**4. Novel nonwoven fabric bioabsorbable spacer for particle radiotherapy**

The purpose to produce a bioabsorbable nonwoven fabric spacer is to overcome problems associated with nonabsorbable GORE-TEX spacer [19]. The nonabsorbable GORE-TEX spacer might cause serious complications after the completion of particle therapy. On the other hand, although previous investigators have reported usefulness of gel spacers for separation of prostate and rectum [20, 21], those spacers are inappropriate for the upper abdomen, which contains lots of free space. Therefore, at present, a nonwoven fabric bioabsorbable spacer is necessary and appropriate for the separation of tumor and adjacent organs of upper abdomen malignancies.

The process for producing the nonwoven fabric involves entangling threads in 3 dimensions with a needle-punching process and other methods [22, 23]. Spacer placement during radio‐ therapy is a promising method designed to allow increased tumor dose while limiting radiation exposure to adjacent organs. The spacer exhibits excellent properties related to bioabsorbability, biocompatibility, thickness retention, and water equivalency according to physical and animal experiments (Figure 2). The reason and the advantage for the use of PGA to construct this nonwoven fabric spacer are that PGA is one of the most widely studied polymers and has excellent mechanical properties and biological affinity [24, 25]. Historically, PGA has played a central role in surgery since its development as the first synthetic absorbable

suture material in 1962 [26]. The PGA is absorbed in 60–90 days after insertion in the body. It is hydrolyzed without any phagocytosis, which results in a weaker immune response than that of absorbable organic sutures [27]. The degradation of PGA generally involves random hydrolysis of their ester bonds. Under physiologic conditions, PGA is also degraded by certain enzymes, especially those with esterase activity [28, 29]. The attractiveness of PGA as a biodegradable polymer in medical application is that its degradation product glycolic acid is a natural metabolite [29]. The glycolic acid is nontoxic and can enter the tricarboxylic acid cycle, after which it is excreted as water and carbon dioxide. Part of the glycolic acid is also excreted in urine [26, 28].

As for PGA sutures, Chu et al. have reported a simple degradation mechanism via homoge‐ neous erosion [30–32]. The degradation process occurs in two stages, the first involves the diffusion of water into the amorphous regions of the matrix and simple hydrolytic chain scission of the ester groups. The second stage of degradation involves largely the crystalline areas of the polymer, which becomes predominant when the majority of the amorphous regions have been eroded. It is important to note that PGA nonwoven fabric spacer is com‐ prised of the PGA suture mainly with a 3-dimensional needle-punching process. Therefore, the degradation mechanism of the content of the PGA spacer could be same as PGA sutures. However, it is possible that because the volume of PGA polymers in the PGA spacer is quite large, inflammatory biological responses might be different from that of relatively small volume of PGA suture (Figure 3).

**Figure 1.** A case of pancreatic body cancer in which a GORE-TEX spacer was placed before the application of protonbeam therapy. The left panels show perisurgical images during the placement of the GORE-TEX spacer. The right pan‐ els show computed tomography images, including tumors, liver, adjacent organs, and the GORE-TEX spacer (arrows)

**4. Novel nonwoven fabric bioabsorbable spacer for particle radiotherapy**

The purpose to produce a bioabsorbable nonwoven fabric spacer is to overcome problems associated with nonabsorbable GORE-TEX spacer [19]. The nonabsorbable GORE-TEX spacer might cause serious complications after the completion of particle therapy. On the other hand, although previous investigators have reported usefulness of gel spacers for separation of prostate and rectum [20, 21], those spacers are inappropriate for the upper abdomen, which contains lots of free space. Therefore, at present, a nonwoven fabric bioabsorbable spacer is necessary and appropriate for the separation of tumor and adjacent organs of upper abdomen

The process for producing the nonwoven fabric involves entangling threads in 3 dimensions with a needle-punching process and other methods [22, 23]. Spacer placement during radio‐ therapy is a promising method designed to allow increased tumor dose while limiting radiation exposure to adjacent organs. The spacer exhibits excellent properties related to bioabsorbability, biocompatibility, thickness retention, and water equivalency according to physical and animal experiments (Figure 2). The reason and the advantage for the use of PGA to construct this nonwoven fabric spacer are that PGA is one of the most widely studied polymers and has excellent mechanical properties and biological affinity [24, 25]. Historically, PGA has played a central role in surgery since its development as the first synthetic absorbable

after the surgery.

208 Non-woven Fabrics

malignancies.

**Figure 2.** Macroscopic features of a novel nonwoven-fabric bioabsorbable spacer composed of surgical polyglycolic acid (PGA) sutures in the rat abdomen. The left panels show images of formalin-fixed specimens showing the abdomi‐ nal wall, implanted PGA spacer, and adjacent intestines. The right panels show images of the same specimens stained with hematoxylin and eosin (HE). These images indicate that the PGA spacer was absorbed gradually.

**Figure 3.** Microscopic features of the PGA bioabsorbable spacer implanted in the rat abdomen. In the surface of the PGA spacer, inflammatory response was observed. There were degraded PGA fibers, lymphocyte, and macrophage observed in the surface. Interestingly, very few cell infiltrations were observed in the body of the PGA spacer

Although there is no clear definition of a bioabsorbable nonwoven fabric spacer reported, an optimal bioabsorbable spacer must have thickness-retaining and water-equivalency properties according to treatment duration [19]. In particle therapies, because treatment protocols take 2–7 weeks [5, 33], a spacer that maintains its thickness for the duration of or slightly longer than the treatment period seems to be ideal. From our investigation, the percentage of thickness retention of the PGA spacer varies according to the concentration of the PGA sutures. The PGA spacer product with 0.2 g/cm3 of PGA sutures retains more than 90% thickness for 8 weeks. Therefore, this PGA spacer could be applied in various protocols in particle therapy. The bioabsorbable PGA spacer might become a useful device and expand indications for particle therapy.
