การตรวจสอบข้อจำกัดเครื่องกำบังรังสีแบบซี่ขนาดใหญ่ในการฉายรังสีร่วมพิกัดบริเวณศีรษะ

Abstract

Stereotactic Radiosurgery (SRS) and Stereotactic Radiation Therapy (SRT) are radiotherapy techniques by multiple and non-coplanar beam used for treating intracranial lesions. C-arm based linear accelerators (C-arm linacs) are widely used for these techniques due to their compatibility for shaped beam with aperture devices such as multi-leaf collimators (MLC). Various leaf widths of this platform may impact the radiation dose coverage on the lesion, particularly a small lesion. This retrospective study then investigated the limitation of the large MLC on the C-arm linac in intracranial stereotactic radiosurgery by various sizes of Planning Target Volume (PTV) through dosimetric quality parameters and Gamma analysis by Gamma Passing Rate (GPR). GPR was performed in Patient specific quality assurance (PSQA) by created on a single lesion of 69 treatment plans that were treated by the SRS/SRT. The SRS phantom was inserted the 2D array detector (SRSMapCHECK®) that measured the dose delivery. Various dosimetric parameters, such as Conformity index (CI), Gradient index (GI), and treatment planning complexity were used to evaluate the significant correlation among each other, including the GPR. This study examined PTV sizes ranging from 0.34 – 30.42 cm3 which was 8.69 ± 8.38 cm3 (mean ± SD). The CIICRU and CIPaddick were 1.29 ± 0.17 and 0.77 ± 0.10, respectively, while the GI was 5.24 ± 2.18. The treatment planning complexity calculated by monitor units (MU) in terms of Modulation Factor (MF) and treatment planning complexity calculated by the standard deviation of photon in terms of Fluence Map Variation (FMV) were 3.38 ± 0.87, 0.10 ± 0.03, respectively. The study found significant correlations (p < 0.001) were found between PTV sizes and CIICRU (r = -0.676), CIPaddick (r = 0.670), GI (r = -0.913), and FMV (r = 0.444, p < 0.001) whereas MF (r = 0.363) was a significant correlation with sizes of PTV at p = 0.002. The GPR2%/2mm values were 92.42 ± 3.74 on absolut dose (AD) and 96.38 ± 3.24 on relative dose (RD), respectively for criterion. In case GPR2%/1mm values were 82.03 ± 6.69 and 89.64 ± 7.26 for AD and RD, respectively. No significant correlations were found among FMV, MF and GPR values. Confidence limits (CL) were also calculated. The Confidence limits (CL) values of GPR2%/2mm were 85.09% and 90.03% whereas the CL values of GPR2%/1mm were 68.92% and 75.41% for AD and RD, respectively. This study demonstrated that the large MLC has no limitations in SRS/SRT techniques. The decreasing CI and GI values in small PTVs, especially in PTVs with target volume less than 5 cm3. The increasing PTVs can improve the value of CI and GI. By increasing the PTV volume, the treatment plan complexity was also increased. This increasing volume, however, was not impacting on the GPR. In GPR2%/1mm, GPR value yields lower values than 2%/2 mm criterion. This may ensure that a PTV margin might be contributed at least 2 mm, particularly small lesions. These can be confirmed by the CL value that only GPR2%/2mm revealed a value above 90% of the RD.