Background and purpose: Gated delivery of radiation during part of the respiration cycle may improve the treatment of lung cancer with intensity modulated radiation therapy (IMRT). In terms of the respiration phase for gated treatment, normal end-expiration (EE) is more stable but normal end-inspiration (EI) increases lung volume. We compare the relative merit of using EI and EE in gated IMRT for sparing normal lung tissue.
Patients and methods: Ten patients received EI and EE respiration-triggered CT scans in the treatment position. An IMRT plan for a prescription dose of 70 Gy was generated for each patient and at each respiration phase. The optimization constraints included target dose uniformity, less than 35% of the total lung receiving 20 Gy or more and maximum cord dose <or=45 Gy. We compared planning target volume (PTV) coverage, mean lung dose, percentage of total lung receiving 20 Gy or more (V(20)) and lung normal tissue complication probability (NTCP).
Results: For 9 of the 10 patients, cord and lung doses were acceptable and PTV coverage was similar for EE and EI, with lung sparing was equal to or slightly better at EI than at EE. For the 10th patient, lung sparing at EI was significantly better. Patient averaged mean lung dose was 15.4 Gy (range: 7.1-20.4) at EI and 16.3 Gy (range: 6.9-21.9) at EE. The average V(20) was 23.8% (range: 13-36.4) at EI and 25.3% (range: 13-37.3) at EE. The average NTCP at EI was 8 versus 12% at EE.
Conclusions: Dosimetric indices of lung protection for IMRT plans at EI are better than at EE. For 9 out of the 10 patients in our study, this difference is small. Thus other factors such as reproducibility, reliability and duty cycle at normal end expiration may be more critical for selecting treatment breathing phase.