$450 Posters

approximately 7% less than prescribed dose as a result introducing a small posterior complementary boost field is limited 1-2 fraction.

1073 poster

Radiotherapy planning optimisation for maxillary sinus tumors

R. Pfeffer, D. A/ezra, J. Menhe/, L. Tsvang, Z. Symon, D. Levine

Sheba Medical Center, Oncology, Tel Hashomer, Israel

Introduction: We have recently introduced Intensity Modulated Radiotherapy (IMRT) for treatment of head and neck cancer. Since IMRT is more time consuming than conventional XRT we are evaluating all patients considered for IMRT of head and neck tumors with both conventional and IMRT plans before implement treatment.

Materials and methods: Nine patients (5 squamous cell carcinoma, 2 sarcoma, 2 lymphoma) with tumors in the maxillary sinus and nasal cavities underwent both conformal radiotherapy planning and IMRT planning using Varian Eclipse treatment planning software after mask immobilization. Critical structure delineated on the patient's CTs included the eye, retina and lens. The conformal planning in each case included 4-5 non-coplanar fields avoiding critical structures, including the orbital contents, oral cavity and parotid glands. The IMRT plans entailed 4 coplanar beams.

Results: For all 9 patients the conformal plans were considered preferable to the IMRT plans. IMRT plans did not result in improved tumor coverage and usually delivered higher doses to critical structures, in particular the intra- orbital contents. The time required to prepare a non-coplanar conformal plan was considerably shorter than that for an optimized IMRT plan.

Conclusions: For sites in the maxillary and nasal sinuses conventional conformal radiation resulted in more optimal plans than IMRT. This may be due to the special situation of lesions lying close to both orbits in which there is a particular advantage to using non-coplanar fields. Caution should be taken to avoid automatic use of IMRT in all treatment sites.

1074 poster

Dosimetric verification of customised tissue equivalent boluses in postmastectomy breast wall irradiation with electrons

K.H.H. Ny.qaard, O.H. Odland, J.I. Heggdal, B. Nygaard, R. Hafslund, L.P. Muren

Section of Medical Physics, Department of Oncology and Medical Physics, Bergen, Norway

Purpose: Postmastectomy breast wall irradiation is often performed using electrons. A customized bolus is placed on the breast wall mainly to compensate for irregularities in the chest wall thickness and to reduce the dose in underlying critical organs. The sharp edges of the bolus inside the irradiated field are sloped to minimise the dose perturbation in the dose profiles below the bolus. In this study dose distribution below multi-layered boluses with different angles on the sloped edge were measured using both a diode in a water phantom and a CCD-camera system.

Materials and methods: Perspex plates of 20x20 cm 2 size and tehicknesses between 5 and 30 mm were used. The sides of these plates were sloped with angles of 90, 60, 45 and 30 degrees. The central axis of a 10x10 cm 2 field was placed at the toe of the edge, i.e. the plate covered half the field. The distance from the source to the toe of the edge and

the water surface was 100 cm. The edge was aligned with the x-axis and all measurements were performed along the y- axis. With the diode (Sun Nuclear Corporation, Florida), Dose profiles in both 9 and 15 MeV were measured from the surface to 40 mm and 65 mm depth, respectively. Doses were normalized to the maximum dose in the corresponding open beam, and isodose distribution were generated using routines written in Interactive Data Language (IDL, Research Systems Inc.). When measuring with the CCD-camera (Scanditronix-Wellhoefer Beam Imaging System-2G) the Perspex bolus was placed on solid water plates to obtain data in 20 mm depth. The profiles from the CCD-camera were normalized to the open part of the field as measured with the diode. Minor misalignments of the beam profiles were corrected for by securing coincidence of the 50% dose measured with the two systems.

Results: For both 9 and 15 MeV the isodose distribution showed an increasing maximum dose with increasing slope of the bolus. For a 5 and 10 mm thick bolus in a 15 MeV electron beam, the area covered by the hotspot caused by a 90 degree angle was not considerably larger or different than in a 60, 45 and 30 degree angle, except for a thin area of 110% (Figure 1). With increasing bolus thickness, the maximum dose at the toe of the 90 degree bolus increased to 130% for a 30 mm thick bolus in a 15 MeV beam. For a 9

M e V beam a maximum dose of 115% was found below a 20 mm bolus with a 90 degree angle. With a 45 degree angle, the maximum dose did not increase considerably with increasing thickness and the area covered by the 90% and 100% isodose remained more or less constant for the 15 MeV beam. For a 9 MeV beam, no significant maximum dose was detected when increasing the angle or thickness of the bolus. However, an area of minimum dose appeared under the 90 degree bolus edges for boluses thicker than 10 mm. Similar results were found with the CCD-camera (Figure 2). Comparison with treatment planning system calculations were performed and will be presented at the meeting.

Conclusion: For boluses of less than 10 mm thickness the impact of sharp edges is small, and the layers can be used directly without sloping when using a 9 or 15 MeV beam. For boluses exceeding 10 mm thickness, thus increasing the impact of the 90 degree angle, sloping is necessary to avoid doses over 110% for a 15 MeV beam and doses under 90% for a 9 MeV beam. The BIS-2G system may be used for rapid verification of dose distribution under customized patient boluses in depth.

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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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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