Dosimetric comparison of different treatment modalities for stereotactic radiotherapy

Abstract

BackgroundThe modalities for performing stereotactic radiotherapy (SRT) on the brain include the cone-based linear accelerator (linac), the flattening filter-free (FFF) volumetric modulated arc therapy (VMAT) linac, and tomotherapy. In this study, the cone-based linac, FFF-VMAT linac, and tomotherapy modalities were evaluated by measuring the differences in doses delivered during brain SRT and experimentally assessing the accuracy of the output radiation doses through clinical measurements.MethodsWe employed a homemade acrylic dosimetry phantom representing the head, within which a thermoluminescent dosimeter (TLD) and radiochromic EBT3 film were installed. Using the conformity/gradient index (CGI) and Paddick methods, the quality of the doses delivered by the various SRT modalities was evaluated. The quality indicators included the uniformity, conformity, and gradient indices. TLDs and EBT3 films were used to experimentally assess the accuracy of the SRT dose output.ResultsThe dose homogeneity indices of all the treatment modalities were lower than 1.25. The cone-based linac had the best conformity for all tumors, regardless of the tumor location and size, followed by the FFF-VMAT linac; tomography was the worst-performing treatment modality in this regard. The cone-based linac had the best gradient, regardless of the tumor location and size, whereas the FFF-VMAT linac had a better gradient than tomotherapy for a large tumor diameter (28 mm). The TLD and EBT3 measurements of the dose at the center of tumors indicated that the average difference between the measurements and the calculated dose was generally less than 4%. When the 3% 3-mm gamma passing rate metric was used, the average passing rates of all three treatment modalities exceeded 98%.ConclusionsRegarding the dose, the cone-based linac had the best conformity and steepest dose gradient for tumors of different sizes and distances from the brainstem. The results of this study suggest that SRT should be performed using the cone-based linac on tumors that require treatment plans with a steep dose gradient, even as the tumor is slightly irregular, we should also consider using a high dose gradient of the cone base to treat and protect the normal tissue. If normal tissues require special protection exist at positions that are superior or inferior to the tumor, we can consider using tomotherapy or Cone base with couch at 0° for treatment.

Dosimetric Comparison of 6 MV Flattening Filter Free and 6 MV Stereotactic Radiosurgery Beam Using 4 mm Conical Collimator for Trigeminal Neuralgia Radiosurgery

“…A lower GI value was reported in our study for 6X FFF beam using 4 mm Varian conical collimator. Hsu et al compared the dosimetric comparison between Cone based LIN-AC, FFF VMAT LINAC, and tomotherapy for SRS and concluded that cone-based LINAC had the best dose gradient for tumours located near the critical organs such as brainstem [35]. The organ at risk in TN treatments is the brainstem which is very near to trigeminal nerve.…”

Technological quality requirements for stereotactic radiotherapy

“…However, in connection with helical treatment, especially for SRS, the inherent coplanarity of this modality (and for others, too) might be an additional disadvantage [155][156][157]. When compared with other stereotactic radiotherapy treatment platforms, helical radiotherapy per-formed worse in some of the studies [158,159], though not in all cases. In a planning study comparing forwardplanned stereotactic conformal radiation therapy (SCRT), IMRT, and helical radiotherapy, the results demonstrated that helical radiotherapy was superior to SCRT, including a better hippocampal sparing for close target lesions [159].…”

“…Additionally, another study claimed that sparing the hippocampus was feasible with four treatment platforms, including one for helical radiotherapy [160]. Furthermore, in one study, helical radiotherapy was even favored over conebased gantry-based linear accelerators in cases where OAR prevented the use of non-coplanar beams [158]. Likewise, another study could not show superiority (or inferiority) of helical radiotherapy over gantry-based systems for arteriovenous malformations in general, but found advantages for the former for targets with special geometry [161].…”

Prediction of conical collimator collision for stereotactic radiosurgery

“…The use of stereotactic cones began in the 1980s in favor of (a) tertiary collimation system to provide small field sizes, (b) superior lateral penumbra compared to rectangular collimators, (c) spherical isodose distributions conformal to the tumor shape 1 . Although multileaf‐collimator (MLC)‐based SRS became prominent recently, cone‐based SRS is still being used to treat small intracranial targets such as small benign or metastatic tumors, or functional diseases like trigeminal neuralgia 2–4 …”