Prospective comparative evaluation of planning target volume margin for brain intensity modulated radiotherapy utilizing hybrid online imaging modalities

Background: A new advancement in daily monitoring of patient positioning is the use of hybrid technologies where two separate online imaging modalities are integrated to achieve precise treatment delivery. Our center has a set-up that integrates Elekta Linear accelerator device (EPID) with BrainLAB ExacTrac imaging for the first time in the world. We calculated planning target volume (PTV) margin for brain radiotherapy with thermoplastic mask immobilization with conventional EPID and BrainLAB ExacTrac image guidance system. Materials and Methods: EPID (iViewGT) and ExacTrac verification images of 32 patients in total 784 radiotherapy sessions were acquired and analyzed. Systematic (Σ) and random errors (σ) were calculated in cranio-caudal, lateral and anteroposterior directions. PTV margins calculated using van Herk (2.5 Σ +0.7 σ) formula for each imaging system. Result: Of total 784 sessions EPID image were obtained in 723 sessions, ExacTrac obtained in 431 sessions. In cranio-caudal direction, the systematic error, random error, and the calculated PTV margin were 0.09 cm, 0.12 cm, and 0.31 cm, respectively, with EPID image and 0.17 cm, 0.13 cm, and 0.51 cm, respectively, with ExacTrac. The corresponding values in lateral direction were 0.11 cm, 0.15 cm, and 0.40 cm with EPID and 0.16 cm, 0.10 cm, and 0.47 cm, respectively, with ExacTrac image. The same parameters for anteroposterior were 0.10 cm, 0.13 cm, 0.37 cm with EPID and 0.144 cm, 0.10 cm, and 0.43 cm with ExacTrac image. Pearson's correlation coefficient was found to be 0.66, 0.67, 0.62 in these three directions. Conclusion: With dual imaging modalities, our calculated adequate PTV margin for brain radiotherapy cases are 0.51 cm, 0.47 cm, is 0.43 cm in cranio-caudal, right-left, and anteroposterior directions, respectively.