Measuring Out-of-Field Dose to The Whole Brain in Radiotherapy

INTRODUCTION: Purpose: The aim of this study is to measure out-of-field lens and thyroid doses in whole brain radiotherapy with thermoluminescence dosimeters (TLD) and compare them with treatment planning system (TPS) calculation values.
METHODS: Materials and Methods: Before Computed Tomography (CT), TLDs were placed on the lens and thyroid surfaces of the random phantom and then the phantom was scanned with computed tomography. Data was transferred from CT to RayStation™ TPSs and then target volume and critical organs were determined. The treatment plan was created. TLDs were placed on the lens and thyroid for out-of-field dose measurement.
RESULTS: Results: For the right lens, the mean values of TLD measurement were 188.3±2.2 cGy and the mean values of TPS measurement were 192.0±0.2 cGy. The average TLD measurement for the left lens was 190.2±0.5 cGy, and the average TPS dose reading was 192.0±0.1 cGy. For doses in the thyroid region, which is further from the target, the TLD measurement and TPS reading averages were 44.9±5.2 cGy and 40.9 cGy, respectively. Accordingly, right lens point doses calculated from TPS were 1.93% higher than TLD measurements. For the left lens, this difference in the same direction was determined as 0.93%. Within the thyroid region, TLD measurements were observed to be higher than TPS readings.
DISCUSSION AND CONCLUSION: Conclusion: We measured out-of-field doses via TLDs and found that TPS calculations for thyroid were 8.90% lower than the measured dose. The results we obtained from our study are guiding in estimating out-of-field lens and thyroid doses in 3DCRT whole brain irradiation.