Room: AAPM ePoster Library
Purpose: MRI provides superior soft tissue imaging useful in delineating treatment targets but is challenging to implement in skin brachytherapy. MRI affords the ability to track plaques in the dermis and to modulate treatment depth versus the standard 3mm. However, diagnostic MR images typically are not analogous to treatment position; in addition, applying and securing a surface applicator to patient skin can deform the region, making appropriate MR registration with the planning CT difficult to achieve. A departmentally housed MR-simulator offers full control of patient positioning and sequences. Hence, we piloted such a new MR-simulator as an imaging tool for our skin brachytherapy practice.
Methods: Patients undergo normal simulation with the target marked on skin and covered with a flap applicator. CT is obtained after securing the flap with an ace bandage, marking the first catheter with a BB, and placing dummy wires. Without removing the applicator, the BB and dummy wires are removed and replaced with MR compatible markers. With the hand in the same treatment position, sequences are taken to highlight the underlying lesion. The CT and MR are registered to aid in contouring, and the MR dummy wires were evaluated. Registration accuracy was evaluated as part of the QA process.
Results: Patient positioning has matched well between CT and MRI. Enhanced soft tissue definition permitted better target contouring, highlighting plaque regions to be included in the CTV. The MR dummy wires were clearly visible and tracked closely with the CT dummy wires. Representative dosimetry from a squamous cell carcinoma of a hand with an 8 catheter flap and 6 interstitial needles: CTV D90=100.13%, Bone D2cm^3=94.09%.
Conclusion: A procedure for MR-guided skin brachytherapy was successfully introduced to the clinic. Further work will evaluate the accuracy of the MR dummy wires, which may allow omitting the CT scan entirely.
IM/TH- MRI in Radiation Therapy: MRI guidance - Brachytherapy