*2.5. Determination of Photon Beam Quality Index (TPR20,10—Tissue Phantom Ratio)*

Samples of DSF-based particleboard and solid water phantoms were mounted and aligned on the central axis of the beam, followed by the insertion of the IC into an electrometer (Model PTW-Unidos ET10008/081134) at depths z = 20 cm and z = 10 cm below the water surface at 10 cm × 10 cm field size and 100 cm SSD, as depicted in Figure 2. Before taking any reading, the IC and electrometer were warmed up for 10 min. For each of the phantom samples, three exposures were made at the two depths, and the average charge collected was evaluated. The expression related to the charge collected at the two depths can be expressed as:

$$TPR\_{20,10} = \frac{Q\_{20}}{Q\_{10}}\tag{10}$$

where *Q*<sup>20</sup> and *Q*<sup>10</sup> are the respective charge (nC) collected at depths z = 20 cm and z = 10 cm for DSF-based particleboards, water, and solid water phantoms, respectively.

**Figure 2.** Measurement setup for TPR20,10: (**a**) depths z = 20 cm and (**b**) depth z = 10 cm below the water surface at field size (10 cm × 10 cm) and SSD (100 cm).

#### *2.6. PDD Evaluation Using IC*

Samples of 15 cm thickness were placed on the LINAC couch to establish the photon and electron beams with backscatter. The calibrated IC with an inner volume of 0.6 cm3, connected to the electrometer, was placed in the chamber slot to acquire PDDs in the DSF-based particleboards, water, and solid water phantoms, as displayed in Figure 2. The slabs of solid water were selected for the phantom material, as it was found to be appropriate for dosimetry of high energy photon and electron beams [30]. The IC and phantoms were positioned in an isocentre distance of the LINAC at an SSD of 100 cm using the front pointer device, and the field size was set at 10 cm x 10 cm on the surface in accordance with the calibration parameters in the dosimetry protocol of IAEA TRS-398:2000 [31]. Exposures were rendered using photon beams of 6 and 10 MV and electron beams of 6, 9, 12, and 15 MeV with 100 monitor units (MU). Particleboard slabs were added above the IC to assess the ionization at depth below the surface of the phantoms, and the SSD and field size were subsequently readjusted. During these measurements, both the gantry and collimator angles were set to zero degrees. The PDDs were measured from the phantom surface at 0 cm until a depth of approximately 20 cm was reached along the central axis, with a measurement interval of 1 mm from the surface to 2 cm depth followed by 2.5 cm and then 3 cm up to 25 cm. The PDD determination for each depth took 6 s. Exposure using electron beams was achieved by adopting an applicator to the LINAC treatment head. After each exposure, a time delay of 120 s was applied before the next phantom slab was inserted in order to take proton production into account. The PDD values were expressed as a percentage of the absorbed dose at a given depth D to the absorbed dose at a specified reference depth (maximum depth) D" along the central axis of the phantom samples (Equation (11)). The discrepancy in the calculated PDD was estimated as a percentage (D%), as given in Equation (12):

$$PDD = \frac{\mathbf{D}'}{\mathbf{D}'} \times 100\% \tag{11}$$

$$D(\%) = \frac{PDD\_{(DSF)} - PDD\_{(water/solid\ water)}}{PDD\_{(water/solid\ water)}} \times 100\tag{12}$$

where *PDD*(*DSF*) is the PDD for the constructed DSF-based *R.* spp. particleboard phantom samples and *PDD*(*water*/*solid water*) is the PDD for the water and solid water phantoms. The PDD curves were plotted for 6 and 10 MV photons, as well as for 6, 9, 12, and 15 MeV electrons.

#### *2.7. PDD Evaluation Using Gafchromic EBT3 Radiochromic Films*

The Gafchromic EBT3 radiochromic film sheets (Lot #: 05161903), with dimensions of 20.3 cm × 25.4 cm, were inserted between the DSF-based particleboard phantoms and solid water phantoms in a portrait orientation due to their near tissue- and waterequivalent characteristics, and 10 cm of phantom material was placed under the film to ensure sufficient backscatter. Phantom slabs were inserted above the film, and the SSD and field size were readjusted afterward. The measurements were repeated until a depth of almost 20 cm, and the results were compared with that of water and solid water phantoms. All films were marked with reference points to indicate the film orientations relative to the gantry. Irradiation was made parallel to the beam for a static 10 × 10 cm2 field size at 100 cm SSD and with a dose ranging from 0 to 700 cGy. Three films were exposed for each photon and electron energy. The irradiated films were kept at room temperature for 24 hrs post-irradiation to allow time for the polymerization reactions in the film to stabilize and produce a stable optical density measurement [32]. The films were then processed with an EPSON Expression 10,000 XL flatbed scanner. To acquire images, a desktop computer was interfaced with the scanner, and VeriSoft® software 5.1 was used for image scanning and capture. The experimental setup for the PDD evaluation is highlighted in Figure 3. The PDD data were normalized to the maximum dose, expressed as a percentage, and the percentage variation was measured, as indicated in Equations (11) and (12).

**Figure 3.** Experimental setup used for PDD evaluation.

#### **3. Results and Discussion**
