SU-F-T-621: Impact of Vacuum and Treatment Couch On Surface Dose in Stereotactic Body Radiation Therapy With and Without a Flattening Filter.

PURPOSE: When treating lung cancer patients with stereotactic body radiation therapy (SBRT), better immobilization is needed for accurate delivery of high-dose radiation. However, using a treatment couch (TrueBeamTM) and vacuum bag (BlueBAGTM) may increase the surface dose and skin toxicity. This study investigated the influence of couch and vacuum bag on the surface dose.

METHODS: The relative surface dose (D0 /DMAX) was measured in an ion-chamber (Markus-type PTW, 0.05cm(3) ) with a solid water phantom and SSD to 100 cm. A comprehensive comparison of different parameter settings, including the different energies (6MV-FFF, 10MV-FF, and 10MV-FFF), field sizes (3 X 3 cm(2) , 5 × 5 cm(2) , 8 × x cm(2) , 10 × 10 cm(2) , and 15 × 15 cm(2) ), thickness of the vacuum bag (5mm, 15mm, 30mm, 39mm and 55mm), and couch (with and without), was performed.

RESULTS: The FFF increases the surface dose as compared to FF mode. In a similar setting with field of 10 × 10 cm(2) , FFF mode increases the surface dose from 26.0% to 32.8% for 6 MV, and 17.4% to 21.5% for 10 MV. When the beam passes through the couch, the surface dose increases to 3.6, 4.6, 2.9, and 3.7 times for 6 MV-FF, 10 MV-FF, 6 MV-FFF, and 10 MV-FFF, respectively. At the same energy, the surface dose increases to 3.93, 4.11, 4.23, 4.16 and 4.24 times at 5 mm, 15 mm, 30 mm, 39 mm and 55 mm thickness of the vacuum, respectively.

CONCLUSION: Using a couch and vacuum significantly increases the surface dose. For SBRT with a superficial target close to the couch and immobilization vacuum, reduction of vacuum thickness and careful attention to skin dose in planning would be helpful in avoiding severe skin toxicity.