Three-dimensional conformal radiation therapy (3D-CRT) gives more certainty about organ position, so higher doses can be delivered to the target volume without adjacent organs receiving excessive radiation.
The target volume and adjacent organs are outlined by the clinical oncologist on a planning CT scan. Margins are added to allow for organ movement and daily setup error. The physics team then uses software to add radiation beams and calculate the dose each organ will absorb. The beams are altered to achieve optimal dose distribution.
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The linear accelerator has inbuilt shielding (multileaf collimators) that shapes the beam and treatment is delivered from three or four successive positions (treatment fields), all directed at the prostate.
IMRT is an extension of 3D-CRT but uses more treatment fields, within which the multileaf collimators can adopt different configurations, often moving while the beam is on, thus varying the dose intensity. Tomotherapy uses a type of linear accelerator that produces a narrow fan beam and rotates 360 degs around the patient while the beam is on. Tens of thousands of beamlets ‘intensity modify’ the dose.
Both types of IMRT involve complicated, time-consuming planning and rigorous quality assurance, but have the advantage of sparing more normal tissue, particularly the rectum, from the maximum dose. This allows a higher dose to be given to the prostate, or the treatment of larger volumes.
The target volume moves a little from day to day in relation to changes in bladder and bowel filling. Image-guided radiotherapy attempts to circumvent this by daily monitoring of the target volume position with scanning of implanted gold prostatic markers. Whether this improves outcomes in terms of disease control and reduced morbidity is not yet known.
Dose calculation is critical and is measured in Grays (Gy). 1Gy is equivalent to one joule of radiation energy absorbed per kilogram of organ or tissue weight. The rad is the old unit of absorbed dose, and 1Gy is equivalent to 100 rads.
The administration of doses above 72Gy in 2Gy daily fractions leads to improved cure rates4 and NICE recommends patients receive a minimum of 74Gy in 2Gy fractions.1 Future dose escalation is likely.
The Conventional or Hypofractionated High Dose Intensity Modulated Radiotherapy for Prostate Cancer (CHHiP) trial, funded by Cancer Research UK, is in the process of evaluating dose escalation by delivering larger doses per fraction in fewer total fractions. This technique is based on research that suggests prostate cancer may be more sensitive to larger doses per fraction.
The practicalities for organising treatment involve the patient attending a planning CT scan (takes one hour), usually performed supine with a full bladder to displace the small bowel and bladder from the treatment field. Tattoos are then made for accurate on-treatment setup. Treatment is planned as described above. This process takes two to three weeks. Before treatment commences, the patient undergoes verification (a dummy run) to ensure the setup is correct. This takes about one hour.
Treatment begins shortly afterwards and is given each weekday for seven to eight weeks, with each session taking five to 10 minutes to set up and five minutes to treat. The patient sees the oncologist weekly throughout the treatment to monitor side-effects. Once treatment starts, it should not be interrupted, because unplanned gaps significantly reduce efficacy.
Radical brachytherapy
Low-risk patients are most suitable for brachytherapy, although intermediate-risk patients are increasingly being treated, owing to new developments in technique. Derived from the Greek root meaning ‘close proximity’, brachytherapy involves the implantation within the prostate of radioisotope sources emitting gamma rays.
Low dose rate (LDR) brachytherapy is the most established method. Seventy to 150 iodine-125 seeds are inserted under ultrasound guidance into the prostate. These deposit an intense dose of radiation (more than 145Gy) in the prostate with a steep dose fall-off at the capsule edge. This minimises the dose to adjacent structures. Computer-assisted dosimetry software generates a plan for the optimal placement of seeds that is specifically designed for each patient’s anatomy and is used to verify accurate insertion. The specified radiation dose is emitted over a period of four to 10 months and the seeds remain in place permanently. The iodine-125 has a half-life of about 60 days. High dose rate (HDR) brachytherapy involves a similar process–radioisotopes with greater activity are temporarily positioned in the prostate before removal. HDR is usually used as a boost after EBRT, rather than as a monotherapy.
