code: public PdfAppearance getAppearanceRadioCircle(boolean on) {
          PdfAppearance app = PdfAppearance.createAppearance(writer, box.getWidth(), box.getHeight());
          switch (rotation) {
              case 90:
                  app.setMatrix(0, 1, -1, 0, box.getHeight(), 0);
                  break;
              case 180:
                  app.setMatrix(-1, 0, 0, -1, box.getWidth(), box.getHeight());
                  break;
              case 270:
                  app.setMatrix(0, -1, 1, 0, 0, box.getWidth());
                  break;
          }
          Rectangle box = new Rectangle(app.getBoundingBox());
          float cx = box.getWidth() / 2;
          float cy = box.getHeight() / 2;
          float r = (Math.min(box.getWidth(), box.getHeight()) - borderWidth) / 2;
          if (r <= 0)
              return app;
          if (backgroundColor != null) {
              app.setColorFill(backgroundColor);
              app.circle(cx, cy, r + borderWidth / 2);
              app.fill();
          }
          if (borderWidth > 0 && borderColor != null) {
              app.setLineWidth(borderWidth);
              app.setColorStroke(borderColor);
              app.circle(cx, cy, r);
              app.stroke();
          }
          if (on) {
              if (textColor == null)
                  app.resetGrayFill();
              else
                  app.setColorFill(textColor);
              app.circle(cx, cy, r / 2);
              app.fill();
          }
          return app;
      }
docs: Gets the special field appearance for the radio circle.
  @param on <CODE>true</CODE> for the checked state, <CODE>false</CODE>
  otherwise
  @return the appearance