumor.

Electronic Poster: Brachytherapy: Anorectal

EP-2249 A novel rectal applicator for contact radiotherapy with 192Ir HDR sources M. Bellezzo 1 , M. Berbee 1 , B. Reniers 2 , A. Verrijssen 1 , G. Fonseca 1 , E. Van Limbergen 1 , F. Verhaegen 1 1 Department of Radiation Oncology MAASTRO, Clinical Physics Research, Maastricht, The Netherlands 2 Hasselt University, Centre for Environmental Sciences CMK, Hasselt, Belgium Purpose or Objective Dose escalation to rectal tumors leads to higher complete response rates and may thereby enable omission of surgery. Important advantages of endoluminal boosting techniques, such as 50 kV contact x-ray radiotherapy (CXRT) or brachytherapy (BT), include the possibility to apply a more selective/localized boost than using external beam radiotherapy. Compared to the currently existing BT techniques, favorable characteristics of CXRT include: more accurate positioning; sharper dose-depth profile and a more selective treatment surface. Unfortunately, the widespread introduction is hampered by the limited availability of the device. Hence, a novel applicator was designed to deliver a dose distribution similar to the one of CXRT, but using commonly available A multichannel applicator with lateral shielding was designed to position a HDR 192 Ir source perpendicular to several points on the tumor surface. The applicator was designed to shape the resulting radiation dose distribution to optimally cover the tumor while protecting the organs at risk. The model was achieved by Monte Carlo modelling, validated experimentally with film dosimetry, and by comparison with results found in the literature. An in-house inverse planning system was used to optimize the source positions and dwell times for the final model of the applicator. The model for the applicator was compared with the Papillon 50 (P50) CXRT device regarding the applicator dimensions, dose distribution and treatment time; however, the aim was to be able to deliver a more conformal dose distribution to the tumor than the CXRT device can deliver. Results The shape of the dose distribution is primaril y dictated by the distance of the sources to the surface of the applicator and the number of channels used. For a 26mm diameter model using 9 channels, sources at 5 mm from the surface (Figure 1) will yield a dose distribution similar to the one from a P50 with the 25mm rectoscope. Adding more channels to the applicator results in a more uniform dose distribution at the surface of the applicator; however, it also increases the treatment time due to the time required to send the dummy source in each catheter. An applicator with 9 channels was found to be the best compromise considering dose uniformity and total irradiation time, being able to deliver 20 Gy at the surface under 9 minutes for a source of 5 Ci. 192 Ir HDR BT sources. Material and Methods

Results The CCA1364 plaque placed centrically and the CCB1256 eye plaque give similar doses at the apex while showing a similar coverage over the tumor volume. It is observed that the CCA1364 placed eccentrically delivers to the tumor apex a maximum dose 23% lower than the dose delivered for the same plaque placed centrically. It is also observed that the maximum dose absorbed by the eye lens is the lowest when the CCA1364 plaque is placed eccentrically. It is 69% lower than the CCA1364 placed centrically and 61% lower than the CCB1256 plaque placed equatorially.

Conclusion Using a general homogeneous CCA eye plaque taking as reference the irradiation time given by the manufacturer leads to an overdosage over the tumor volume. The fact of disregarding the 2 mm safety margin is not decisive to achieve the objective of 100 Gy at the tumor apex. At the same time, an eccentric placement could be positive in order to be more conservative with respect to the eye lens. Even though the radiation time is lower, there is no advantage in using a CCB1256 eye plaque in comparison