creating a 3d sphere (as a mask) on a 3D image

I am using a 3D raw image (size of PET image(float) is [84*71*103] with spacing size of 4.07*4.07*3 mm). what I want to do is, using the voxel coordinates that correspond to center of the tumor(defined by me) [116,177,86], create a sphere in which the center of a sphere will be the same as the center of the tumor. The diameter of the sphere I want to change to a maximum of a little bigger than the tumor itself. So the sphere is basically going to be serving as a mask which is overlaid with the image and I can get the statistics of the values of the image within that spherical shape. I hope I have made it clear enough. I would really appreciate your help.My code is not working correctly, I am very confused in how to proceed. {

fid_1 = fopen('I:\PatientData\PET1_DIFF.img','r','l');
pet1_diff = fread(fid_1,'float32');
pet1_diff = reshape(pet1_diff, [84 71 103]);
pet1_diff = imrotate(pet1_diff,90,'nearest','crop');
figure(1);
imagesc(pet1_diff(:,:,30)); title('PET1_DIFF');
fclose(fid_1);

imageSizeX = 84;
imageSizeY = 71;
imageSizeZ = 103;
[columnsInImage rowsInImage pagesInImage] = meshgrid(1:imageSizeX,             1:imageSizeY, 1:imageSizeZ);
%creating the sphere in the image.coordinates of sphere center(tumor center)
centerX = 116;
centerY = 177;
centerZ = 86;
radius = 5;
sphereVoxels = (rowsInImage - centerY).^2 ...
+ (columnsInImage - centerX).^2 + (pagesInImage - centerZ).^2 <=    radius.^2;
% sphereVoxels is a 3D "logical" array.
% Now, display it using an isosurface and a patch
fv = isosurface(sphereVoxels,0);
patch(fv,'FaceColor',[0 0 .7],'EdgeColor',[0 0 1]);
view(45,45);
axis equal;
title('Binary volume of a sphere')
}