[Adaptive IG-IMRT for prostate cancer].

BACKGROUNDS

Adaptive image-guided intensity-modulated radiation therapy (IG-IMRT) is a perspective method for the treatment of localized prostate cancer. Validate optimal protocols for IG-IMRT using kilovoltage cone-beam CT (CBCT) are required.

PATIENTS AND METHODS

Seventy-six patients with prostate cancer were treated using adaptive IG-IMRT. Based on the CBCT performed during the first 10 fractions of radiotherapy, an average prostate position in relation to the pelvic bones was determined in antero-posterior AP, supero-inferior SI, and right-left axes. An adapted treatment plan for the second phase of the treatment included an isocenter shift into its average position (correction of the systematic error sigma). A margin between a clinical and planned target volume (CTV-PTV) was adjusted according to the magnitude of random error sigma. During the second phase of radiotherapy, set-up of patients was performed daily on pelvic bones using kilovoltage skiagraphic imaging in two projections (kV-kV). Follow-up CBCTs were repeated once a week.

RESULTS

An average isocenter position differed from the position of a reference planning CT isocenter in at least one axis in 63 patients (83%). Major changes were recorded in AP axis - shift > or = 2 mm in 33 patients (43%), shift > or = 5 mm in 7 patients (9%). PTV for the second phase of radiotherapy was in the range of 6-10 mm in AP axis, 6-8 mm in SI axis, and 6 mm in RL axis. Mean sigma value in the AP axis was smaller in patients with a specific diet compared to patients without the diet (2.2 mm vs. 2.7 mm, p = 0.05). We evaluated 446 follow-up CBCT images from the second phase of radiotherapy (66 patients had 6 CBCT, 10 patients had 5 CBCT). Set-up error exceeding CTV-PTV margin occurred in 4 cases with no more than once per patient. Safety margin was sufficient in 72/76 patients (95%).

CONCLUSION

IG-IMRT protocol integrating CBCT and kV-kV imaging provided adequate coverage of the target volume and proved to be compatible with departmental workflow. Margin reduction around the CTV is a prerequisite for dose escalation aimed at a intraprostatic lesion.