Normalization for 6 MV photon simulation

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1

Dear FLUKA team,

I am performing a 6 MV photon 3D-CRT treatment plan recalculation for a real brain patient using FLUKA.

I performed a simulation of the clinical LINAC and obtained a dose of:

x=9.3×10⁻⁷ Gy/primary.

According to the procedure I am following, I calculated:

NP/MU = 1/(100*x)

which gives approximately:

NP/MU=10753 primary/MU.

I then used this NP/MU value in the Beam Model File for the patient treatment-plan simulation.

I would like to confirm the following:

  1. If x is already expressed in Gy/primary, is the calculation NP/MU = 1/(100*x)correct?

  2. Since x is already in Gy/primary, is it correct that no additional conversion factor such as 1.602×10⁻⁷ is required?

  3. Where should I obtain the number of irradiation particles from? Is it obtained from the Beam Model File, the clinical MU, the RT Plan, or somewhere else in the FLUKA simulation?

  4. What is the recommended equation for converting the FLUKA dose per primary into the absolute patient dose in Gy?

For example, if a patient field has MUi​, should the number of primary particles be calculated as:

N_i = (NP/MU) × MU_i

and then calculate the absolute dose as:

D_absolute = D_FLUKA/primary × N_i

Or is this conversion already handled automatically by the Beam Model File, in which case applying this multiplication again would result in double normalization?

I would also like to clarify this because, in my patient simulation, I obtain a CTV dose from USRBIN DOSE of approximately:

4.0341×10⁻⁷

while the corresponding TPS dose is approximately:

31.592 Gy.

I want to make sure that I am not applying an incorrect additional normalization factor when comparing the FLUKA DVH with the TPS.

Thanks in advance very much for your help and clarification.

Best regards,

3

Dear @walaa.khalil88,

  1. Can you describe what NP and MU are? As a general rule, please define all acronyms you are using.

If NP is number of particles and MU is Monitor Units, then you have to provide/know the conversion based on your accelerator/monitors. In this case, we cannot know if NP/MU = 1/(100*x) is correct or not, this is for you to verify with the irradiation facility.

  1. You seem to have several simulations.

How did you perform this simulation, also with FLUKA? The standard FLUKA output: Doses will be expressed in GeV/g per unit primary weight. To obtain dose in Gy, multiply GeV/g by 1.602176462E-7.

  1. You have to define the number of irradiation particles or monitor units. In FLUKA, you can provide the number of primary particles used to obtain your dose/primary results, but the number of primaries are used for statistical reasons and the FLUKA output is an average over the number of primaries. The more primaries you use, the more statistically significant your result, but this is not a priori related to number of irradiation particles.

I will get back to you regarding Question 4.

As a general rule, please upload also your inp files.

Best,
Daniel

4

Dear @dprelipc

Thank you for the clarification. I understand now that the number of primaries used in the FLUKA simulation is for statistical purposes and is not necessarily the number of particles delivered during the clinical irradiation.

Yes, exactly.
NP refers to the number of primary particles, and MU refers to Monitor Units.

In my case, the clinical calibration at the hospital is 1 Gy per 100 MU. Using my FLUKA LINAC simulation result of x = 9.3×10⁻⁷ Gy/primary, I obtained:

NP/MU = 1/(100*x) ≈ 10753 primary particles/MU.

I am now trying to understand how this NP/MU value should be used for the final absolute dose normalization in the patient simulation.

I have two simulations:

1. LINAC calibration simulation: I obtained approximately x = 9.3×10⁻⁷ from the FLUKA DOSE scoring and used a 100 MU reference to calculate NP/MU = 10753 primary particles/MU.

2. Patient simulation: I used the resulting Beam Model File for the patient treatment-plan recalculation and used USRBIN with DOSE. For the CTV brain, the USRBIN result is 9.0341 ×10⁻⁷. and “rt femur” is 4.5504×10⁻⁷ while the corresponding TPS dose for CTV and rt femur are 55,36 GY

1-Could you please clarify whether the 9.0341 ×10⁻⁷ value should first be converted from GeV/g to Gy using 1.602176462×10⁻⁷, and then multiplied by the number of clinical irradiation primaries corresponding to the MU of each field?

