SIMULATION OF 3D-TREATMENT PLANS IN HEAD AND NECK TUMORS AIDED BY MATCHING OF DIGITALLY RECONSTRUCTED RADIOGRAPHS (DRR) AND ONLINE DISTORTION CORRECTED SIMULATOR IMAGES

Background and purpose: Simulation of 3D-treatment plans for head and neck malignancy is difficult due to complex anatomy. Therefore, CT-sim ulation and stereotactic techniques are becoming more common in the tr eatment preparation, overcoming the need for simulation. However, if s imulation is still performed, it is an important step in the treatment preparation/execution chain, since simulation errors, if not detected immediately, can compromise the success of treatment. A recently deve loped PC-based system for on-line image matching and comparison of dig itally reconstructed radiographs (DRR) and distortion corrected simula tor monitor images that enables instant correction of field placement errors during the simulation process was evaluated. The range of field placement errors with noncomputer aided simulation is reported. Mater ials and methods: For 14 patients either a primary 3D-treatment plan o r a 3D-boost plan after initial treatment with opposing laterals for h ead and neck malignancy with a coplanar or non-coplanar two-or three-f ield technique was simulated. After determining the robustness of the matching process and the accuracy of field placement error detection w ith phantom measurements, DRRs were generated from the treatment plann ing CT-dataset of each patient and were interactively matched with on- line simulator images that had undergone correction for geometrical di stortion, using a landmark algorithm. Translational field placement er rors in all three planes as well as in-plane rotational errors were st udied and were corrected immediately. Results: The interactive matchin g process is very robust with a tolerance of <2 mm when suitable anato mical landmarks are chosen. The accuracy for detection of translationa l errors in phantom measurements was <1 mm and for in-plane rotational errors the accuracy had a maximum of only 1.5 degrees. For patient si mulation, the mean absolute distance of the planned versus simulated i socenter was 6.4 +/- 3.9 mm. The in-plane rotational error in both pla nes was <3 degrees with one exception. Three large field placement err ors (two patients with 11.5 and 16.0 mm distances of the planned versu s simulated isocenter, respectively and one patient with a 7 degrees r otational error) were detected and, as with the smaller errors, were i mmediately corrected. Conclusion: On-line image matching of treatment planning CT-derived DRRs and distortion corrected treatment simulator images is a precise and reliable method to reduce field placement erro rs in the simulation of complex 3D-treatment plans for head and neck m alignancy and thus enhances accuracy in the first step of the treatmen t preparation/execution chain. However, out-of-plane rotational errors could not be assessed and assumedly they are comparatively small sinc e due to rigid fixation, detected in-plane errors were small. (C) 1997 Elsevier Science Ireland Ltd.