Question Placement of a 3d texture

Hi everyone, I am new here so I hope to write a question in the right place.
I am programming an ecography simulator and I have an interface that allow the user to place a 3D object of possible internal organs inside a human body.
The rendering of the object is done using a shader over the object and a 3D texture.
This is the shader code (if can be useful)

Code (CSharp):

1. Shader "Custom/Texture3D"
2. {
3.     Properties
4.     {
5.         _MainTex ("Albedo (RGB)", 3D) = "" {}
6.     }
7.     SubShader
8.     {
9.         Tags { "RenderType"="Opaque" }
10.         LOD 200
11.  
12.         Pass{
13.             CGPROGRAM
14.  
15.             #pragma vertex vert
16.             #pragma fragment frag
17.             #include "UnityCG.cginc"
18.             #pragma target 3.0
19.  
20.             sampler3D _MainTex;
21.  
22.             struct vertInput
23.             {
24.                 float4 pos : POSITION;
25.                 float3 uv : TEXCOORD0;
26.             };
27.  
28.             struct v2f
29.             {
30.                 float4 pos : SV_POSITION;
31.                 float4 pos_w : TEXCOORD1;
32.                 float3 uv : TEXCOORD0;
33.             };
34.  
35.             v2f vert (vertInput input)
36.             {
37.                 v2f o;
38.                 o.pos_w = mul(unity_ObjectToWorld, input.pos) ;
39.                 o.pos = UnityObjectToClipPos(input.pos);
40.                 o.uv = input.uv;
41.                 return o;
42.             }
43.  
44.             half4 frag(v2f i) : SV_Target
45.             {
46.                 half4 mainColor = tex3D(_MainTex, float3(i.pos_w.x, i.pos_w.y, i.pos_w.z));
47.                 return mainColor;
48.             }
49.            
50.             ENDCG
51.         }
52.     }
53.     FallBack "Diffuse"
54. }

It seems to work well, the problem is that using a shader the 3D texture is always placed in the origin (0,0,0).
Is there a method to assign new world coordinates to the texture position?
Any kind of alternative solutions are wisely accepted, thanks