Water calorimetry as a primary measurement standard for proton therapy
Radiotherapy aims at an as high as possible tumor control probability combined with an as low as possible probability of long term radiation induced complications significantly affecting the quality of life of the patients. As compared to conventional radiotherapy, proton therapy offers, because of the finite penetration depth of the proton, a significantly higher conformity of the dose distribution with the tumour volume, which should result in a reduction of the probability and severity of radiation induced complications.
However, to realize this expected clinical benefit the deposited radiation dose has to be known with at least the same 1% accuracy as currently achieved in conventional radiotherapy. Taking into consideration the on-going development towards more personalized treatment an even better accuracy will be required in the not too distant future. Given the fact that in proton therapy also secondary radiation such as neutrons and gamma rays contribute to the delivered dose at a level non-neglible as compared to the required accuracy, attaining this accuracy requires very detailed knowledge of the radiation field and the effect of the selected measurement technology on the outcome of the measurements.
The most direct measure for dose (unit Gy = J/g) is temperature rise in a material in which the dose is deposited, which has led to the widespread use of calorimetry as a primary standard for deposited dose. As human tissues are composition-wise reasonably similar to water, it is a logical choice to use water as the reference material: the production of and dose deposition by the secondary particles will closely resemble the situation in the patient.
Over the last six years we have studied several issues that affect the accuracy that can be achieved in reference dosimetry of protons with water calorimetry:
- Radiation-induced chemical reactions may produce or absorb heat, thus compromising the calibration results. Our simulations and experiments have demonstrated that by saturating the water with H2 this effect can be completely suppressed by performing a pre-irradiation with a relatively low dose, even in the Bragg peak for dose delivery with pencil beam scanning where the dose rates are very high.
- Heat transfer through diffusion at the timescale of individual dose deposits reduces the measured temperature increase due to the irradiation. We have experimentally validated the detailed finite-element modelling of this phenomenon, so that the measurements can be corrected for this effect. The simulations and experiments have also demonstrated that small scale spatial and temporal fluctuations of the dose deposition significantly affect the quality of the measurement. This implies that the stability and homogeneity requirements for calorimetry measurements with pencil beam scanning are significantly more stringent that for patient treatment.
- The contribution of secondary neutrons and gamma rays to the deposited dose has for the first time been determined by a combination of experiments and simulations. The measurements show that the difference in sensitivity for neutrons of the ionization chambers used in clinical practice and the calorimeter will become a limiting factor for the attainable accuracy when accuracies significantly below 1% are required.
In the coming years we intend to continue the development of reference dosimetry based on water calorimetry for pencil beam scanning dose delivery of protons. A versatile beam scanning system is currently under construction and possibilities to improve the stability of the proton beam will be studied in the near future.
This is a joint project of the KVI-CART, UMCG, the Dutch Metrology Institute VSL and the Swiss Federal Office of Metrology METAS.
|Last modified:||25 January 2019 10.54 a.m.|