Abstract
Mesh brachytherapy is a permanent implant with low energy radioactive seeds in an absorbable mesh sutured into the tumor bed immediately after surgical resection. Traditionally, process entails real-time treatment planning while the patient is under GA. Mesh applications have been limited to a few premier institutions utilized mainly for lung, head and neck and truncal sarcomas only. I-125 is used commonly but Cs131 offers shorter half-life and more intense energy deposition. Dose delivered varies depending on multiple factors, such as post-surgery recovery and distortions in the implant shape over time. We developed a process for the Intraoperative Cs 131 mesh brachytherapy utilizing techniques which can be easily translated into any hospital setting with minimal resources and careful training. This publication is to highlight the simplified pretreatment planning process with post implant dosimetry verification.
Patients were selected after multidisciplinary discussion with concerns for positive margins in planned curative intent surgery. Patients underwent thin slice treatment planning CT to include the target site and all surrounding OARs. The GTV and OARs were delineated with diagnostic imaging eg- CECT, MRI. Likely area for positive margin was delineated in consultation with the surgeon and radiologist to generate the CTV. The doses to the target were prescribed per AAPM TG 222. OAR tolerance was defined per the published literature and expert recommendations. The CTV was used for planning, utilizing Varian Brachytherapy TPS v15.6. Plan was generated modeling the seeds as line sources and placing them in the target volume while modifying strength to achieve target coverage with the prescribed dose while sparing OARs. The CTV dimensions were utilized to predict the mesh dimensions with 10% added to each 2D parameter. The thickness of tissue from the mesh to spare the OAR was calculated. 1cm seed spacing was selected to avoid hot or cold spots and allowing intra-seed distancing to tailor the mesh intraoperatively if needed. Intraoperatively, frozen sections were reviewed and positive margin was confirmed. The positive margin area was templated on a vicryl mesh. The template mesh was used to measure the shape and dimensions of the area that needed to be implanted. The sterile mesh (after calibration) was custom tailored with the previously designed template allowing additional margin to suture the implant and cover the target adequately. The implant was sutured into the tissue after ensuring adequate spacing with OAR with gelfoam and Tisseal/ floseal to hold the implant in place. Total seeds implanted and unused were accounted for. The patient was monitored post implant during hospital stay and radiation safety ensured per guidelines until discharge. Post implant dosimetry was completed on CT and verified compared to preplan. Imaging performed periodically to assess seed migration.
Patient 1: Dedifferentiated mediastinal liposarcoma post EBRT 50 Gy with positive margin attached to the brachiocephalic vein and aorta. Patient 2: Anal carcinoma recurrence post concurrent chemotherapy and radiation for complete pelvic exenteration with positive margin on pubic symphysis Both sites were prescribed 60 Gy with the intent to keep Aorta/ brachiocephalic vein< 100 Gy and the VRAM flap <100 Gy respectively. Post implant dosimetry (fig) confirmed the dose to the OARs as well as target coverage. There was nonsignificant seed migration at 1 month.
This preplanning process for Cesium 131 mesh brachytherapy allowed translation of the preplanning dosimetry to the implant with adequate target coverage and OAR sparing. Longer followup is needed to confirm long term clinical outcomes.