Radiotherapy is the use of ionizing radiation for the treatment of cancer and 50 – 60 % of patients require radiotherapy at some point during their treatment[1]. It is delivered in the form of external beam radiotherapy (EBRT) using linear accelerators (linacs), or internally, known as brachytherapy (BT).
Brachytherapy is further divided into Low Dose Rate (LDR), where the radioactive sources, known as seeds, remain implanted permanently, and High Dose Rate (HDR), where higher activity radiation seeds, with dose rates > 12Gy/hr, are temporarily implanted.
- In the case of LDR-BT (see figure 1a), these seeds are usually Iodine-125, with apparent activities of the individual seeds typically between 0.357mCi and 0.42mCi respectively, and have a half-life of 59.43 days.
- In HDR-BT (see figure 1b), a computer driven device, known as a remote afterloader, pushes the radioactive source, typically Iridium-192 or Cobalt-60, which is attached to a wire, through the applicators to the treatment site. The radioactive source is moved in millimetre increments and remains in a certain position (dwell position) for a predetermined amount of time. Once the planned treatment dose has been achieved, the remote afterloader automatically withdraws the radioactive source so there is no residual radiation or radioactivity in the patient. Each treatment typically takes 10 – 15 minutes and is delivered in a number of sessions, known as fractions. In cervical cancer for example, a patient would typically receive 1 – 10 fractions of 3 – 20 Gy depending on the indication.
[1] Rosenblatt, E., Acuña, O., & Abdel-Wahab, M. (2015). The Challenge of Global Radiation Therapy: An IAEA Perspective. International Journal of Radiation Oncology• Biology• Physics, 91(4), 687-689.