Purpose: Interstitial HDR brachytherapy uses MRI for the clinical target volumes (CTV) and the organs at risk (OARs) delineation for treatment planning. CT is utilized for needle localization, as opposed to MRI, which often does not provide sufficient contrast to visualize needles. An optimized ‘Pointwise Encoding Time Reduction with Radial Acquisition’ (PETRA) MRI sequence is a promising candidate for implementing an MRI-only treatment planning (MRTP). The feasibility of administering MRTP in gynecological patients was evaluated against the clinical treatment plan (CTP).
Methods: Five gynecological cancer patients with interstitial needle implants were treated with the CTP generated using the standard CT/MRI-based method employing 3D SPACE images. To generate a plan using MRTP approach, an optimized 3D axial PETRA MRI (TR\TE 3.3\0.07) was acquired on a 3T MAGNETOM Vida (Siemens Healthcare, Erlangen, Germany). The MRI-only needle digitization was performed on Oncentra Brachy (Elekta Brachytherapy, Netherlands) Treatment Planning System using both PETRA and SPACE images for better visualization of the needles through tissues and the interstitial template, respectively. The MRI-based plan was generated by activating and assigning the same dwell times as in the CTP. The quality of the MRI-based plan was evaluated against the CTP by comparing the DVH and EQD2 metrics for the CTV and the OARs (rectum, bladder and sigmoid). D90 and D2cc were used for the CTV and OARs, respectively.
Results: In these five patients, the maximum relative differences in the DVH metrics were 5%, 11%, 12%, and 5% for the CTV, rectum, bladder and sigmoid, respectively. These differences correspond to the maximum relative EQD2 metric difference of 6% for all CTVs and OARs.
Conclusion: The optimized PETRA shows the potential to make MRTP feasible in this pilot cohort of gynecological cancer patients. A larger cohort of patients is required to confirm these preliminary observations.