Close-up of low-power, red argon lasers, directed through 4 fibre-optics waveguides to treat a cancerous tumour in a woman's throat. Unlike other types of laser devices, the treatment does not involve the use of heat energy generated by the laser to remove the growth: the low-power beams activate a drug (HPD - haemoporphyrin derivative, previously injected into the patient). This drug does not affect normal cells, but is strongly absorbed by cancerous ones & the products of the photochemical reaction initiated by the laser are toxic to cancerous cells. The technique is also used on tumours of mouth, oesophagus & lung (with aid of bronchoscope).