146 Radiation Oncology, Biology, Physics Volume 21, Supplement 1

years. Patients were immobilized in a WMRI/PET compatible system prior to pre-operative/pre-chemotherapy treatment planning CT and MRI studies. Post-operative repeat CT-treatment planning was performed in order to compare the effect of maxillectomy cavities upon lateral scatter dosimetry within resected volumes. Beam’s Eye View designed blocking was performed in order to assure adequacy of target treatment. Resulting alternative plans were compared with attention to uniformity of dose throughout the interorbital area, orbital apex, and cavernous sinus.

Results: The complex anatomy of paranasal sinus irradiation has demanded precise immobilization, accurate shielding, and attention to disease within the interorbital area. Using both pre-operative CT and MRI-treatment planning incorporating Beam’s Eye View displays, improvements to conventional opposed-lateral, wedged-pair, and 3-field beam orientations have been achieved. Comparison of alternative 34ield plans, with and without interorbital electron fields, has been performed, resulting in a preference of electron nasal boost for patients at high-risk for disease in the interorbital area. Anteriorly-weighted plans are reserved for patients at risk only for microscopic disease between the eyes. Accurate assessment of optic chiasmal dose requires careful weighting of anterior photon and electron fields. The dosimetic consequence ofamaxillectomy cavity upon paranasalcavity surface treatment studied with 3-dimensional dose calculations supports the irradiation of patients with maxillectomy prosthesis in place during irradiation. CT-treatment planning with prosthesis inserted allows electron density measurements of the bolus effect of the palatal appliance.

Conclusion: Although head and neck irradiation relies heavily upon routine anatomic referencing using lateral radiographs, 3- dimensional treatment planning is of utility to patients with disease approximating the orbit. This dosimetric study of a group of patients with advanced paranasal disease has resulted in improvements in targeting and uniformity of dose throughout spatially irregular tumors. Consistent treatment planning techniques have been developed with the use of 3-dimensional planning which may be applied to standard 2- dimensional planning of these advanced lesions.

61 CONFORMAL THREE DIMENSIONAL TREATMENT PLANNING MAY IMPROVE THE THERAPEUTIC RATIO OF HIGH DOSE RADIATION THERAPY FOR LUNG CANCER.

J. Armstrong, C. Burman, S. Leibel, D. Fontenla, 0. Kutcher, and 2. Fuks. Memorial Sloan-Kettering Cancer Center. New York.

Purpose: Conformal three dimensional radiation therapy (C3DRT) is designed to improve dose delivery to the target and minimize the dose to normal tissues. If these objectives were realized C3DRT would be a useful tool with which to assess the clinical impact of dose escalation in lung cancer. To determine its suitability we used the endpoints of target dose coverage and normal tissue dose to compare the delivery of high dose radiation using C3DRT to conventional treatment planning (CTP).

Materials and Methods: Lung cancer patients were simulated in custom molds and then 1 cm thick CT slices were taken from below the larynx to the upper abdomen. Slices 5 mm thick were taken through the levels containing gross disease. were outlined using mediastinal windows.

Elective nodal target volumes

outline gross disease volumes. Mediastinal and lung windows were used to

The objective was to devise plans delivering 50.4 Gy to elective nodal volumes and 70.2 Gy total dose to gross disease. physicist) devised plans using CTP and the other team used C3DRT.

One team (physician +

computerization, customblocking, Both systems utilized

scatter correction factors, andluns densitv correction. In addition the C3DRT system has a beams eye view facility,.

* more accurate representation

of anatomy, and more sophisticated lung and tissue density correction. The 3-D system also utilizes dose volume histograms (DVHs) and visual displays to objectively compare candidate plans. volumes,

Plans were compared to assess adequacy of dose delivery to target DVHs for normal tissue (lung, heart, esophagus, cord), and derived normal tissue

complication probabilities (NTCPs).

Results: The mean minimum dose to 95% 66 Gy with CTP.

of the gross target volume was 72 Gy with C3DRT and Significant underdosing occurred in 2% of the gross target volume with

C3DRT vs 15% with CTP. Mean lung dose was 18.1 Gy with C3DRT and 20.8 Gy with CTP; 0 corresponding NTCPs were 9% vs 206.

of dose to the uninvolved lung. This advantage of C3DRT was largely due to reduction

Esophagus NTCPs were 22% with C3DRT vs 33% with CTP. The NTCPs for heart and cord were negligible with both systems.

Conclusions: This preliminary analysis suggests that C3DRT may be superior to CTP in delivering high doses to gross disease without a corresponding increase in NTCPs. Furthermore, lung and esophageal NTCPs were reduced with C3DRT suggesting that this approach may have the potential to improve the therapeutic ratio of radiation for lung cancer.