RTOG 0915-compliant patient specific QA for lung stereotactic body radiotherapy using the new PTW 1600SRS detector...

An area of focus in radiotherapy is the treatment of oligometastatic lung cancer using highly conformal techniques such as SBRT, performed using VMAT that involves flattening filter free (FFF) beams. This study proposes a new calibration procedure for PTW Octavius 1600SRS detector array and was designed to also evaluate clinical and dosimetric aspects of a patient-specific quality assurance (PSQA) for lung SBRT patients.

The cohort consists of 20 patients, treated for lung metastases using SBRT with 50 Gy dose in 5 fractions (10 Gy/fr). The proposed calibration method uses only one calibration factor determined at maximum dose rate of 6MV FFF photon beam. The dosimetric accuracy of achieving a high dose gradient was analyzed using the RTOG 0915 protocol and was confirmed by PSQA procedures using the PTW Octavius 1600SRS detector.

Conformity index, gradient index, maximum dose at 2 cm and V₂₀ parameters were evaluated with clinical favorable results, with only two plans with lesions situated in the inferior lobe exceeding the deviation allowed for the gradient index. Gamma passing rates using the new calibration method were 98.93% and 99.38% for different gamma criteria of 2 mm/2% and 1 mm/3%, respectively.

The proposed method for calibration using one calibration factor at maximum dose rate for the involved photon beam shows clinically acceptable gamma passing rates. Employing the RTOG 0915 protocol for lung SBRT treatment plan evaluation brings important dosimetric information about treatment plan quality and dose gradient fall-off which can be correlated with the results achieved during the pretreatment verification procedures.