Development and characterization of the Hydra: a novel multi-headed, single optical channel scintillation detector

<i>Objective.</i>Plastic scintillation detectors (PSDs) possess many properties (small size, water-equivalence, real-time reading) that make them well suited for dosimetric applications in radiotherapy. Multi-point PSDs (mPSDs) further enhance capabilities of PSDs, incorporating multiple detection points cascaded within a single collecting optical fiber in order to provide more spatial information about the radiation field. The goal of this study was to develop and characterize a novel mPSD design, called Hydra, that possesses multiple optical fiber heads connected to a single optical fiber. Each head tip includes a single scintillating element, allowing to measure the dose simultaneously at three independent locations in space.<i>Approach.</i>The Hydra design consists of three Medscint proprietary scintillators with emission peaks at 434, 428 and 530 nm. The optical fibers of each head and the collecting fiber are made of PMMA (ESKA GH-4001, Mitsubitshi Chemical Co., Tokyo, Japan). The Hydra was used to measure output factors, dose profiles and percent-depth-dose (PDD) curves in water under a 6 MV photon beam, individually for each head as well as with all three heads in the irradiation beam.<i>Main results.</i>The deviation on output factors, when all three heads are in the irradiation field, remains within ±1.1% as compared to when only 1 head is in the irradiation field. For the profile and PDD curves, the deviation with the reference curves remains within ±2% in the high-dose and low-gradient regions.<i>Significance.</i>The Hydra offers a good control on the uniformity of light intensity coming from each scintillator, a freedom to measure independently in a non-contiguous space, and an easier and more accurate calibration procedure compared to previous mPSD designs. These advantages open the door to new possibilities for multi-point dosimetry applications.