Dosimetric Comparison of a-Si 1200 and a-Si 1000 Electronic Portal Imager for Intensity Modulated Radiation Therapy (IMRT)

Aim: This study investigates the dosimetric impact of a-Si 1200 over a-Si 1000 portal imager for 6 MV photon beam of a Varian Amorphous Silicon Electronic Portal Imaging Device (a-Si EPID) installed on Edge and Novalis TxLinac. Materials and Methods: The performance of an a-Si EPID 1200 was assessed and compared to its performance with a-Si EPID 1000 and dose measurements using an ionization chamber. This study was conducted for 6MV photon energy and the EPID performance was assessed as function of the delivered dose, dose rate, field size, SDD effect, Ghosting effect, Backscatter arm shielding impact and intensity-modulated radiation therapy fields. Results: The dose response for a-Si 1200 was within 0.5% for low MU (2-10) as compared to 1.4% for a-Si 1000 portal imager and less than 1% for both the imager above 10 MU. The output factor variation for 25 × 25 cm2 was 1.3% for a-Si 1200 and 3.8% for a-Si 1000 when compared with ionisation chamber. The ghosting was measured to be 0.2% for a-Si 1200 as compared to 0.8% for a-Si 1000 portal imager. There is a significant improvement in a-Si 1200 portal image due to backscatter shielding material attached to the back of the panel of a-Si 1200 portal imager. Conclusion: The new aS1200 detector showed a significant dosimetric improvement when compared with previous aS1000 hence providing more accurate measurements for pre-treatment patient specific Quality Assurance (QA).