Abstract
Purpose: Planned dose distribution for IMRT or VMAT could be altered by tissue changes during treatment course as in head and neck patients. Thus the aim of our study is to investigate a simple method that guides decision for re-planning. We will correlate changes in exit fluence tracked through entire treatment course with the anatomical changes. Methods: Fifteen patients were planned for IMRT; weekly CT scan was registered to original CT. Volume changes of different structures were assessed for each week. Frequently integrated images were acquired for all fields twice weekly via portal imager. The delivered fluences were compared with reference images. Gamma analysis (3% and 3 mm) including maximum γ, and percentage of points with γ≥ 1, were calculated for all fields. Results: Continuous reduction in GTV, CTV1 and parotids volumes during treatment was found [median value of 5.3 (1.3–20.8), 23.8 (3.0–128.7) and 1.7 (0.5–6.3) cm3 respectively] with significant change on week 2 (p= < 0.005). Also the value of gamma area >1 parameter continued to increase, significantly (P < 0.005), starting from second week (1.8 ± 1.9%) reaching 4.1 ± 1.6% by the end of treatment. Most of these changes in the exit fluences are significantly correlated to patient anatomy variations. Pearson correlation coefficient was −0.984 (P = 0.002) and −0.984 (P = 0.014) for the maximum gamma value and the value of gamma area >1, respectively. Conclusion: The changes in exit fluences are reflecting the corresponding anatomical changes. We proposed a method to track the changes in the averaged maximum gamma value and the gamma area > 1 value for the exit fluences during treatment. Maximum gamma value of 2.6% and Maximum gamma area > 1 value of 1.8% would be useful boarder lines that if exceeded a re-planning would be recommended.