Page 26 - ARNM-1-1

Advances in Radiotherapy
& Nuclear Medicine Mathematic modeling of PDD for FF and FFF in photon

the depth of dose maximum) along the central axis of All exposed films of depth dose curve were then
the beam. scanned with an Epson Expression 11000XL scanner in the
(iv). S for radiation beams of two photon energies of 6 and 48-bit RGB mode (16 bits/color), and the data were saved
c
10 MV in FF and FFF were measured [12,13] . as tagged image file format and analyzed by the VeriSoft
(v). High energy photon PDD curves in FF and FFF of two imaging processing software. A red filter was placed on
energies were modeled by the buildup-tail function top of the Gaf EBT3 films before scanning to increase the
generated in this study. slope of the H-D curve, thereby raising the resolution of
(vi). The PDD in FF and FFF beams of two energies was the dose-OD curves .
[16]
modeled by the buildup-tail function by adjusting the The depth dose curve derived from the ionization
parameters n and μ to get the best fitting. depth curve from the parallel-plate chamber for FF and
(vii). The S in FF and FFF beams of two energies of 6 and FFF beams of two energies 6 MV and 10 MV was then
c
10 MV was expressed by the modeling parameters n compared with the depth dose curves measured by Gaf
and μ.
The details of each step are described in the following EBT3 films.
sections. Absolute output and machine quality assurance were
performed before conducting the measurements of percent
2.2. PPD measurement ionization depth by the parallel-plate chamber as well as
PDDs in FF and FFF beams of two photon energies of PDD curve by Gaf EBT3 films.
6 and 10 MV were acquired with a PTW MP3-T Water 2.4. Measurement of collimator scatter factor, S
Phantom (PTW-Freiburg GmbH) at Wuwei Heavy Ion c
Center, Wuwei Cancer Hospital, Gansu, China (WHICH). Collimator scatter factors, S , for the radiation beams
c
PDDs were measured with a PTW Semiflex parallel-plate in FF and FFF of two energies of 6 and 10 MV were
ionization chamber (type 31010, volume 0.125 cm ). measured using PTW Semiflex parallel-plate ionization
3
For the acquisition of PDDs, the chamber position chamber (type 31010, PTW-Freiburg GmbH), coupled
was automatically corrected to the effective point of to either a PTW UNIDOS or a Scanditronix-Wellhofer
measurement. The measurements of PDD for two photon Dose1 (IBA Dosimetry, Germany) electrometer. The
energies were performed at 100 cm SSD in the sequence chamber and phantom fulfill the suitability criteria for
of square field sizes of 5 × 5, 10 × 10, 15 × 15, 20 × 20, this type of measurement according to recommendations
[2]
and 40 × 40 cm . PDDs were normalized to 100% at d max by the AAPM TG-74 report . The chamber was placed
2
3
depth. Since the parallel-plate chamber has a small plate in an aluminum (ρ = 2.7 g/cm ) mini phantom at 3.9 cm
separation, it is explicit that the point of measurement is above the chamber, a measurement depth beyond d max
the front surface of the cavity. The depth curve measured equivalent to a depth of 10 cm water for sufficiently
by the parallel-plate chamber was then compared with the avoiding contaminant electrons. The phantom and
PPD curve measured by films. chamber axes were vertically aligned to the beam central
axis, and the chamber reference point was set at 100 cm
2.3. The comparison of PPD converted through source-to-chamber distance. All measurements were
parallel-plate chamber ionization curve with normalized to the reference field reading of 10 ×10 cm .
2
Gafchromic EBT3 film S values were measured for a selection of square fields
c
The PPD measurements were made using Gafchromic from 3 × 3 to 40 × 40 cm with the chamber in mini
2
EBT3 (Gaf EBT3) chromic films (Ashland Specialty phantom.
Ingredients GP, NJ USA; Lot # 04022001) and compared 2.5. PPD numerical equation
with the PPD measured by plane-parallel ion chamber.
The film processing and dose profile measurements There are two home-generated numerical equations
followed international protocols . A pre-exposure describing the PPD curves of high energy photon beam,
[14]
technique was used for the calibration curve derivation buildup function, and tail function. The buildup function
[14]
by giving each film a priming dose of 2 Gy to homogenize was a quadratic function in the form of d with two
the film density using WHICH facilities with a dose of d + n
2
1 Gy at the photon energy of 6 MV. We then measured the main parameters of d (depth in water) and n, while the tail
dose homogeneity using a densitometer. Graded doses of function was in the form of e −µ d and composed of an
5, 10, 15, 40, 60, 80, 100, 150, and 200 cGy were given to exponential function with main parameters of d and μ. The
the Gaf EBT3 chromic film to obtain the Hurter-Driffield modeled PDD was the product of the buildup-tail function,
calibration curve (H-D curve) . as in Equation I.
[15]

Volume 1 Issue 1 (2023) 3 https://doi.org/10.36922/arnm.0314