Development of end-to-end test phantom for stereotactic radiosurgery using alanine dosimeter

Abstract

Alanine dosimeters are generally used in high-dose dosimetry, while accurate and precise dose delivery verification procedures are important in stereotactic radiosurgery (SRS). Hence, an end-to-end test is required. In addition, no end-to-end SRS test employing an alanine dosimeter has been established in Thailand. The objective of this study was to determine the dose accuracy of SRS VMAT plans in end-to-end test phantom using alanine dosimeter. The electron paramagnetic resonance (EPR) parameters were optimized in the 1-2000 cGy dose range. Following that, suitable parameters were used to investigate the alanine characteristics and estimate the uncertainty in radiation therapy dose range. The alanine dosimeter system was then validated for output measurements in eight radiotherapy centers in Thailand at 1000 cGy at 6MV-FFF energy. Finally, a head phantom was fabricated and validated using an alanine dosimeter for end-to-end test in SRS. The optimized EPR parameters are microwave power of 2 mW, modulation amplitude of 7.018 G, and time constant of 40.96 ms. The expanded uncertainty (k=2) was 2.2% in the 100-2000 cGy dose range. The uniformity and reproducibility of the alanine dosimeter was studied in 6MV-FFF, with the percentage of CV being 0.61 and 0.22, respectively. The EPR Intensity demonstrated a linear proportion to the dose from 0 to 3000 cGy (R2 = 0.9998). The repetition rate response, energy dependence, and directional dependence were 0.80%, 0.80%, and 0.30%, respectively. Furthermore, the fading in three months was 0.98%. The output survey in nine machine radiotherapy facilities was accurate to within ±1%. The phantom was made of cast nylon since it has a CT number of about 80-100 HU, which is close to soft tissue and is also a more commercially priced phantom. Volumetric modulated arc therapy (VMAT) SRS plans target percentage dose differences were ranging from 0.01% to 1.16%, while the percentage dose difference in the brain stem (OAR) ranged from 3.65% to 11.95%. A high variation of dose was found in Position 3 of TPS were SD=19.7, 17.8 and 21.5 in plan 1, plan 2 and plan 3, respectively. This contributes to a large dose difference when low dose at brainstem were performed. In conclusion, this study presented the fabricated cast nylon end-to-end test head phantom suitably designed to image and irradiate during an end-to-end test for stereotactic radiosurgery using alanine dosimeter.