Radiotherapy

Mary PW Chin 钱碧慧博士
PhD (Wales), MSc (Surrey)
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RADIOTHERAPY IN CONTEXT
a slideshow

Slide-by-slide narrative
  1. This is a map of Penang Hill (locals call it Bukit Bendera) from the foot (bottom right) to the peak (left). As we go higher, successive contours indicate increasing elevation. Each contour joins points of the same elevation.
  2. Recall geography classes at school, when we learned contour maps.
  3. Here is a hypothetical example (no longer of Penang Hill) showing a contour plot side by side with its corresponding elevation plot. Points A, C and E are peaks — as we approach the heart of each point, the elevation increases. Points B and D are valleys — as we approach the heart of each point, the elevation decreases. The surrounding area is between 30 and 35. Point A is 70. Point B is 20. Point C is 50. Point D is 5. Point E is 50.
  4. Two routes are shown: green and blue. The green route: contours with loose space between indicate a slow change, we call this a low gradient. The blue route: contours closely packed together indicate a drastic change, we call this a high gradient. Our lungs and hearts and muscles would need to work far harder if we take the blue route.
  5. Contours on a geographical map show elevation; without the contours we know which place is where but we won't know how high is each place. In radiotherapy, contoured dose maps take centre stage. Patients need to wait for the physicist to complete treatment planning before they can receive treatment — because the physicist needs time to optimise the dose contours. We like high dose gradients. That is, rapid dose fall-off as we move out from the tumour (our target) to the surrounding healthy tissues. Depending on tissue type, some tissues can withstand more radiation than the rest. We call this attribute radiosensitivity, of a subject called radiobiology (radiation biology).
  6. Here's the Google exercise. Google for 'external-beam radiotherapy dose contours', click on the 'Images' tab. On each of the image, try to identify the four items.
  7. Here is the Google find.
  8. External-beam radiotherapy may be delivered using different modalities: cobalt-60 (which is less common today), standard linear accelerator (which is most widely available), Tomotherapy and Cyberknife (which are a bit exclusive and are not available to the general population).
  9. We usually aim radiation at an area slightly beyond the tumour visible on CT and/or MR images. This margin is applied to account for microscopic tumour extensions, setup and beam uncertainties, day-to-day (e.g. weight-loss and bowel filling) and moment-to-moment (e.g. breathing) changes in the patient's body which affects the alignment of the body with respect to the radiation beam.
  10. In external-beam radiotherapy, the beam enters from the outside into the patient's body. As the beam enters the body and makes its way to the tumour, it inevitably harms some cells along its way. We call this the entrance dose. As opposed to external-beam radiotherapy delivered by a linear accelerator, brachytherapy is a modality where the radiation beams from the inside out. Yes, from within the body. This is made possible via insertion of radioactive seeds into body cavities (the most common being the vagina). The dose to surrounding healthy tissues is thus lowered. Brachytherapy may be administered via different modes: remote-afterloading exposes staff to less dose; high dose-rate allows the treatment to be delivered in an outpatient basis.
  11. In external-beam photon radiotherapy, the beam enters the body, makes its way to the tumour, then makes its way out. As it makes its way out, it inevitably harms some healthy cells. We call this the exit dose This happens in all photon radiotherapy modalities — Tomotherapy and Cyberknife included. In fact, Tomotherapy and Cyberknife tend to lead to higher entrance dose and higher exit dose compared to conventional external-beam radiotherapy. Hadrontherapy is revolutionary in terms of low entrance dose and lose exit dose. The Bragg peak is far higher than the enrance and exit doses. Hadrontherapy includes proton and carbon therapies.
  12. Now we come to another Google exercise. Google for 'radiotherapy depth dose'.
  13. And here's the Google find. Note that advertising banners usually under-represent the entrance dose in hadrontherapy. A quick check is to look at the peak — if the peak is really a point, then the entrance dose at the start of the same curve is only true for a monoenergetic beam, which is not the way hadrontherapy is administered. Hadrontherapy is administered via a polyenergetic beam because tumours are hardly ever as narrow as the Bragg peak. A polyenergetic beam would produce a spread out Bragg Peak (SOBP) so that the peak adequately covers the tumour. Polyenergetic beams, when superimposed, would lead to higher entrance dose than that of a monoenergetic beam.
  14. Japan and Italy are leading in BNCT (Boron Neutron Capture Therapy), which conceptually promises low entrance and exit doses. BNCT is not yet widely available, not that it is exclusive for the wealthy, but that the concept has yet to be fully translated into practice. Lots of radiopharmacy research still needs to be done. It works a bit like 18F-FDG PET (Positron Emission Tomography), in that a tumour seeker is bound chemically to a radioisotope (10Boron instead of 18Fluorine). The tumour seeker leads the way in the blood stream, and brings the radioisotope to the site of action (the tumour). Before reaching the site of action, the radioisotope does nothing, ensuring no entrance dose. Only after reaching the site of action, the radioisotope delivers a dose with a range short enough to ensure no exit dose. This action is triggered by an external neutron beam. Note that BNCT is particle therapy, not neutron therapy. It is lithium and alpha particles which deliver the dose. Neutrons act only as a trigger. Neutron therapy is a bit of a taboo and is better not mentioned, following some horrific clinical trials decades ago.
  15. In almost all modality, not all particles lose all their energy in the patient's body. There are always some surviving particles exiting the patient's body. We can always scavenge these for verification purposes, as they often bear tell-tale signs of how the beam deposited energy in the body, and so become invaluable treatment record. I have done scavenging work of this sort for conventional photon therapy, BNCT, proton and carbon therapies.
  16. Therapy and imaging used to be distinct branches in medical physics. The demarcation is increasingly blurred as we move forward with IGRT (Image Guided Radiation Therapy). Contemporary treatment machines now incorporates an imaging capability of some sort (e.g. cone-beam CT, MRI), though not of full diagnostic quality, for guiding and verifying radiotherapy.

Classifying radiotherapy modalities

There is a variety of radiotherapy modalities. The first step in understanding the field of clinical radiotherapy is to be able to put each modality in the context of the full picture, to be able to identify each modality by different classifications.

Radiotherapy
    |
    |-- External                  
    |
    |-- Internal                  
            |
            |-- Brachytherapy     
            |       |
            |       |-- Outpatient
            |       |
            |       |-- Implant 
            |       
            |-- Nuclear medicine
      
Chart 1: Classification of radiotherapy according to delivery mode.
Radiotherapy
    |
    |-- Photon and electron
    |              
    |-- Hadrons               
            |
            |-- Proton              
            |
            |-- Alpha 
            |
            |-- Lithium 
            |
            |-- Beryllium 
            |
            |-- Carbon     
            |
            |-- Oxygen 
            |
            |-- Nitrogen 
        
Chart 2: Classification of radiotherapy according to particle type.
Radiotherapy
    |
    |-- Generated
    |       |
    |       |-- Accelerator
    |       |
    |       |-- Cyclotron
    |              
    |-- Natural radioactivity
            |
            |-- Static       
            |
            |-- Stepping     
            |
            |-- Liquid 
        
Chart 3: Classification of radiotherapy according to source type.