Traceability of absorbed dose in radiotherapy: from primary standard to patient treatment

摘要

Treatment of patients affected by cancer is performed by different techniques that can be used jointly or independently: surgery, chemotherapy and radiotherapy (external or internal). In France, about 185 000 patients were treated in 2006 by external radiotherapy (in about 170 centers). Radiotherapy treatment unit is mainly based on linear accelerators (LINAC) able to produce either photons or electrons of different energies (from a few MeV to about 25 MeV). Very few treatments are still delivered by means of Co-60 sources. Standard quantity of radiation delivered to the patient is defined as "absorbed dose". The purpose of the traceability is to obtain the lowest achievable uncertainty and the correct absorbed dose to properly treat the patient. To do that, many steps are required from the primary standard to the final absorbed dose to the patient. LNE-LNHB (Laboratoire National Henri Becquerel), the French primary standard laboratory for ionising radiation (located near Paris) defines and maintains the primary standards of absorbed dose to water and transfers it to the end user (medical physicist). This paper presents four main steps from the establishment of the primary standard to the absorbed dose delivered to the patient: - establishment of the reference absorbed dose to water from the primary standard (graphite calorimeter associated with calculations and chemical dosimetry), - transfer to the end user by means of a transfer instrument (vented ionization chamber), - calibration of the beam of the LINAC for the treatment, - calculation of the ballistic of the treatment and the duration of the irradiation. This step is performed by a software called TPS (Treatment Planning System). More emphasis has been put on the work of the LNE-LNHB (first and second steps).