Using digitally reconstructed radiographs from MRI (MRI-DRR) to localize pelvic lymph nodes on 2D X-ray simulator-based brachytherapy treatment planning

Purpose: Many of the available brachytherapy treatment planning systems in developing countries are not equipped with CT [or MRI] simulator; therefore, 3D treatment planning cannot be performed. In this project a new procedure has been introduced for utilizing the 2D digitally reconstructed radiograph from MRI images in brachytherapy treatment planning. This procedure enables us to localize the tumor volume and delineate the extent of critical structures in vicinity of tumor volume. Methods: Pelvic lymph node chain position was delineated from transverse MRI images, and transferred into Digitally Reconstructed Radiograph [DRR] and then onto the X-ray images obtained from conventional simulator unit. These images were then imported to Brachytherapy treatment planning system to evaluate the dose to be applied to these organs in cervix Brachytherapy. The accuracy of the matching process was evaluated by phantom study, having known 3D geometric information and landmark assertion. Results: The statistical variations obtained from distance mismatch in phantom and patient studies were in the range of clinically applicable error of registration [< 2mm]. The results showed a large variation of the nodal dose when dose calculation is performed based on point B dose which is the geometrical reference point for calculating the dose to the pelvic lymphatic system. The result also shows that the dose to point B is usually underestimated to represent external iliac maximum dose, and overestimated for representation of external iliac minimum dose. Conclusion: The results indicated that the DRR images can produce comparable accuracies in tumor localization reported in 3D MRI or CT based treatment planning procedures. Therefore, this technique could be used as a feasible approach where a 3D treatment planning is not available