1. A method for probing a tumor of a patient using positively charged particles, comprising the steps of: sequentially delivering from an output nozzle, connected to a first beam transport line, to the patient: a first set of the positively charged particles comprising a first mean energy; and a second set of the positively charged particles comprising a second mean energy, the second mean energy at least two megaelectronVolts different from the first mean energy; after transmission through the patient, sequentially detecting: a first residual energy of the first set of the positively charged particles; and a second residual energy of the second set of the positively charged particles; and determining a water equivalent thickness of a probed path of the patient using the first residual energy and the second residual energy.