**1. Introduction**

Phantoms have become essential for quality assurance (QA) and quality control (QC) in a variety of medical procedures involving radiation. The earliest phantoms consisted of water or wax, but wax phantoms had a number of issues. Wax formulations differed greatly depending on the type of wax used and, at low energies, deviated from tissue equivalence [1]. On the other hand, water has been described as the standard and most universal phantom material for dosimetry measurements of photon and electron beams. As the use of liquid water can prove to be challenging and inconvenient in certain situations, because of its surface tension and the uncertainty in positioning the detector near the surface, solid homogeneous phantom materials have achieved substantial recognition [2]. The benefit of these phantoms allows the measurement of the interaction of ionizing radiation in the human body, which enables the range of doses in various organs and tissues to be measured according to their sensitivity. The most widely used tissue-equivalent material (TEM) are those that are both easy to work with and relatively inexpensive. The usage of natural, readily available, and cheap phantom material, such as wood (*Rhizophora* spp.), is often of interest.

**Citation:** Samson, D.O.; Shukri, A.; Hashikin, N.A.A.; Zuber, S.H.; Aziz, M.Z.A.; Hashim, R.; Yusof, M.F.M.; Rabaiee, N.A.; Gemanam, S.J. Dosimetric Characterization of DSF/NaOH/IA-PAE/*R.* spp. Phantom Material for Radiation Therapy. *Polymers* **2023**, *15*, 244. https://doi.org/10.3390/ polym15010244

Academic Editor: Edina Rusen

Received: 17 October 2022 Revised: 15 November 2022 Accepted: 17 November 2022 Published: 3 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

*Rhizophora* spp. (*R.* spp.) has received increasing attention for industrial applications due to its fast-growing nature, high productivity, quick maturity, and high strength, with advancement in processing technology and increased market demand. The chemical composition of *R.* spp. is very similar to those of TEM normally utilized as phantoms for radiation therapy when compared with other wood species [3–8]. Moreover, *R.* spp. possesses convenient morphological characteristics and physiological adaptations, with moisture content ranging from 5–10% and basic physical density within 0.90–1.04 gcm−<sup>3</sup> [9]. Various researchers have shown that *R.* spp. is a highly attractive material for use as an effective TEM for a wide range of benefits, including high-energy photon and electron radiation therapy, as well as X-ray imaging [5–7,10–12]. However, due to its shortcomings, such as the tendency to be warped, cracked, degraded, and weakened over time, the usage of appropriate resins with unique characteristics in the development of *R.* spp. particleboards has been reported [5–8,11,12]. The type of these curing resins and their chemical properties are also crucial criteria that should be well-decided for particleboard phantom formation and structure.

Among the various forms of modifying resins, synthetically-based ones are the most commonly adopted [13], but prolonged human exposure to non-renewable resources has been shown to cause chronic toxicity, myeloid leukemia mortality, and lymphohematopoietic malignancies [14–16]. In relation to TEM studies, synthetically based resins were also found not to be compatible with the intended density and radiation attenuation properties (RAPs) of *R.* spp. particleboards as compared to water [17]. On the other hand, bio-based materials, such as soy protein (DSF—defatted soy flour) developed in wood resin, have been validated through specific independent studies because of their ready availability and low cost, coupled with the fact that they are biodegradable, biocompatible, and eco-friendly [6,7,15,16,18–20].

DSF is a highly oxygenated carbon compound, which makes it attractive for use in the development of phantom materials equivalent to tissue and water, and it can be appropriate either as an uncured or a cured bio-based adhesive [18]. Uncured DSF was, however, identified as a weak adhesive, and a chemical change is needed to break the internal bonds and disperse the polar protein molecules [6,16,18–24]. The most commonly used cross-linking agents for DSF are itaconic acid polyamidoamine-epichlorohydrin (IA-PAE), epoxy, formaldehyde, glutaraldehyde, and glyoxal. Since some of these curing agents also have a deleterious environmental impact, as well as being non-renewable, IA-PAE has been considered as an alternative cross-linker for DSF-based adhesives [6,18–20,24]. The cross-linking reaction of DSF with NaOH/IA-PAE resins is highly regarded for their incomparable multifunctionality, enhanced physical and mechanical characteristics, stable water resistance, and good wood-bonding ability [6].

The current study aims to construct and examine the dosimetric characterization of biobased particleboard phantoms for radiation therapy by integrating DSF-based resins—*R.* spp. particles of size ≤74 μm, NaOH (10 wt%)—and four different treatment levels of IA-PAE (0, 5, 10, and 15 wt%). High energy photon attenuation measurements were ascertained using a Ludlum setup with 137Cs and 60Co sources with effective photon energies of 0.662 and 1.250 MeV. A linear accelerator (LINAC) was utilized to determine the dosimetric characteristics of the DSF/NaOH/IA-PAE/*R.* spp. particleboard phantoms. This is done using a cylindrical Farmer-type ionization chamber (IC) (NE 2581/334) and Gafchromic EBT3 radiochromic films to evaluate the tissue–phantom ratio (TPR20,10), percentage depth dose (PDD) and beam profile of the samples for high energy photon (6 and 10 MV) and electron (6, 9, 12 and 15 MeV) beams. The findings were compared with those of appropriate standard phantom materials (water and solid water) utilized in radiation therapy.

#### **2. Materials and Methods**

#### *2.1. Preparation of Bio-Based Adhesives*

As previously reported [6,7], the synthesized IA-PAE solution had a solid content of 55.96 ± 0.01 wt%, a pH of 6.68 at 27.58 ◦C, and an apparent viscosity of 100.40 ± 0.25 mPa.s, comparable to commercial PAE-soy protein (C-PAE-SF) and IA-PAE reported by Gui et al. [19]. The DSF-based bio-adhesives were prepared at room temperature by dissolving 35 g of DSF under steady mechanical stirring in distilled water (65, 50, 45, and 40 g) for 0.5 h following the procedure described by Samson et al. [6]. Various concentrations of the prepared IA-PAE (0, 5, 10 and 15 wt%) were then applied to the uniform mixtures and moderately stirred for 0.5 h. The cured DSF/IA-PAE slurry mixture was maintained at pH 11.0 with 2N of NaOH (10 wt%) solution, since pH 11.0 is the optimum condition for cross-linking reactions [25].