2-In particular, does the Beam Model File already apply the NP/MU and field MU weighting, or do I need to apply the conversion from MU to primary particles during post-processing?

thanks in advance

Best,

5

Dear @walaa.khalil88,

  1. The raw output of FLUKA for dose is in GeV/g - so to obtain Gy one has to multiply using 1.6x10⁻⁷.
  2. First, since you are doing photon beam simulations (and not proton beam), this is why your facility has to provide to you how many photons (NP) correspond to one monitor unit (MU). From you, I understand that you are trying to obtain this conversion based on the simulated dose, which is one way to do it, but you should double check this result experimentally.

Second, the Beam Model File gives you the result normalized per primary particle, meaning for NP=1. There, you have the option to specify the NP/MU conversion.

Best,
Daniel

6

Dear Daniel

Thank you very much for your reply and for taking the time to carefully explain the details to me.

First, regarding the calculation of NP, I obtained the following result from my simulation:

(1/(100 × 9.31 × 10⁻⁷×1.602176462 × 10⁻⁷)) = 67 × 10⁹

This is the result I obtained from my simulation to determine NP. However, the hospital informed me that the actual value of NP is 3 × 10⁹.

The difference between these two results may be because I performed the simulation without simulating the head of the linear accelerator. However, I used NP = 3 × 10⁹ to create the beam model file.

As I understand it, NP/NU is used to calculate the weight contribution for each spot, while the final FLUKA result is always expressed per primary particle. Therefore, in the end, I need to normalize the result according to the number of particles delivered during the irradiation (NP = 3 × 10⁹).

My problem is the following: in the MONACO TPS, for example, the PTV structure has a maximum dose of 55.5 Gy. However, the same PTV structure has a dose of 7.45 × 10⁻⁷ GeV/g per unit primary weight in my FLUKA simulation.

According to the FLUKA documentation, doses expressed in GeV/g per unit primary weight can be converted to Gy by multiplying by 1.602176462 × 10⁻⁷.

Therefore, my understanding is that, to normalize this result using NP = 3 × 10⁹, I should calculate:

7.45 × 10⁻⁷ × 1.602176462 × 10⁻⁷ × 3 × 10⁹

However, this gives a result that is much lower than the 55.5 Gy from the MONACO TPS.

Could you please clarify whether I am applying the normalization correctly? In particular, should 3 × 10⁹ be used directly as the number of primary particles for this normalization, or is there another factor related to the beam model, spot weights, or NP/NU that I need to take into account?

Or I can used the factor (55.5 /7.45 × 10⁻⁷ )?

the final factor give me logical result

Thank you very much for your help.

Best regards,
Walaa

7

Dear @walaa.khalil88,

Question: is the NP of 3 × 10⁹ is to give you the number of particles to obtain 1 Monitor Unit (MU), or the total number of particles for the total pacient irradiation?

If the former, then it makes sense that that you need more MUs to reach the total dose to be delivered, thus explaining why your current result is much lower than what you expect.

Best,
Daniel

8

Dear Daniel

thank you for your reply and for taking the time to carefully explain the details to me.

Yes, that is the number of particles required to produce 1 MU, but for a machine other than mine. I searched for the number of primary particles for my machine currently used in patient treatment and found it to be 1.5e12. Also, the total number of MUs in the treatment plan is 336. Therefore, I performed the following calculation: 1.5e12 * 1.602e-7 * 4.3e-7 * 336, which resulted in a maximum tumor dose of approximately 33 Gy. This closely matches the maximum dose obtained from the TPS calculations.

This calculate is right?

and what is the meaning of the total number of particles for the total pacient irradiation?

and where I can find it ?

thank you again

Best

Walaa