A Dosimetrical Analysis of the Magnitude of Isocenter Shifts Using CT-on-Rails Guided Online Adaptive Planning for Left Breast Cancer Patients Treated by Intensity Modulated Radiation Therapy

Introduction: This study was conducted to check the accuracy and validity of an automated adaptive planning (AAP) method which combines automated re-contouring and re-planning to check for inter-fractional anatomical changes in breast cancer patients receiving intensity modulated radiation therapy (IMRT) based on a weekly computed tomography (CT)-on rails images dosimetrically viewing the magnitude of difference in isocenter shifts and the degree of dose deviations regarding target coverage and dose received by organs at risk. Methods and Materials: Twenty left sided conservative breast cancer patients were selected for IMRT treatment. Automatic generation of the contours on each repeat CT image was performed by mapping the contours from the planning CT image using deformable image registration. 3D- CRT and IMRT plans were automatically recalculated for each patient. The magnitude of isocenter shifts in postero-anterior (PA), cranio-caudal (CC) and right-left (RL) directions was calculated and the degree of dose deviations regarding target coverage and dose received by organs at risk. Results: In the PA,CC and RL directions, 9.2%, 11.4% and 6.4% of the 100 CT scans didn’t require any position adjustments. The mean isocenter shifts and standard deviation (SD) were 0.5 & 0.6 +/- 1.1mm, 0.3 &0.4 +/- 0.5mm and 0.2 & 0.4+/- 0.6mm for the IMRT and 3D-CRT plans in the PA,CC and RL directions, respectively. 74.1%, 65.1%, and 55.8% required a shift of 1 - 3 mm; 10.5%, 23.9%, and 33.2% required a shift of 3 - 5mm; and 1.5%, 3.3%, and 5.4% required a shift of more than 5 mm, respectively. With respect to target coverage, the D90[Gy] for the breast CTV was 46.2 Gy and 45.6 for the IMRT and 3D-CRT plans. The mean doses to the heart and left lung were reduced by 5.5 %, 3.7 Gy and 4.1%, 2.6 Gy using the AAP method. Conclusions: The fully automated AAP method using CT on rails for online re-planning was effective in reducing the magnitude of isocenter shifts and compensating for target volume coverage underdosing and higher doses perceived by risk organs caused by anatomical inter-fractional changes in breast cancer patients.