Topics covered in this article include:
- Introduction
- Current Limitations
- Calculation Notes
- Calculation of R50 values from I50 values
- Temperature-Pressure Correction: PTP
- Polarity and Ion Recombination Correction Factors
- Corrected Readings
- Dose Delivered to Water at d=measurement depth (photons)
- Dose Delivered to Water at d =dmax (Photons, SAD)
- Dose Delivered to Water at d =dmax (Photons, SSD)
- Reference Depth dref (electrons)
- Gradient Effective Correction (electrons)
- Dose Delivered to Water at d =dref (electrons)
- Dose Delivered to Water at d = dmax (electrons)
- Target Reading for Adjustments
- End Notes
Introduction
The TG-51 capabilities built into Total QA are based on the original TG-51 document^{1} and the later 2014 amendment^{2}.
The approach followed is to, as much as practical, integrate the workflow of the TG-51 forms into the familiar Total QA workflow to minimize the learning curve for existing Total QA users. Thus, TG-51 pre-built templates for photons (SAD and SSD) and Electrons can be scheduled for any Linac in a similar way that irregular QA templates are apple. The general workflow of completing, saving, commenting, and finalizing will be familiar to physicist users of the system.
The current focus of the TG-51 system is for photon and electron TG-51 reports using cylindrical and Farmer type chambers.
Many of our users gave generous and valuable feedback during the development of this project. Thank you. Many of your suggestions made it into the software already and rest assured that the others will guide us as we evolve the TG-51 capabilities.
Current Limitations
Parallel plate chambers are not currently supported. We do plan on adding those as we gather more test data.
Currently, calculated k_{Q} and k'_{R50} factors are entered into the system but not calculated within Total QA. We plan to add the capability to calculate those factors within the system.
The current workflow allows for working on a single energy for a given machine in a report at a time. We are looking at ways to be able to be working on multiple energies at once.
Calculation Notes
Calculation of R_{50} values from I_{50} values
Users may opt to enter I_{50} values and have the corresponding R_{50} values calculated.
R_{50 }= 1.029I_{50} - 0.06 (cm) (for 2 ≤ I_{50} ≤ 10 cm)
or
R_{50 }= 1.059I_{50} - 0.37 (cm) (for I_{50} > 10 cm)
See equations 16 and 17 in Almond et al(1999). I_{50} values < 2 cm are not permitted.
Temperature-Pressure Correction: P_{TP}
The temperature - pressure corrections are calculated using the pressure units specified for the account.
P_{TP }= (T_{W }/ 295.15°K × (Reference Pressure/ Measured Pressure) where T_{W }= T(℃) + 273.15
Reference Pressure Unit |
Reference Pressure |
mmHg |
760 mmHg |
hPa |
1013.3 hPa |
kPa |
101.33 kPa |
Polarity and Ion Recombination Correction Factors
The polarity correction (P_{pol}) is based on equation 9 in Almond et al(1999). The value is calculated from the average raw electrometer readings entered in nC.
Note that the assumption is that Mh1 is the reading corresponding to the charge collected for the reference dosimetry measurements in the clinic and which should be the same as for the chamber calibration^{3}.
The ion recombination correction is based on equation 12 in Almond et al(1999).
Note the assumption is that the reading are in the range where P_{ion} < 1.05 where the linear form of the saturation curve holds^{4}.
Corrected Readings
Photon SAD and SSD
The fully corrected charge reading for the ion chamber reading is based on equation 8 in Almond et al(1999) with the addition of the P_{rp} factor to take account of the variation of the radial dose distribution that is averaged by the detector^{5}. This factor becomes important when measuring FFF fields
M = P_{TP}P_{elec}P_{pol}P_{rp}M_{raw}
Electron
The fully corrected charge reading for the ion chamber reading is based on equation 8 in Almond et al(1999) .
M = P_{TP}PionP_{elec}P_{pol}M_{raw}
Dose Delivered to Water at d=measurement depth (photons)
The dose delivered at the measured depth is calculated according to equation 3 in Almond et al(1999). Note that the N^{60Co}_{D,w} is recorded in units of cGy/nC within the Total QA system and the results of the equation are displayed in units of cGy.
D_{w}^{Q} at d_{measured-depth} = Mk_{Q}N^{60Co}_{D,w}
Dose Delivered to Water at d =d_{max} (Photons, SAD)
To calculate dose at d_{max} the dose at the measured depth is divided by the clinical TMR for the appropriate depth, field size, and energy entered for the Linac^{6}. Note that TMR is recorded in the Total QA system as a proportion (0 to 1).
D_{w}^{Q} at d_{max} = D_{w}^{Q} at d_{measured-depth} / TMR_{(depth, fieldsize)}
Dose Delivered to Water at d =d_{max} (Photons, SSD)
To calculate dose at d_{max} the dose at the measured depth is divided by the clinical percent depth dose for the appropriate depth, field size, and energy entered for the Linac^{7}. Note that percentage depth dose is recorded in the Total QA system as a percentage (0 to 100%).
D_{w}^{Q} at d_{max} = D_{w}^{Q} at d_{measured-depth} / (%ddR_{(depth, fieldsize)} / 100%)
Reference Depth d_{ref }(electrons)
The reference depth is based on equation 18 in Almond et al. (1999).
d_{ref} = 0.6R_{50 }- 0.1 (cm)
Gradient Effective Correction (electrons)
The correction for the gradient effect is based on equation 21 in Almond et al(1999).
Dose Delivered to Water at d =d_{ref} (electrons)
The dose delivered at the reference depth is based on equation 6 in Almond et al.(1999) . Note that the is recorded in units of cGy/nC within the Total QA system and the results of the equation are displayed in units of cGy.
D_{w}^{Q} at d_{ref} = MP^{Q}_{gr}k_{ecal}N^{60Co}_{D,w}
Dose Delivered to Water at d = d_{max} (electrons)
The dose at d_{max} is derived by dividing the dose at the reference depth by the PDD value for the field size and measurement depth and then by the MUs delivered^{8}. Note that percent depth dose recorded in percentage in the Total QA system.
D_{w}^{Q} at d_{max} = D_{w}^{Q} at d_{ref} / (Clinical %dd / 100%) / MUs
Target Reading for Adjustments
The target reading attempts to predict the electrometer reading required to achieve a dose at d_{max} of 1 cGy/MU.
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