Characterization of a novel 3D printed patient specific phantom for quality assurance in cranial stereotactic radiosurgery applications

Abstract

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient’s planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as ‘bone’) and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

Validation of PTV margin for Gamma Knife Icon frameless treatment using a PseudoPatient® Prime anthropomorphic phantom

“…The results showed a dramatically reduced passing rate at 2%/1 mm in all scenarios. This is attributed to the uncertainty of the film dosimetry protocol, as Makris et al reported the film‐to‐CT image registration uncertainty to be in the order of 1.5 mm 16,22 …”

“…The film was calibrated using a single‐channel protocol with the red color channel 15 . The irradiated films were digitized with an EPSON flat‐bed color scanner (Perfection V850 Pro, Nagano, Japan) using the scanning parameters described by Makris et al 16 After the film scans, the net optical density values were converted into the absolute dose values. The spatial resolution of the film was 0.169 mm.…”

Robustness of single-isocenter multiple-metastasis stereotactic radiosurgery end-to-end testing across institutions

“…The primary advantage of gel dosimetry in an anthropomorphic phantom is that, unlike other patient-specific QA, it does not rely on a recalculation of the plan on a phantom nor on the process with which to reconstruct a 3D dose distribution. Rather, the measurement in this phantom can be directly compared with the patient's calculated dose distribution as was demonstrated by Makris et al 13 . Finally, for the most realistic execution of an end-to-end test, an anthropomorphic phantom is the best substitute to approximate the entire process.…”

“…A combined 1σ dosimetric uncertainty of 2.3% was estimated for the dose level of 8 Gy which was the prescription dose. According to Pappas et al (2017) 16 study, a spatial 1σ uncertainty of 1.5 mm is resulting from the spatial registration procedure between scanned film images and the CT dataset of the film phantom following a registration technique described elsewhere 13,16 .…”

Technical Note: Development of 3D‐printed breast phantoms for end‐to‐end testing of whole breast volumetric arc radiotherapy

“…Recent dispersion of low-cost three-dimensional (3D)-printing technology has allowed for the development of (patient) specific phantoms in radiotherapy. [6][7][8] This work describes the development of a series of end-to-end left breast attachments to the CIRS SBRT thorax phantom, using a mid-range 3D printer, allowing for commissioning of breast radiotherapy treatment techniques. All phantoms are fully compatible with surface scanning technology for accurate positioning and have either a film or IC insert centrally located in the breast.…”