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

Abstract

The Gamma Knife Icon allows the treatment of brain tumors mask‐based single‐fraction or fractionated treatment schemes. In clinic, uniform axial expansion of 1 mm around the gross tumor volume (GTV) and a 1.5 mm expansion in the superior and inferior directions are used to generate the planning target volume (PTV). The purpose of the study was to validate this margin scheme with two clinical scenarios: (a) the patient’s head remaining right below the high‐definition motion management (HDMM) threshold, and (b) frequent treatment interruptions followed by plan adaptation induced by large pitch head motion. A remote‐controlled head assembly was used to control the motion of a PseudoPatient® Prime head phantom; for dosimetric evaluations, an ionization chamber, EBT3 films, and polymer gels were used. These measurements were compared with those from the Gamma Knife plan. For the absolute dose measurements using an ionization chamber, the percentage differences for both targets were less than 3.0% for all scenarios, which was within the expected tolerance. For the film measurements, the two‐dimensional (2D) gamma index with a 2%/2 mm criterion showed the passing rates of ≥87% in all scenarios except the scenario 1. The results of Gel measurements showed that GTV (D100) was covered by the prescription dose and PTV (D95) was well above the planned dose by up to 5.6% and the largest geometric PTV offset was 0.8 mm for all scenarios. In conclusion, the current margin scheme with HDMM setting is adequate for a typical patient’s intrafractional motion.

End to end comparison of surface‐guided imaging versus stereoscopic X‐rays for the SRS treatment of multiple metastases with a single isocenter using 3D anthropomorphic gel phantoms

“…This hollow phantom can be printed with bone-mimicking materials based on anatomical structures of a past patient, based on CT data or scans, and it is filled with water and a 3D polymer gel dosimeter insert to simulate soft tissue equivalence. 30 Irradiation of this gel phantom provides the ability to evaluate 3D dose distributions for SRS. 18,31,32 The custom phantom is designed with a glossy white exterior, which is incompatible with the SGRT cameras.…”

“…30 Irradiation of this gel phantom provides the ability to evaluate 3D dose distributions for SRS. 18,31,32 The custom phantom is designed with a glossy white exterior, which is incompatible with the SGRT cameras. A nude makeup foundation (Stay Matte, Rimmel, London) was applied to the phantom's surface to simulate a skin tone visualizable by the scanner units.…”

Ramifications of Setup Margin Use During Frameless Stereotactic Radiosurgery/Therapy With Gamma Knife Icon Cone-Beam Computed Tomography (CBCT): A Dosimetric Study

“…Also, a more practical approach might be to choose a value close to the recommended amount from this work, say 1 mm, for instance, to implement as setup margin, but which is similar to that predicted by the VHMF or Parker formulae (Table 4). Utilizing a similar approach of 1 mm for PTV margin (1.5 mm superior and inferior), a recent phantom study demonstrated acceptable dose delivery even under the worst-case scenario for frameless radiosurgery with Gamma Knife Icon [27]. In any situation, rational margin application should seek to balance the risk of normal tissue injury from radiation treatment with the risk of injury to the same tissue due to uncontrolled disease [28].…”

Brain stereotactic radiosurgery using MR‐guided online adaptive planning for daily setup variation: An end‐to‐end test

“…The phantom is a 3D‐printed anatomical replica created from a CT image of a human head. The hollow phantom with internal anatomical bony structures can be filled with water and has an insert to hold an ionization chamber for point dose measurement 17 . Recently, RTsafe developed a new delineation insert that fits into their Prime phantom with two predefined brain metastatic lesions near brainstem that are visible on MRI (Figure 1a).…”

“…The hollow phantom with internal anatomical bony structures can be filled with water and has an insert to hold an ionization chamber for point dose measurement. 17 Recently, RTsafe developed a new delineation insert that fits into their Prime phantom with two predefined brain metastatic lesions near brainstem that are visible on MRI (Figure 1a). The superior target has a volume of 4.8 cm 3 , the inferior target has a volume of 1.4 cm 3 , and the density of both targets is 1.03 g/cm 3 .…”