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I was booked to start radiation therapy 6 weeks after surgery. The Orthopeadic Surgeon wanted to check that I was free of infection before referring me on to the radiation oncologist.
The Radiation Oncologist has overall responsibility for the patient’s safety, medical care and general cancer management during radiation therapy.
Lyn Oliver AM PhD
Head and Neck Radiotherapy
Just as the Orthopeadic surgeon relies on his theatre team for a successful operation, the radiation oncologist works with the department’s planning and treatment team for an optimum radiation therapy outcome. The radiation therapists and medical physicists provide the doctor with that team support.
To read an overall description of how the radiotherapy patient services are organised, see:
Radiotherapy Megavoltage Treatment
and
Radiotherapy Treatment Pathway
Regina Bromley MSc
Deputy Director of Medical Physics, Northern Sydney Cancer Centre, Royal North Shore Hospital
Radiation therapy technology and methods of treatment improved significantly during the 2000 – 2010 decade. Computer planning using dose optimisation by inverse calculations and intensity-modulated radiation therapy (IMRT) provided better cancer target beam shaping with less dose to the surrounding critical organ structures.
Combined with the new image-guided radiation therapy (IGRT) technique, uncertainties of internally moving tumours and normal organs were reduced and delivered a more accurate treatment. By 2010, radiation therapy could claim to be providing a much-improved quality of patient service.Â
And, since my retirement as a medical physicist in 2010, more improvements in radiation therapy technology and techniques have continued to evolve.
My treatment plan was a volumetric arc treatment (VMAT1). The 6MV X-ray beam source rapidly sweeps through two complete circles (720 degrees) about the patient (shaping the radiation beam as it goes) to treat my tumour and spare my surrounding critical structures (Figure 1.).
Figure 2. shows how the muti-leaf collimator produces different shaped X-ray exposure as the machine rotates through 360-degrees. It then repeats one after the other a second 360-degree exposure, with different X-ray beam shapes. The dose rate is a very high 800 mu/minute. Note that, for some angles, the collimator leaves are shadowing the tumour. This is deliberately calculated to limit the prescribed X-ray dose that critical structures receive from this angle.
Even though a number of CT and MRI scans were arranged during my hospital surgical care, radiation therapy still needed to CT scan my body using their dedicated CT Simulator. I was set-up on the CT simulator (Figure 3.) lying the same as I would be for treatment on the linear accelerator.
1. Head and neck patients have a personalised mask made for them. It is to ensure (i) the patient’s treatment set-up is the same for each treatment and (ii) the mask immobilises the head and neck during the X-ray exposure.
2. The therapist, Craig (Figure 3.), lay me down on the treatment couch and extended my chin up to stretch the neck area. A sheet of thermoplastic was immersed in hot water to soften it and was then firmly placed over my face and neck. Once the sheet cooled, it remained a rigid shape of my face and neck.
3. The mask was fitted onto me for the CT simulator scan. A lead marker is attached to the cross visible on the side of my mask (Figure 4.). The lead marker shows up on the CT simulator images and becomes an important point in space. It is registered by the planning computer software and used by the radiation therapists when they set me up for each treatment (see Treatment Set-up).
4. The CT simulator images become the baseline template for the computer treatment plan, and later, comparing it with CT type images (called cone-beam CT scans) taken on the linear accelerator before each treatment.
5. During a joint meeting with the planning radiation therapist and medical physicist, the radiation oncologist reviewed all the hospital scan images and compared them to the CT simulator images.
For my treatment, the radiation oncologist prescribed a tumour dose of 40 Gy in 20 treatments, a dose limit of 40 Gy to the oesophagus and 42 Gy to the spinal cord. It’s to the patient’s benefit if the dose to the critical structures is much less than the prescribed dose-limit.
Outlining the target and critical organs is a lengthy, labour-intensive process.
To help the doctor see the extent of the cervical-6 tumour when outlining the target volume, the pre-surgery MRI scan (Figures 5. and 6.) was overlaid onto my head and neck CT simulator images (Figure 7.).
The tumour volume (referred to as the target) is more clearly shown in Figure 7. The small blue line outline indicates the tumour mass seen on the previous MRI. The target includes the whole of the remaining vertebra (red outline) to ensure that any remaining tumour cells are adequately destroyed. The translucent green volume in Figure 6. is the 100%, 40 Gy dose.
The CT simulator image is overlaid on the dose plan to show the critical structures and surrounding anatomy (Figure 8.)
Program software for overlaying (or fusing) image data from different scans is a recent innovative technique. It’s referred to as medical image registration.
There are technical difficulties in registering different image data. The patient may not be laying the same way for each scan and internal organs may also have moved in subtle ways. Programs written to auto-outline internal organs have improved considerably. But are not yet perfect.
There’s extensive international work underway on Medical Image Registration R&D, practice protocols, workshops, and webinars.
The Australian-New Zealand medical physicists have set-up a Medical Image Registration Special Interest Group which is open to all disciplines to become involved.
