Supervisors info:
Παναγιώτης Παπαγιάννης, Αναπληρωτής Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Ελευθέριος Παππάς, Μεταδ. Ερευνητής, Ιατρική Σχολή, ΕΚΠΑ
Παντελής Καραίσκος, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Summary:
Technological evolutions in medical imaging and radiation oncology have paved the way towards an increased application of Stereotactic Radiosurgery (SRS) techniques. Although the proven superiority in the clinical efficacy of SRS techniques compared to conventional ones, the dosimetric and geometric accuracy as well as the radiobiological mechanisms are crucial factors for the efficient and safe implementation of an SRS treatment. Therefore, quality assurance procedures and accurate radiobiological estimations play an important role in the clinical outcome.
Given the increasing role of Stereotactic Radiosurgery (SRS) in the management of multiple brain metastases, there is a need for implementing advanced Quality Assurance (QA) tools, adapted to the multi-target nature of such treatments, and associated spatial accuracy requirements. In this work, the dosimetric characteristics of a novel 3D dosimeter coupled with optical CT (oCT) readout, are evaluated for multi-target SRS QA.
The materials and methods used were a cylindrical container filled with a radiochromic gel based on Pluronic F-127 hydrogel and a formulation optimized for SRS dose characteristics. For spatial registration purposes, fiducials were placed on-top of markers. The dosimeter was then fixed inside a water-filled prime head phantom (RTsafe, Greece). Treatment planning involved 5 targets distributed over the entire available volume, irradiated by 5 Gamma Knife single-collimator shots (ELEKTA, Sweden). 20Gy were delivered to the 50% isoline. Dose readout was performed 20hr after irradiation by a Vista 15 oCT scanner (Modus, Canada). Measured and calculated relative dose distributions were spatially registered and compared.
By evaluating shot centroids in 3D, a spatial uncertainty of 0.8mm was estimated. Dose-response characteristics for the specific formulation and oCT scanner are: (i) adequate sensitivity to measure relative doses of >10%, (ii) no saturation effects observed, (iii) excellent signal-to-noise ratio with (iv) a suitable for SRS spatial resolution of 0.5mm.
However, the comparison also revealed a systematic enlargement of all measured full-width at half-maximums (FWHMs) by 0.7 - 1.9mm, depending on the collimator size, which can be attributed to unacceptable radiochromic signal diffusion rate. The investigated formulation and methodology demonstrate favourable characteristics for SRS QA, provided that signal diffusion is mitigated, potentially by minimizing the time interval between irradiation and oCT scanning.
Keywords:
Stereotactic Radiosurgery (SRS), Quality Assurance (QA) tools, Optical CT (oCT), 3D dosimeter, Multi-target SRS QA
