Page 80 - ARNM-3-1

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

2. The principle and dose calculation model
of ZND-A Smart Knife

2.1. Principle and components
The ZND-A Smart Knife is designed for SRT of solid
tumors in the head and body as well as lesions that require
radiosurgery. The equipment is shown in Figure 1.
ZND-A Smart Knife utilizes Cobalt-60 radioactive
sources arranged in multiple concentric circles. By utilizing
triple rotating and focusing of collimator, gamma-ray
module, and drum, the system focuses 1.25 MeV average
energy gamma rays onto the tumor or other pathological Figure 1. Gamma beam stereotactic radiotherapy system
tissues. The therapeutic goal is achieved by leveraging both
the damaging and inhibitory effects of gamma rays on circular pattern and installed in the gamma-ray module.
tumors as well as the inherent radiosensitivity difference The gamma rays produced by the radioactive sources are
between tumor and healthy cells. Medical physicists focused for the 1 time by the collimator. The gamma-ray
st
make treatment plans based on tumor radiobiological module rotates around its own rotational axis. Due to the
principles, ensuring that normal tissue doses remain angular offset between the collimator axis and the module’s
within acceptable levels. The Smart Knife’s control system rotational axis, a secondary focus is formed during
precisely positions the patient’s tumor or other pathological rotation. As the module follows the rotating drum around
tissue at the gamma-ray focal point based on the treatment drum’s rotational axis, a tertiary focus is generated within
plan made by Treatment Planning System (TPS). During the drum. Through this three-stage gamma-ray focusing,
repositioning, the stereotactic positioning system, guided the equipment achieves a high focal-to-skin dose ratio and
by the image-guided system, ensures accurate realignment a steep dose gradient. 15
of the target. The radioactive sources deliver planned
conformal irradiation at the target point. This integrated 2.2. Dose calculation model
workflow ensures that tumors or other pathological tissues The ZND-A Smart Knife uses isocentric irradiation, and
are precisely treated while sparing healthy structures. 3 the dose calculation is based on the Tissue Maximum
With the triple rotating and focusing technology, the Ratio (TMR) and Off-Axis Ratio (OAR) dose calculation
maximum energy is deposited within the lesion region, model. The TMR and OAR are parameters decided by the
while minimizing radiation exposure to normal tissue collimator, source-to-axis distance, and machine-specific
along the beam path. parameters in an actual installed machine. Thus, the TMR
and OAR data have to be measured on the actual machine
In the interaction between gamma-ray and lesion, the in the commissioning process. Then, the measured TMR
relationship between the intensity of gamma rays and and OAR baseline data would be imported to TPS for the
the depth of the medium is represented by the following TMR-OAR dose calculation model.
equation:
The coordinates of the dose calculation model are
I t = Ie − d (I) shown in Figure 2. The dose D p(X, Y, Z) of point P(X, Y, Z)
0
in the radiation field of ZND-A Smart Knife is represented
where I and I are the intensity of gamma rays before by the following equation :
4
t
0
and after attenuation by medium, μ is the mass attenuation
(
,,
(00
Z
coefficient of the absorber medium, and d is the depth of DX YZ,, ) = D (00 Z )∆ TMR ,, )
m
m
measurement in medium (path length in medium).  Z  2
(
Z
(0
∆  f + + Z ∆ OAR ,, )∆ OAAR X,,0 ) (II)
YZ
As demonstrated by Equation Ⅰ, the intensity of gamma  f m  
rays gradually decreases with increasing penetration
depth, depositing energy in the passing-through medium. where D (0,0,Z ) is the dose at the maximum dose
m
m
Therefore, to deposit the maximum energy (dose) in the point depth on the central beam axis; TMR (0,0,Z) is the
lesion region while minimizing radiation exposure to ratio of the dose at depth Z on the central beam axis in
normal tissue along the beam path, the ZND-A Smart phantom to the dose at the same source-point distance
Knife radioactive sources are arranged in a concentric and the reference depth of the maximum dose (Z ); OAR
m
Volume 3 Issue 1 (2025) 72 doi: 10.36922/arnm.6280