What is Proton-Beam Therapy (PBT) and how is it used?
PBT1 is a treatment that utilizes beams of protons to treat the tumor. The advantage compared to standard x-ray radiotherapy is, that the interaction of the protons and the tissue is different.
The power deposition of the protons will be more focused and hence it is possible to give less dose to normal tissue and more dose to the tumor. Because of the lower dose to normal tissue initially, the main treatment group for PBT were children.
However, the group of indications treated with PBT is steadily growing and the available capacity is also growing.
The power deposition of the proton-beam is described as the Bragg peak. The big advantage of the power deposition compared to photons, is that the power at the surface where the beam enters into the patient is very low and the power when the beam exits the patient is also very low. The majority of the power is deposit in the Bragg peak and the position of this can be controlled with the energy of the beam.
Due to the size of the particle gantries, the distance between the x-ray imaging source and the detector is much larger than for standard IGRT equipment. This has a negative impact on the image quality and this can potentially limit the accuracy. Hence, fiducial markers can be an enabler for improving the accuracy of the image guidance in PBT. However, the use of fiducial markers has been limited due to the high relative stopping power of the metal fiducials in the proton beam. BioXmark® has been found to be suitable for PBT2.
BioXmark® is under development and not yet CE marked.
A new PBT treatment method is being investigated at the moment; it is called FLASH. The idea is to increase the doserate of the delivery system, hence the dose can be given extremely fast. Radiobiological studies show, that this has a beneficial impact.
The tumour cells are more sensitive and the normal cells are less sensitive. The concept of FLASH is then to deliver the total dose in one fraction and this can then be done in seconds due to the extremely high doserate. The advantages are:
• The radiobiological benefits
• Less sensitivity to intra fraction movements as the dose is delivered in seconds not minutes
• No inter fraction movements, since the total dose is delivered in one fraction
Image courtesy of Varian Medical Systems, Inc. All rights reserved.
1. Journal of the ICRU, ICRU Report no. 78, Prescribing, Recording and Reporting Proton-Beam Therapy
2. Scherman Rydhög J. et al. Liquid fiducial marker applicability in proton therapy of locally advanced lung cancer. Radiotherapy and Oncology 122 (2017) 393–399