Session Item

Percentage Depth Dose curve (PDD) measurement is an important step of the QA process in radiotherapy[1]. It is routinely implemented by using computer-controlled water phantoms, which need a time-consuming set-up. To facilitate this task, we propose an instrumented solid-water phantom to allow a quick and highly resolved PDD measurement or check without using a water tank. Its high depth resolution also makes it possible to measure PDD in heterogeneous cavity conditions.

The proposed PDD phantom is shown in figure 1. It consists of a plastic scintillating fibre ribbon (SciFi) inserted into a RW3 water-equivalent block. The scintillating fibre outputs are coupled to three arrays of 256 silicon photodiodes, with signal readout and processing modules. Each detected signal from fibre ribbon outputs corresponds to integrated dose over the irraidated fiber segment length. The relationship between signal readout and integrated dose has been established by testing. Signal processing allows on-axis PDD retrievment. The RW3 phantom has one air cavity that extends on either side of the fibre ribbon and can be fitted with inserts made of RW3 or other materials equivalent to cortical bone or lung, respectively.         
The designed and fabricated PDD phantom has been tested at university hospital of Lyon on a  6MV photon beam delivered by a Novalis TRUBEAM STX Linac. The obtained results have been compared with measurements from computer-controlled water phantom (equipped with a PinPoint Ion Chamber). On the other, primo MC simulations have been performed and compared.

The designed phantom provides a 250µm depth resolution, which is suitable for measurements in built-up areas close to surface or near a heterogeneous cavity. 
Fig.2 compares simulated and measured PDD curves in homogeneous (with an RW3 insert) and heterogenous (with an air cavity) conditions, respectively. 

We have designed, fabricated and tested a high-resolution phantom for QA process. The proposed instrumented phantom can be used for quick PDD measurement or verification. It can also serve to study inhomogeneity effect on PDD.

References:

[1] Smilowitz, J.B., Das, I.J., Feygelman, V., Fraass, B.A., Kry, S.F., Marshall, I.R., Mihailidis, D.N., Ouhib, Z., Ritter, T., Snyder, M.G. and Fairobent, L. (2015), AAPM Medical Physics Practice Guideline 5.a.: Commissioning and QA of Treatment Planning Dose Calculations — Megavoltage Photon and Electron Beams. Journal of Applied Clinical Medical Physics, 16: 14-34.