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Figure 1A: The isodose distribution under a 15 MeV electron beam with a 10 mm thick bolus with a 90 degree angle. Figure 1B: The isodose distribution of a 15 MeV electron beam with a 10 mm thick bolus with a 45 degree angle. The bolus Was positioned as illustrated in the upper part of each figure.

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Figure 2: A 15 MeV beam on a 20 mm thick bolus with a 90 degree edge. The measurements were done under 20 mm build-up on the BIS-2G and 20 mm depth in the water tank using a diode. The bolus was positioned as illustrated in the upper part of the figure.

1075 poster

Single vocal cord irradiation: comparison of treatment plans with Monte Carlo calculations and gafchromic film dosimetry S. van der Hout, M. Essers, M.J. Simons, J.P.A. Marijnissen, P.C. Levendag, B.J.M. Heijmen

Erasmus MC - Daniel den Hoed, Radiotherapy, Rotterdam, The Netherlands

Routinely, T1N0 laryngeal cancers are irradiated with 2 lateral 4- or 6-MV beams with field sizes around 5x5 cm 2. In this way, a relatively large volume of healthy tissue is also irradiated, leading to reduced voice quality as well as deglutition difficulties. In our institution, we are working on improved treatment techniques for these lesions, which should result in reduced side effects without jeopardising local control. For very small lesions, the ultimate goal will be to irradiate a single vocal cord, leading to improved voice quality.

With field size reduction, underdosage of the lesion, both due to lack of electronic equilibrium at the tissue-air-interfaces as well as due to the field penumbras, is a potential danger, and ways to correct for that will have to be investigated and implemented. In addition, patient setup must be very accurate, and lesion motion must be corrected for or taken into account.

The present study concentrates on dosimetrical aspects. For this purpose, a special phantom, based on CT data of a patient, was constructed, to precisely determine the dosimetry in the vocal cord area. Dose distributions calculated with our Cadplan and Pinnacle systems were compared with Monte Carlo calculations (BEAM) and Gaf Chromic (HD801) film measurements, both for conventional treatment plans as well as for improved treatment plans with much smaller field sizes. Large discrepancies between actually delivered doses and calculated doses have been observed. Results of the comparisons and possible solutions for improved treatment techniques will be presented.

1076 poster

MRI simulation for conformal radiation therapy of prostate cancer D. PasquieP '2, N. Betrouni 3, B. Castelain 1, E. Lartigau 1'2, J. Rousseau 2,3

1Centre 0 Lambret, Radiotherapy, Lille, France 2Facult6 de medecine, Lille, France 3ERT 23, Lille, France

Thanks to its excellent soft tissue contrast, MRI ensures better delineation of target volumes and organs at risk in many locations, such as the prostate. MRI is only used, in most cases, in conjunction with Computerized Tomography (CT). MRI simulation would eliminate the localization errors introduced by image matching and also makes it possible to dispense with an additional imaging examination that is costly, time consuming, which generates additional irradiation and which is difficult to coordinate with the MRI examination. The obstacles mentioned are geometrical distortion and chemical shift, measurement of electron densities, and the compatibility of some treatment planning systems (TPS).

We have carried out geometrical distortion measurements on two standard 1.5 T MR scanners, a Magnetom Vision (Siemens ®, Erlangen, Germany) and a Gyroscan Intera (Philips ®, Eindhoven, Netherlands). The phantom used, measuring 400 mm x 300 mm x 210 mm, was composed of 730 glass spheres immersed in a 1,2-propandiol solution. The phantom was imaged with a body coil with a T2- weighted sequence conventionally used for prostate imaging