Contribution of CT to quantitative radiation therapy planning

The contribution of computed tomography (CT) in radiotherapeutic treatment of lung cancer was evaluated. Radiation therapy ports prepared for 50 patients on the basis of routine diagnostic radiographs and without CT scan information were reviewed after CT scanning. Of 50 patients, 13 (26%) required alteration of treatment ports on the basis of the additional CT scan information. In 11 patients (22%) the changes resulted in an increase in field size to cover the tumor adequately, and in two patients (4%-) the field size was reduced to spare normal tissue. A three-phase study was completed to evaluate the impact of CT on quantitative parameters of two-dimensional treatment plans on 25 of the 50 patients. Treatment plans in the absence of CT scan and without lung transmission correction were compared with treatment plans where CT information was used and lung transmission corrections were performed. Numerical results for local efficiency and nonuniformity factor were compared for conventional, CT unoptimized, and CT optimized plans. Of the 25 patients, 14 (56%) had poorer local efficiency and 16 (64%) had nonuniformity factor exceeding 5% when treatment was planned without CT information. CT-optimized plans improved local efficiency in 16 (64%) of 25 patients and reduced nonuniformity to within 5% in 21 (84%). It is suggested that in the treatment planning of patients with lung cancer, CT scan information is essential for accurate determination of dose distribution and optimization of therapy.