3-dimensional scintillation dosimetry for small irradiation fields control in protontherapy

by Gautier Daviaux

X-rays have been used in radiotherapy to treat cancer since the end of the 19th century. But this method has a drawback: it delivers a greater dose on the entry point of the body than onto the tumor. To compensate this handicap, complex treatment ballistics were developed. For similar reasons, proton-therapy was also developed in the middle of the 20th century. Protontherapy can reach the tumor in the body with smaller doses delivered in its path compared to X-ray radiotherapy. The proton-therapy’s drawback is the higher complexity of the radiation facility needed, leading to higher cost. In France, only 3 centers are certified to do protontherapy.

To this day, tumor smaller than 3 cm cannot be treated. The issue is the lack of guaranty on the treatment planning on such a small volume. The treatment planning is the construction of the radiation ballistic: how many beams will be used, what energy, direction and intensity each beam will have. This planning ensures that the tumor receives the correct amount of dose and healthy tissue are preserved as much as possible. This process is extremely important for the safety of the patient and is the cornerstone of any cancer therapy.

The aim of my thesis is to create a dosimetry setup allowing medical physicists to control their treatment plans for small tumors geometrically and dosimetrically. The setup is composed of an cubic plastic scintillator, a fast camera, a mirror allowing the side view of the cube and a casing preventing parasitic lights to disrupt the scintillation measurement.

The setup will have two operating modes: the first one is used to control the beam characteristics: energy, position and intensity. The second mode is the 3D dosimetry reconstruction, this mode enables the operator to check the dose distribution computed by the treatment planning system.