Attachment_A_Proton_Therapy_Facility_Dosimetry.pdf

Date: 2019-07-19 16:20 Page 1 of 2

Attachment A: Radiation Effects Dosimetry at Proton Therapy Facilities

The following text is intended to serve as useful background / context information as well as a series of practical dosimetry examples that may help prospective vendors better understand NASA requirements.

Proton therapy facilities generally use a calibrated ion chamber and a readout with a recycling integrator to serve as a continuous monitor of the beam flux during radiation exposures. The readout is in Monitor

Units, also called MU. They measure the energy of the proton beam by stopping it in water. In proton therapy treatments, dose and dose rate in water (e.g. Gy(H2O) and Gy(H2O)/s) delivered to the patient as the beam stops are the quantities of interest. In radiation effects testing, the number of protons/cm2-s, called flux, and the number of protons/cm2, called fluence, for an exposure in which the protons pass through a device under test (DUT) are the quantities of interest.

For radiation effects testing at a proton therapy center, using any beam delivery system – single scatter, double scatter, uniform beam, etc. – the test team needs the facility to provide the following:

1) Dose in water per Monitor Unit, Gy(H2O)/MU,

2) Dosimetry calibration, and

3) Proton energy at the DUT location.

The proton therapy community uses the SI unit gray (Gy) for absorbed dose, where 1 Gy = 1 joule/kg.

However, the radiation effects community frequently uses the deprecated CGS unit rad, where 1 rad =

100 ergs/g = 1 cGy. Like grays, it needs to be defined as a function of the target material, e.g. Gy(Si) or rad(Si). This supporting document shows how to take Gy(H2O)/MU, MU rate, and the energy at the DUT to calculate the fluence and flux at DUT.

Dose is defined as the energy absorbed per unit mass and is the product of the fluence and the linear energy transfer (LET), which is a function of the kinetic energy of the incident particles, the target material the energy is transferred to, and the particle species. LET is well known for protons and can be obtained from SRIM or the NIST PSTAR tool. For convenience, we recommend using the NIST PSTAR web-based tool, which relies on ICRU Report 491. There are known small differences in LET values from these sources.

Test case example:

- 200 MeV protons at the DUT location

- Dosimetry calibration = 6.54x10-4 Gy(H2O)/MU at DUT

- Exposed at 500 MU/sec for 20 s

Conversions and constants:

1 Gy = 6.2414x106 MeV/mg = 0.1 krad

1 https://icru.org/home/reports/stopping-power-and-ranges-for-protons-and-alpha-particles-report-49 http://www.srim.org/ https://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html https://icru.org/home/reports/stopping-power-and-ranges-for-protons-and-alpha-particles-report-49

Date: 2019-07-19 16:20 Page 2 of 2

1 krad = 6.2414x107 MeV/mg

Charge of proton = 1.6022x10-19 C

LET(H2O, p, 200 MeV) = 4.491x10-3 MeV-cm2/mg

LET(silicon, p, 200 MeV) = 3.627x10-3 MeV-cm2/mg

Convert calibration from Gy to MeV/mg:

- 6.54x10-4 Gy(H2O)/MU x 6.2414x106 MeV/mg = 4080 MeV/mg(H2O)-MU

Calculate fluence/MU:

- fluence/MU = 4080 MeV/mg(H2O)-MU/4.491x10-3 MeV-cm2/mg = 9.08x105 protons/cm2-MU

Calculate flux/MU/s:

- flux/MU/s = 9.08x105 protons-s/cm2-s-MU

For the example, there are 500 MU for 20 s, or 10000 MU total for the exposure.

- So, the fluence is 1x104 MU x 9.08x105 protons/cm2-MU or 9.08x109 protons/cm2 and the flux is

(500 MU/s) * (9.08x105 protons-s/cm2-s-MU) = 4.54x108 protons/cm2-s

The dose in water is 6.54x10-4 Gy(H2O)/MU * 10000 MU = 6.54 Gy(H2O)

The dose in silicon is fluence (protons/cm2) * LET(Si, p, 200 MeV) MeV-cm2/mg = 9.08x109 protons/cm2

* 3.627x10-3 MeV-cm2/mg = 3.29x107 MeV/mg or (3.29x107 MeV/mg) / (6.2414x107 MeV/mg/krad) =

0.527 krad(Si) or 587 rad(Si).