When a solid tumor is formed and coupled to the blood vessel system (carcinoma in-situ), it starts to effuse tumor cells in the blood stream. Although lymph nodes acts as “filters”, it is proven that some cancer cells are able to pass through the lymph nodes. The lifetime of these cells in the blood is often very short, but it also can be very long (many years). It was found that patients with breast cancer have > 1000 cancer cells circulating in the bloodstream. (Silverstein M.J et al 1996) The basic idea of the PDS blood screening method is to control the blood with respect to the concentration of free circulating cancer cells. If the blood can be kept cancer cell-free, the probability of metastasis formation will be strongly reduced. Metastasis is the leading reason for the resultant mortality of patients with cancer. Cancer cells detected by the PDS screening can be easily removed by Chemotherapy or Photodynamic Therapy. This removal process again can be controlled by PDS. Consequently, the cancer patients can overcome the psychological uncertainty about their lifetime prognosis after primary surgery and chemotherapy.
The prerequisite for interpretation of the PDS blood screening spectra is the difference between the reference-spectrum and the verum-spectrum as result of the digital subtraction. A misinterpretation can only occur, when the reference-spectrum contains the emission of a cancer cell. In this case, no additional photon bursts would be detected, the result of the blood screening could be too positive and probably wrong. The probability that the reference-spectrum contains the photodynamic emission of a cancer cell depends on the concentration of cancer cells and can be estimated. When one single cancer cell is contained in the whole 5l blood, this cell will pass the laser spot during the verum screening period one times, because in 1000s the whole blood volume has passed the laser spot one times. The reference-spectrum is performed in 1 millisecond, the verum spectrum is measured in 1000s. Therefore, the probability that a cancer cell is passing the laser spot during the 1 ms reference-screening time is 1 ms /1000 s = 10 -6
The probability that a cancer cell emission is recorded during the reference-scan therefore is 1 to 1 million. Despite the very low probability, the PDS-verum scan is always repeated two times so that a total screening time is of 2000 s. That means that the probability of misinterpretation is further reduced to 1:2 Mill., a value which can be neglected for the clinical applications of the PDS-blood screening.