The present invention provides a kit for immobilization of a patient body part for radiotherapy applications. The kit includes two moldable thermoplastic sheets, a lower sheet for covering the anatomical contours of a first area of the body part and an upper sheet for covering the anatomical contours of a second area of the body part which is not covered by the lower sheet. The upper and lower sheets form a double shell mask enclosing the body part. The sheets may be connected and retained by a fixation device. The sheets have different physical and visual properties.
