Page 82 - ARNM-3-1

Advances in Radiotherapy
& Nuclear Medicine Cone beam-focused GK dosimetric analysis

and MATLAB in-house program were used to process and radiation center and mechanical center coincide at one
analyze the data. point. All implementations of the technique are based on
Based on the IAEA Technical Report of Series no. 277 the isocenter position, so the accuracy of the isocenter is a
Absorbed Dose Determination in Photon and Electron prerequisite for ensuring the accurate dose delivery to the
Beams - An international Code of Practice (TRS277) and tumor.
JJG 1013-2006 Verification Regulation of γ Radiation Source Using the PMMA standard phantom, the EBT3
Used in Head Stereotactic Radiosurgery Therapy, Equation Gafchromic film was placed in the phantom to separately
(V) was used for calculation of absorbed dose to water. measure the XOY and YOZ planes. The isocenter pinprick

D = M×N ×(1−g)×K ×K ×K ×K ×K ×S ×P ×P on the film holder was used to create reference holes on the
water k att m TP h s w,air u cel irradiated film, serving as the mechanical center reference
(V)
where M is the reading of the dosimeter; N is the point. The irradiated film was then scanned with an Epson
k
calibration coefficient in terms of air kerma by ionization V850Pro scanner, exporting images in 16-bit grayscale.
Figure 5 shows the film images of 1# collimator’s XOY
chamber (unit = Gy); g is the fraction of the energy of the and YOZ planes, with the punched hole at the mechanical
secondary electrons lost to bremsstrahlung in air, which center, which serves as the positioning reference point.
is 0.0030 for Cobalt-60; K is the correction factor for the The threshold segmentation function of ImageJ was
att
attenuation and scattering of photons in the ionization used to extract the radiation field center coordinates and
chamber material (including the buildup cap); K is the the punched hole coordinates (mechanical center and
m
correction factor for the lack of air equivalence of the
ionization chamber material; K is the temperature and positioning reference point). The deviations along the X,
TP
pressure correction factor, with a value of 0.997 when Y, and Z axes were acquired to calculate the positioning
5,6
the relative humidity is 20% – 70% and the temperature reference point deviation (∆R) using Equation VI.
is between 15°C and 25°C; K is the factor to correct the ∆ = ∆R X 2 +∆Y 2 +∆Z 2 (VI)
s
response of an ionization chamber for the lack of complete
charge collection (due to ion recombination); S w,air is the 4.1.2. Nominal focal point dose rate
stopping power ratio of water to air which is 1.136 for
Cobalt-60; P is the perturbation correction factor; P is the The PMMA standard spherical phantom was mounted
cel
u
factor that corrects the response of an ionization chamber on the positioning frame, with its center aligned to the
for the effect of the central electrode during in-phantom positioning reference point. The radiation detector was
measurements in high-energy photon, electron, and then inserted into the spherical phantom at a preset
proton beams. 9,16 position, ensuring that the effective measurement point
aligns with the phantom center. The treatment couch
Due to the very small field sizes of Smart Knife’s 1# and moved the spherical phantom to the preset irradiation
2# collimators, the microDiamond detector (PTW60019) position, after which it was irradiated for 2 min with
was calibrated by the ionization chamber (PTW31010) each collimator sequentially. The dosimeter readings
using the 4# collimator for measurements of small fields. 14 were recorded and the absorbed dose to water at the

3.2. Dosimetric performance test focal point D was calculated. The absorbed dose rate
to water at the focal point was calculated according to
China has national standards for gamma-ray beam SRT Equation VII.
equipment for head and body treatments: YY 0831.1 –
2011 and YY 0831.2 – 2015. The dosimetric performance D  = D (VII)
11
10
of the ZND-A Smart Knife was tested, including the t
nominal absorbed dose rate at the focal point, positioning
reference point deviation, radiation focusing field size, dose where is the absorbed dose rate to water at the focal
gradient, and comprehensive error in dose calculation. The point of this focusing field (unit = Gray per minute,
test items and nominal values are summarized in Table 1. Gy/min); D is the cumulative absorbed dose to water at the
focal point of this focusing field in time t (unit = Gray, Gy);
4. Measurements and results t is the time of irradiation (unit = minute, min).
4.1. Measurements A physicist can use Equation VI to derive the dose rate
to water on the installation date from the dose rate to water
4.1.1. Positioning reference point deviation
on the measurement date.
Gamma beam SRT is a technology used in clinical practice

×
to treat tumors based on isocenter irradiation, where the D  0 = D e 0.693 ×∆ /tT 1/2 (VIII)
Volume 3 Issue 1 (2025) 74 doi: 10.36922/arnm.6280