After completing the computer treatment plan, it is checked by an independent planner or medical physicist and authorised by the Radiation Oncologist. The X-ray treatment details are saved on-line to a central computer database. There’s ready access via the data network to the linear accelerator console from there.
I was booked for treatment on the Ethos 6 MV X-ray linear accelerator (Figure 9., Varian Medical Systems Ltd).
Unlike the conventional dual X-ray high-energy linear accelerators used for pelvic, abdominal and trunk treatments, Ethos is more suited for smaller sized sites such as head, neck, breast and extremity treatments.
The design of moving parts in the Ethos machine is similar to the radiology CT scanner. It has a ring bearing arrangement. The Ethos ring mounting has a short 6 MV linear accelerator X-ray tube (for treatment), a diagnostic KV X-ray tube and opposing detectors (for imaging) mounted on it.
Notable features are:
(i) Rotation of the Ethos CT cone-beam scan and 6 MV X-ray accessories travel through 360 degrees much faster than the bulkier higher energy linear accelerators. They are limited in speed for safety reasons.
(ii) The new Ethos multileaf collimator system overcomes the previous unwanted interleaf X-ray dose leakage. That simplifies patient treatment dose calculations.
Why is a mask needed? Well, in my case, the oesophagus (where I did not want any dose) was very close to the tumour target volume. It was technically difficult to not receive some X-ray dose to the oesophagus while treating the tumour next to it. Small errors in targeting the tumour could mean extra dose to the adjoining oesophagus.
The mask (Figure 10.) plays an important role in fixing the head and neck to be in the same extended position for each treatment. That ensures that all anatomical structures are the same for each treatment.
The mask also keeps the patient and internal anatomy from moving during the X-ray exposure.
The mask is purposely a snug fit. I found it comfortable to wear during the treatment. But, I can appreciate that patients who suffer any form of claustrophobia would not be so comfortable.
To help patients who may have claustrophobic feelings, a mask with the face area open can be used if it still provides sufficient immobilisation.
Alternative methods are designed with the patient held firm by a ‘bite block’ and using optical motion sensors, are commercially available. Other innovative designs are also in development.
Once the mask is fitted, the therapist moves the couch up to align the cross on the mask with the purple laser cross (Figure 10., bottom right.). This is a ‘pseudo’ set up point located at the entrance to the treatment machine’s aperture.
For the ‘final set-up’, the therapist presses the ‘set-up’ button on the side of the machine. The couch and patient are then automatically driven into the aperture of the linear accelerator.
The CT simulator x,y,z coordinates taken from the patient’s plan are aligned with the linear accelerator ‘isocentre’. The Ethos software accurately moves the patient on the couch to the treatment position without therapist assistance. Magic!
How Ethos does this, is not easy to explain in simple terms.
The Ethos imaging system is far more efficient than previous image-guided method. No markers are required and there is very little therapist interaction to make manual adjustments.
A daily image of the patient in the final set-up is automatically compared against the CT Simulator image using the consoles software analysis (Figures 11 and 12). The algorithm matches the two images and calculates any 3-D couch correction to make before treating.
The patient’s treatment dose for that day is calculated from combining the Ethos cone-beam and the planning CT data. Ethos creates a synthetic CT using the tissue positions from the cone-beam CT with the image density from the planning CT. The dose is calculated on the sCT and overlaid on the cone-beam CT (figure 13. left) so that it can be compared against the original dose calculation based on the planning CT (figure 13. right)
The therapist checks the image and dose matching. If the result appears acceptable, the treatment is approved, the couch corrections are remotely corrected by the machine’s couch electronics and the X-ray exposure is activated to follow.
Checking how well the dose for each treatment matches the original dose planned, is an entirely new technique. This feature is particularly valuable for pelvic, abdominal and chest treatments when there can be considerable variation of the tumour and surrounding organs from day-to-day.
Even though my head and neck treatment were easy to immobilise, Figure 14 (shown as a video and as a stepped collection of images) indicates a surprising small variation in each of 20 different treatments.
Before each treatment can begin, the machine’s computer software calculates this change in x,y,z coordinates. The operator then checks and activates the couch to be automatically moved remotely. This is a new feature which provides an improved consistent treatment.
Personally, I am very happy to know the tumour volume remains accurately targeted and my oesophagus is protected from unnecessary excess dose!
This final image technique enables the software to re-calculate the monitor unit dose when it detects different path-lengths of the X-ray treatment beam to provide a more accurate dose too.
While I was left set-up in the treatment position and waiting for something to start, I could hear very little happening. There was no noise during the imaging process. But I did feel the couch make tiny corrections. Some couch movements were a little more noticeable than others.
Then finally, there was no discernible noise as I received the 6MV X-ray treatment. Did they turn it on?
Before I knew it, the therapists had returned. It was just three minutes after leaving the room. Now they were back and helping me off the couch. In fact, it was only ten minutes from the time I entered the treatment room and left again thanking everyone on the way out!
That’s faster than any other linear accelerator treatment, I know.
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Lyn Oliver AM PhD
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