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Cancer Council Australia lists lung cancer as one of the five most commonly diagnosed cancers (Figure 1). It has the poorest 5-year survival cure rate in this group, of 17%.
Whether it be by chemotherapy, surgery, or radiation therapy, there are medical and technical difficulties in treating lung cancer patients.
Ben Cooper, PhD
Qualified Medical Physics Specialist,
Chief Medical Physics Specialist, Medical Physics and Radiation Engineering, Canberra Health Services
Information for the Community
However, recent improvements in modern technology are providing the opportunity to use higher dose with improved accuracy treatments. Modern techniques can provide higher X-ray dose to a smaller, more accurately targeted tumour in the lung, with less complications.
This series of articles briefly describe the challenges when treating lung cancer by radiation therapy. Current research and development work promises smarter and better design methods using existing modern technology.
The use of radiation to treat cancer patients is generally not well understood. Many would respond to this suggestion by asking:
“Doesn’t radiation cause cancer?”
It is true that uncontrolled high levels of exposure to ionizing radiation can cause acute biological effects. Long term, elevated levels of radiation exposure can also be harmful. Medical physicists spend a considerable amount of their professional life ensuring that the medical use of radiation equipment and management of radiation procedures are safe. Regular physics and engineering quality assurance work is designed to ensure that patients and staff are adequately protected when radiation is used for diagnostic or therapeutic purposes.
Provided that radiation equipment is safely designed (according to international standards) and the delivery of X-ray treatment is accurately controlled, it can be effectively used to treat cancers.
Hospital radiation therapy services specialise in utilising the art and science of accurate, safe X-ray treatment. The fundamental goal is to deliver a lethal radiation dose to the tumour while protecting the patient’s surrounding healthy tissues and organ structures.
How do x-rays kill the tumour?
Figure 2. illustrates the X-ray energy as being like a bolt of lightning passing through the cell nucleus and critically damaging the DNA molecule – a direct hit. The ionizing radiation causes molecular changes to the DNA strand and, in some cases, lethal breaks in the DNA. Normal cells are slightly less sensitive to x-ray damage than cancer cells and this effect is exploited in radiation therapy. Additionally, the blood supply and nutrition of the normal cell tissues also provides better capacity for body organs to recover.
As a higher dose of radiation is given to the tumour over a period of time, more and more of the cancer cells are lethally damaged but normal cells have less exposure to x-ray dose and time to repair and recover. After the tumour receives sufficient X-ray dose, it begins to shrink and is eventually destroyed as surrounding normal cells are able to repair the tissues.
Different organs and normal cells of the body have different radiation sensitivity. For instance, lung tissue is much more sensitive to X-rays than normal skin tissue. Unless the X-ray exposure is adequately controlled, too much radiation exposure of the lungs can lead to induced pulmonary pneumonitis with symptoms a bit like pneumonia, except caused by radiation rather than a virus or bacterial infection. The radiation oncologist wants to perfect the treatment technique so that unnecessary pulmonary pneumonitis complications to the remaining healthy lung tissue, is avoided.
Prescribing Tumour Dose and Dose Limits
When prescribing the patient’s X-ray treatment, the radiation oncologist specifies a minimum dose that the tumour must receive and a maximum normal lung dose limit. It is also normal ‘best practice’ to ensure the radiation dose to sensitive structures such as the lung, spinal cord and heart receive as low a dose as practicable when giving the X-ray radiation therapy.
The prescribed treatment cannot be given in one single dose. The patient receives the total prescribed X-ray dose over a number of fractions.
Stereotactic ablative radiotherapy (SABR, for peripheral lung tumours) have a hypofractionation scheme of 48Gy in 4 fractions for a gross tumour volume, GTVs < 2cm from chest wall; or more aggressively 54Gy in 3 fractions for GTVs for more than 2cm from the chest wall (see: Eviq guidelines)
Planning the Treatment
The patient’s a CT scan allows the doctor to “see inside” the patient. Much like a loaf of bread being sliced, the CT scan “slices” cover the area where the tumour is in the lung. This kind of CT is not for diagnosis. It is a planning CT to obtain images that accurately show:
- the internal location of the tumour;
- the size and shape of the tumour target volume, lungs and other sensitive organ structures.
The planning CT scan is used directly in specialized software which allows the radiation therapist and doctor to build a “virtual patient” model. This allows them to see how the X-ray beam exposure can be directed from many different angles around the patient and decide on the best plan. The specialized software calculates and optimises how the tumour can receive the prescribed X-ray dose while avoiding, as much as possible, dose to the sensitive structures (especially the lungs).
Each patient’s treatment plan is individually computed using the patient’s planning CT-scan images, along with previous medical images that they might have had (see also better healthcare technology website: SABR, Hardcastle).
Figure 3. shows the resultant treatment plan. The coloured dose distribution is superimposed on the patient’s CT-scan cross-section. The shaded red area is the prescribed high dose to be given to the tumour in the left lung. The green and blue areas indicate the lower dose distribution to the surrounding normal structures.
Pre-treatment Check
Once the planned treatment is checked by the doctors, radiation therapists, and medical physicists, the patient’s treatment is approved and ready to commence. The patient must be set up in the same position for each treatment. This is checked each time to ensure that the patient’s tumour site is always accurately targeted before the treatment is commenced.
As well as the high energy (megavoltage (MV)) X-rays used for the treatment, the linear accelerator is fitted with an imaging (kilovoltage (KV)) X-ray tube and detector. The latter is to take images of the patient lying in the treatment position. The KV X-ray accessory obtains a special X-ray image similar to a CT scan. It’s called a ‘cone beam CT scan’ (CBCT).
Figure 4. shows the linear accelerator with the MV X-ray beam (yellow arrow) and the orthogonally mounted KV X-ray unit beam (green arrow).
The KV X-ray unit rotates around the patient (blue arrow) to create a 3-D image of the patient. This 3-D image is compared to the “virtual patient” CT planning scan. The comparison verifies that the treatment beam will be accurately targeted and aligned onto the patient’s tumour and that it matches the treatment plan. The radiation therapist can make positional corrections, if necessary, from viewing this image information.
Once the set-up is confirmed, the linear accelerator also rotates about the patient in the blue arrow direction while delivering the MV X-ray treatment beam focused on the tumour target.
The multi-leaf collimator (MLC) is specially designed to be able to dynamically change the ‘shape’ of the X-ray beam while the linear accelerator rotates about the patient. By combining dynamic movement of the MLC and linear accelerator, the patient receives a 3-D shaped X-ray beam to the lung tumour (Figure 5.) while limiting the radiation dose to the surrounding critical anatomy.
The Need for Improvement
Challenge 1.: Adjust X-ray beam targeting to allow for patient breathing
Accurately targeting the tumour, when the patient is breathing during the X-ray exposure, is a well-known problem – it is a moving target problem.
There is a need for better methods of monitoring the movement of the tumour and adjusting the X-ray direction during the X-ray treatment beam exposure.
Challenge 2.: Reduce lung dose from CBCT scans
The problem is, the cone beam CT scan dose is an unwanted extra dose to the patient’s normal lung tissue. But the KV X-ray image is important for ensuring the accuracy of this procedure.
Therefore, the goal should be to obtain sufficient CBCT quality images for checking the set-up with as little as possible KV X-ray dose exposure to the patient’s normal lungs.
The following series of future articles describe progress made to further improve the radiotherapy technique for lung cancer patients:
- Problems of X-ray beam targeting;
- New techniques for X-ray beam targeting; and
- Reducing lung dose from CBCT scans
Ben Cooper PhD, 10 July 2020
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