Lung Cancer Radiation Therapy: New X-ray beam targeting techniques

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Stereotactic Treatment

The success of Stereotactic Radiation Surgery (SRS) and Stereotactic Ablative Body Radiation Therapy (SABR) techniques depends critically on having an adequate collection and algorithmic handling of the image data describing the patient body and organ motion.

Ben Cooper, PhD

Qualified Medical Physics Specialist,

Chief Medical Physics Specialist, Medical Physics and Radiation Engineering, Canberra Health Services

See, for example, Joshua Hiatt’s, basic description of Stereotactic Radiosurgery (SRS):

What is Stereotactic Radiotherapy?

and Nicholas Hardcastle’s more advanced technique:

SABR: A new force in local ablative cancer treatment

X-ray beam gating

Figure 1. The treatment beam is ‘gated’ by switching the beam on/off at a pre-determined part of the breathing cycle. The on-duty cycle of the beam (shaded rectangles) can only occur during the residual pre-set window for target motion.

‘Beam gating’ is a term used to describe the process of switching the X-ray beam on/off during a pre-determined part of the patient’s respiratory cycle. The aim is to reduce normal lung tissue irradiation and limit the size of the tumour target volume as well.

This can be achieved by establishing the location of the tumour target for a specific phase in the breathing cycle (see Figure 3(d) and Figure 4, in the previous article, Lung Cancer Radiotherapy: X-ray beam targeting).

Shortcomings: Executing beam gating in conjunction with the patient’s breathing cycle has technical difficulties caused when there’s a phase shift between internal and external anatomy.

These shortcomings make it questionable whether the beam gating technique would be of benefit for less than 2cm tumor motion.

Tumour Tracking and X-ray Beam Adaption

Tumor tracking is the most recent advanced technology to detect, adapt, and treat a moving tumour. Sensors detecting any tumour movement during the treatment triggers an electronic feedback loop which can then modify the X-ray beam shape and target direction. This is known as ‘adaptive radiation therapy’.

A smaller, more tightly conforming treatment volume is achieved with this technique. The adaptive radiation therapy technique detects tumour motion and modifies the beam collimation as it ‘tracks’ the target in real-time. The technique also removes the need for manually repositioning the patient during treatment.

*Figure 2. The robotic CyberKnife™ system was one of the first devices capable of tracking respiratory motion.*

Notable progress in the research and development of the tumour and beam tracking technique was:

2003: The robotic CyberKnife™ system was one of the first devices capable of tracking respiratory motion.
2012: A feasibility study of MLC linear accelerator based in vivo tumor tracking was first demonstrated in pigs.
2014 – 2016: The first patient clinical trial using a linear accelerator was fitted with:
- Multileaf collimator (MLC) for X-ray beam shaping;
- Transponder implanted into the tumour;
- Electromagnetic positional detectors; and
- KV X-ray accessory for checking tumour imaging

Signals from the motion detectors are computer-processed and drive the X-ray beam defining MLC leaves to remain tumour target aligned during “beam on” treatment (Figure 3.).

Figure 3. Real time tumor tracking feedback loop. Tumor motions are detected by the 3D target position system (Calypso) and processed by the MLC tracking software to automatically adjust the MLC leaf positions in real-time.

The first tumour tracking trial was for prostate cancer patients treated at Royal North Shore Hospital, Sydney (Keall et al, 2014) using a dual-arc VMAT technique. An important finding was that there was a 30% decrease in the rectal dose compared to the original plan.

Then, in 2016, the real-time adaptive radiation therapy technique was developed by Booth et al (also at Royal North Shore Hospital, ref. 5) for treating lung cancer patients. It was an improved method of the Stereotactic Ablative Body Radiotherapy (SABR) technique.

The patient was given a high dose per fraction with less than a normal number of treatments – 48 Gy over 4 fractions to the lung tumour.

The SABR technique is complex and accurately targeting the lung tumour is highly critical. The procedure must be adapted every treatment day because of the very small variations in the tumour location and its relation to the surrounding normal organ structures.

For Nicholas Hardcastle’s overall description of the SABR technique, click on:

SABR: : A new force in local ablative cancer treatment?

Booth et al (2016, ref. 5) reported that real-time adaptive radiotherapy with MLC tracking:

reduced the target volume from 18.7 to 11 cm³;
reduced the mean lung dose from 202 to 140 cGy
reduced lung V20 by 35% and
reduced V5 by 9%.

Adaptive Radiotherapy Tracking References

Dieterich S, Tang J, Rodgers J, Cleary K, editors. Skin respiratory motion tracking for stereotactic radiosurgery using the CyberKnife. International Congress Series; 2003: Elsevier.
Murphy MJ, editor Tracking moving organs in real-time. Semin Radiat Oncol; 2004: Elsevier.
Poulsen PR, Carl J, Nielsen J, Nielsen MS, Thomsen JB, Jensen HK, et al. Megavoltage image-based dynamic multileaf collimator tracking of a NiTi stent in porcine lungs on a linear accelerator. International Journal of Radiation Oncology* Biology* Physics. 2012;82(2):e321-e7.
Keall PJ, Colvill E, O’Brien R, Ng JA, Poulsen PR, Eade T, et al. The first clinical implementation of electromagnetic transponder‐guided MLC tracking. Medical physics. 2014;41(2).
Booth JT, Caillet V, Hardcastle N, O’Brien R, Szymura K, Crasta C, et al. The first patient treatment of electromagnetic-guided real-time adaptive radiotherapy using MLC tracking for lung SABR. Radiotherapy and Oncology. 2016;121(1):19-25.

Ben Cooper PhD, 7 September 2020