S458 Posters

sides of the leaves and increment in 0.4 cm steps. All leaves can interdigitate, so islands of radiation dose can be produced very easily. Those leaves not required for field shaping are 'closed' off to either side of the field beneath a set of fixed, outer diaphragms.

This paper examines some of the dosimetric characteristics (for 6MV photons only) of the first production pilot Beam Modulator@, retrofitted to a standard Elekta SL15X linear accelerator. We also report on its clinical potential from a perspective of interfacing and planning with a commercial treatment planning system.

Materials and Methods: The following physical and dosimetric characteristics were investigated; (a) Leaf Bank Set-up (scaling, centring and alignment), (b) Leaf positioning (X-ray calibration, X-ray to light field coincidence, interleaf offsets and reproducibility), (c) Interleaf leakage and leaf transmission, (d) Leaf and shaped fields penumbra and (e) Percentage depth dose and relative output factors (ROPFs) for open fields. For (a), (c) and (d) X-Omat V film was used at isocentre and dmax (2 and 10 cm depth for the leakage and transmission measurements), analysed using Scanditronix-Wellhofer analysis software. All films were analysed in terms of absolute dose from a calibrated film scanner. (b) was examined using film, standard light field techniques and with a p-type diode in a plotting tank. Various head rotation and gantry angles were examined to test the stability of the system and the influence of gravity on interleaf leakage and leaf positioning reproducibility. (e) was measured using a standard small volume (0.06 cm 3) ion chamber in a water tank and miniphantom.

The clinical potential was explored using Nucletron's PLATO ('Sunrise') treatment planning system. The physical design of the MLC was configured into the system in terms of leaf pitch, movement limits, etc.. Conformal and IMRT plans were simulated on both patient and Rando phantom based CT data. For IMRT, a sequencer was chosen allowing a combination of MLC interdigitation and fixed diaphragm modeling.

Results and discussion: Leafbank scaling and alignment were within 0.2 mm of nominal values; centring was 0.5 mm maximum from the isocentre. Leaf bank calibration was within 1 mm of nominal and the interleaf offsets were found to maintain their calibration throughout the test period. Leaf positioning reproducibility was found to be within 0.1 mm (1 std dev) with a range of < 0.5 mm. Maximum interleaf leakage was 1.7% and the average leaf transmission was < 1%. No significant differences were found with collimator or gantry angle variation. The leaf end penumbra (80-20% dose width) was between 4.5 and 5.0 mm across the full range of leaf travel, and between 2.7 and 3.5 mm for the leaf sides, across a full range of offset fields. The penumbra for shaped fields showed minimal stepping (compared to a standard 1 cm Elekta MLC) with an effective penumbra width of about 4.5 mm for a 450 edge. Percentage depth dose curves, in- water and mini-phantom ROPFs were all extremely similar to the previous machine data with asymmetric collimators. Differences of just over 1% were found only for fields > 16 cm eq. sq.

Conformal and IMRT plans from PLATO could be transferred and delivered on the Beam Modulator@ very easily. Conformal plans showed improved conformation of isodoses around the target volume for all sites, when compared with standard l cm MLC plans. There was a significant improvement in penumbra and the DVHs for prostate and head and neck plans showed reduced OAR doses. Qualitative IMRT segment verification was easily performed using an EPID, with delivered segments comparing very well with the plan.

Conclusions: Overall the production pilot Beam Modulator@ showed very good dosimetric performance. The ease of data interfacing with a commercial TPS was impressive as was the potential clinical gains offered for conformal and IMRT plans.

1091 poster

NTCP of the kidney: comparison of competing irradiation techniques of paraaortic lymph nodes

M. Nevinny-Stickel, 7-. Seppi, K. Poljanc, B. Forthuber, P. Lukas

Innsbruck Medical University, Austria, Dept. of Radiotherapy- Radiooncology, Innsbruck, Austria

Introduction: Partial involvement of both kidneys is an unavoidable consequence for adequate dose delivery in paraaortic lymph node (PLN) irradiation. Depending on total dose ap/pa opposed fields or multi-field techniques are used. A comparison of four common irradiation techniques for the PLN areas on the basis of calculated NTCPs of co-irradiated kidneys will be presented in respect to an optimised treatment of potentially tumour affected PLN combined with tolerable kidney involvement.

Materials and Methods: PTV delineation was performed in CT scans of 21 patients with a lateral safety margin of 3cm to the aorta and 2cm to the vena cava. Ventral and dorsal margins of the PTV were delineated 2cm ventral and dorsal of the vessels. PTV included > 97% of the potentially involved PLN. For each patient four hypothetical treatment plans were created: A) standard ap/pa opposed fields with lateral field margins along the tips of the transverse processes of the vertebral bodies B) individually planned ap/pa opposed fields with lateral field margins according to the PTV C) standard four-field box with lateral width as described for A, with dorsal borders at the center of the vertebral bodies and ventral margins 3cm in front of the vertebrae D) indivi-dually planned four-field box with lateral field margins according to the PTV.Calculation of radiation induced complication probability values for non uniform kidney irradiation were determined for model doses 50.4 Gy, 30.6 Gy and 19.8 Gy according to the Lyman-Wolbarst model (Statistics: U-Test by Mann-Whitney- Wilcoxon).

Results: Individually planned ap/pa opposed fields (B) are accompanied by the highest overall kidney NTCP rates (median 0.68, range 0.00 to 0.99). No statistically significant difference (P > 0.1) in the median overall kidney NTCP was observed among the other techniques (A: median 0.39, range 0.00 to 0.83, C: median 0.27, range 0.06 to 0.35 and D: 0.36, range 0.15 to 0.72).

Conclusion: Regarding to expected overall moderate tissue complication probabilities in both kidneys with optimal inclusion (>97%) of potentially involved PLN the individually planned 4-box irradiation might be superior among the investigated techniques.

1092 poster

Whole scalp irradiation using helical tomotherapy N. Orton, H. Jaradat, J. Welsh, W. Tom~

University of Wisconsin Hospital, Radiation Oncology, Madison, WI, U.S.A.

Homogeneous irradiation of the scalp poses technical and dosimetric challenges due to the extensive, superficial curved treatment volume. Conventional treatments on a linear accelerator use multiple matched electron fields or a combination of electron and photon fields. Problems with this technique include dose inhomogeneity in the target due to