Therapeutic Radiation Technology: 30-year technology update

Recently, I began study of therapeutic radiation technology used to treat cancer with external radiation.  This was a deep update, since the last time I got into the details of X-ray technology was 25 to 30 years ago.  Some of my concerns were based on spatial and spectral resolution of X-ray production and I was delighted to find that several fundamental improvements had been made that allowed fine tuning of both spatial and spectral resolution of the X-rays used for radiation therapy.  This, combined with computer control and sophisticated 3-D simulation of radiation deposition, make the current generation of this technology highly tunable for treating many forms of cancer, particularly brain tumors.  Challenges still remain for treating other forms of cancer, for example, lung tumors.

Long ago, one of the challenges of X-ray technology was the rise-time of the high voltage circuits that drove the X-ray tube.  A slow rise time meant that the harder X-rays were accompanied by soft X-rays during the rise and fall of the voltage.  Different tricks were used 30 years ago to minimize this effect, however, these soft X-rays resulted in poorer resolution of X-ray imaging and lower spatial resolution of therapeutic X-rays.  Spatial resolution is important, because the objective of radiation therapy is to put radiation into the tumor and to avoid irradiating radiosensitive structures near or behind the tumors.  A rotating gantry spreads this collateral radiation around the body, however, the better the spatial resolution, the more manageable the collateral damage becomes.

Current generation therapeutic X-rays machines don’t use transformers to switch the X-rays on and off, but rather, electrons are generated in a linear accelerator, then velocity selected in a magnetic field before they are allowed to hit a water cooled tungsten target.  The Full Width at Half Max (FWHM) of a beam of 6 MeV electrons is then engineered to be small compared to the FWHM of the bremsstrahlung mechanism that generates the X-rays.  This ensures maximum spatial and spectral resolution with suppression of soft X-radiation by orders of magnitude.  The beam of X-rays produced is then further columnated and shaped by dynamic baffles to achieve precise control of the radiation contours within the body.

Other radiation technologies, such as proton beam therapy, that have advantages for certain types of cancer, for example, cancers of the spinal cord.  Unlike X-rays, protons do not irradiate tissue behind the tumor.  For distributed brain tumors, however, proton beams have few advantages over X-rays.  Brain tumors are collocated and interstitial with healthy tissue and require an approach that treats large volumes of the brain with radiation.  In the rare case that a benign tumor can be isolated (e.g., a pituitary tumor), then precise radiosurgery is done, usually with 15 MeV or higher X-rays.

Current generation radiotherapy, radiosurgery and neurosurgery are all computer assisted, using CAT and MRI scans extensively to calibrate and register placement of radiation or surgical instruments within the body.  This approach de-skills the therapy and surgery and ensures a better outcome for the patient.  I was a young physicist when all these technologies were being developed.  It is wonderful to see how far these technologies have come in the last 30 years.  What is your experience with CAT scans, MRI scans, therapeutic X-rays or computer assisted surgery?  What’s your perspective on the most important directions for future development of these technologies?