Breathing-synchronized irradiation using stereoscopic kV-imaging to limit influence of interplay between leaf motion and organ motion in 3D-CRT and IMRT: Dosimetric verification and first clinical experience

Purpose: To verify the technical feasibility of a prototype developed for breathing-synchronized irradiation by phantom measurement and report on the first clinical experience of 3 patients. Methods and Materials: Adaptations to a commercially available image-guidance technique (Novalis Body/ExacTrac4.0; BrainLAB AG, Heimstetten, Germany) were implemented, allowing breathing-synchronized irradiation with the Novalis system. A simple phantom simulating a breathing pattern of 16 cycles per minute and covering a distance of 4 cm was introduced to assess the system's performance to: ( 1 ) trigger the linac at the right moment (using a hidden target in the form of a 3-mm metal beads mounted to the phantom); ( 2 ) assess the delivered dose in nongated and gated mode (using an ionization chamber mounted to the phantom); ( 3 ) evaluate dose blurring and interplay between organ motion and leaf motion when applying dynamic multileaf collimation (DMLC) intensity-modulated radiation therapy (IMRT) techniques (using radiographic film mounted to the phantom). The effect of motion was evaluated by importing the measured fluence maps generated by the linac into the treatment planning system and recalculating the resulting dose distribution from DMLC IMRT fluence patterns acquired in nongated and gated mode. The synchronized-breathing technique was applied to three clinical cases: one liver metastasis, one lung metastasis, and one primary lung tumor. Results: No measurable delay in the triggering of the linac can be observed based on the hidden target test. The ionization chamber measurements showed that the system is able to improve the dose absorption from 44% (in nongated mode) to 98% (in gated mode) for a small field irradiation (3 × 3 cm 2 ) of a moving target. Importing measured fluence maps generated for a realistic patient treatment and actually delivered by the linac into the treatment planning system yielded highly disturbed dose distributions in nongated delivery, whereas the gated delivery showed good agreement with the original theoretical dose distribution. These findings were confirmed by dose–volume histograms. The three different clinical cases revealed three different practical problems that could easily be resolved by minor adjustments to the system. The applied breathing-synchronization technique introduced an increased treatment time by a factor of 3 to 4. Conclusions: Initial tests with the prototype for breathing-synchronized irradiation showed promising results. The use of measured fluence fields, delivered by the linac in nongated and gated mode as imported fluence maps for the treatment planning system, revealed the dramatic impact of dose blurring and interplay between leaf motion and organ motion, as well as the advantage of breathing synchronization to resolve this issue. The latter should, however, be weighed against the increased treatment time.