$476 Posters

reproducible patient set-up in the pelvic area. Results of these tests can be presented later.

1138 poster

Acute effects on hearing after radiotherapy in patients treated for head and neck, and cerebral turnouts H. Quelhas Oliveira 1, A. Costa ~, V. Oliveira 2, F. Borges ~, L Silva 1 ~lstituto Purtugues de Oncologia Francisco Gentil, Radiotherapy, Porto, Portugal 2Hospital Militar, ORL, Porto, Portugal Purpose: To evaluate eventual acute damages in hearing due to radiotherapy, when the inner ear is included in the treatment field.

Materials and methods: Evaluation of fifteen patients with head and neck, and cerebral tumours in the conditions above, treated at the Department of Radiotherapy from Institute Portugues de Oncologia - Porto, during the year 2003. An audiologic evaluation was made prior to treatment, at the time of its conclusion and a month after. The dose in the inner ear was estimated with a computed tomography based in treatment planning system (PLATO 1.8 and 2.2).

Results: Irradiation was given with 6°Co, 4 and 6 Mv photons beams, with a medium dose to the tumour of 67.3 Gy (range 60 to 70 Gy). The dose to cochlea and eustachian tubes ranges from zero to 73 Gy and zero to 72 Gy respectively. Radiodermitis and radiomucositis were present in all patients and were the cause of a mild decrease of hearing.

Conclusions: For the radiotherapist the inner ear is a critical structure in patients with head and neck, and cerebral tumours and the dose-in this structure should be carefully considered when Planning radiation therapy. There is a high probability of hearing loss as a late effect with the increasing of radiation doses as well with the acute complications (otitis, timpanic perfuration, etc.) not resolved.

1139 poster

A comparison of the ISORAD diode and the EDP diode for in vivo dosimetry J. Regnelf, /. Kristensen 2, T. Knd6s 2 ~Lund University Hospital, Dept of Radiation Therapy, Lund, Sweden 2Lund University Hospital, Dept of Radiation Physics, Lund, Sweden In vivo dosimetry is mandatory during external radiotherapy according to Swedish regulations. The experience of the EDP diode is extensive in our department, where it has been used for over 10 years. A drawback is this diode's lower sensitivity when the radiation impinges with an angel not perpendicular in relation to the diode. This is due to the design with the chip mounted perpendicular to the beam (at 0 degrees). We have tested and compared the EDP diode with the ISORAD diode. The latter diode has the chip mounted parallel to the beam thus this diode should exhibit a less pronounced variation in sensitivity with the angle of the incoming radiation.

We have studied the two diodes performance on a polystyrene phantom, using different beam directions. The test was performed for 6MV at five different accelerators in our department.

The results showed, in the worst case, an overestimation of 1% (ISORAD) vs. an underestimation of 2.5% (EDP). The overall performance of the ISORAD, except for 0 degrees, was better then for the EDP diode. A patient study has also been performed, using both diodes at the same time in the

field. Patients where grouped in a) beam direction perpendicular to the skin surface, and b) any other direction. The variation in group b) seems larger than expected.

1140 poster

The electronic portal imaging device (EPID) versus the conventional port film imaging system: the aspect of resorcues involved in the radiation technologist

A. MachtyL M. Kedzia-Tylman, P. Milecki Greatpoland Cancer Centre, Radiotherapy, Poznan, Poland Purpose: The aim of this study was the evaluation of the EPID and a conventional verification film from the point of view of the time spent by radiation technologist on the acqusition of portal imaging.

Material and methods: Between January and March 2004, the time needed for obtaining the traditional cassette portal imaging with an X-ray film which is traditionally used for the Co60 unit and the electronic portal imaging device (EPID) for the therapeutic machine Clinac 2300 D were measured. In our department about 50 patients are daily treated on each of these therapeutic machines. Typically, the irradiated portals geometrical accuracy evaluation during the course of radiotherapy was performed always at the start of RT treatment, in case of any field changes or usually at two weeks intervals. Simple manual preparation of X-ray images have encompassed the following procedures: mounting the cassette with film on the therapeutic machine, transport of cassette, processing of images, and description of the obtained X-rays images.

Results:The procedure of acquisition of simple X-ray images have taken on average 9 minutes (range: 6 - 12 minutes), while the same procedure using EPID only 1.5 minutes (range 1.0 - 2.5 minutes). Taking into account that about 7 patients have carried out daily the portal imaging procedure on each of the evaluated therapeutic machine. The difference in time needed for obtaining porta! images between the two methods is about 50 minutes daily for one radiation technician involved in this task.

Conclusions: Using the EPID allows to reduce the time needed to obtainthe portal images in routine clinical practice which has an important economical aspect apart from many other Such as the quality of images, storage possibility and easy and fast a