PHSOR09 Presentation Time: 1:10 PM

Purpose

Cs-131 has shown promise for the treatment of many cancers with its shorter half-life and more intense energy deposition compared to I-125. Cs-131 mesh has seeds embedded within a vicryl mesh, which keeps the seeds in place for about 30-35 days. Mesh applications have been limited to a few premier institutions. We present our process for the Intraoperative mesh brachytherapy program as a foundation for other new users to build upon.

Methods and Materials

A patient was selected after multidisciplinary tumor board discussion. Radiation Oncologist (RO) provided a written directive with the treatment site and prescription dose (60 Gy). TPS commissioning was carried out for Cs-131 (Cs-1, rev 2, IsoRay Medical Inc., Richland WA) utilizing half-life, source geometry, active source dimensions, dose rate constant, radial dose function, anisotropy function, air-kerma to activity conversion factor, etc. per TG43U1. Treatment planning CT with 1 mm slices was acquired. Target volume and nearby OARs were contoured. Pre-treatment plan was created in Varian Brachytherapy TPS v.15.6. Hand calculations and RadCalc were used to independently verify the TPS dose calculation. Planning CT was reconstructed with 1 cm slice thickness to match the mesh seed spacing. Seeds were modeled as line sources and placed within the target volume while modifying seed strength to achieve target coverage with the prescribed dose while sparing OARs. RO approved pre-plan. Mesh with optimal seed strength and number was ordered. 10% extra loose seeds plus 1 calibrated seed (same activity as determined per pre-plan) were ordered for strength verification. The mesh is received in sterile packaging. After a wipe test of the mesh container to ensure no radiation contamination, source strength for the loose seeds were verified using a well chamber (IVB1000 HO32871) calibrated for Cs-131.Vendor provided radiographs confirmed total seed number. A second independent physicist verified seed strength. Radiation safety training was completed with all personnel involved with the procedure. On implant day, MP and RO were both present. Mesh was taken to the OR after confirming the need for IORT per pathology. MP surveyed radiation levels throughout the procedure. RO and surgeon determined the mesh size to be implanted based on the at risk area. Once the implant was complete, the MP surveyed the OR room and accounted for total seed count. Numbers were verified by residual mesh radiography. Unused seeds were inventoried and stored in a lead container in the hot lab for decay. Post implant, radiation levels at 1 m were monitored periodically by the MP during the patient's hospital stay. Safety information was delivered to patient specific personnel by daily rounds, patient alerts and written instructions placed in the patient chart. MP provided verbal and written radiation safety recommendations to the patient and family members prior to discharge. A postoperative CT scan was acquired within 24 hours of the implant to review final implant dosimetry.

Results

The seed strength required per the pre-plan was 3.0 U. All loose seed strengths were within 2.3% of the vendor specified value. Post implant plan confirmed target coverage. Follow-up images acquired at 2 months confirmed nonsignificant seed migration. The exposure reading at 1 m from the patient was 1.0 mR/h immediately following the implant, and 0.5 mR/h at the day of discharge. The ring and collar badge readings for those directly handling the seed mesh were under regulatory limits.

Conclusions

An intraoperative mesh brachytherapy program has been safely established within our institution with the first patient successfully treated using the process described here.