Benefits of Using IGRT
A large number of patients undergo complete treatment because of lower levels of discomfort and distress with IGRT treatment.
Dr Tejinder Katariya Sethi
X-rays were discovered by Wilhelm Conrad Roentgen in 1895 and the medical use for the diagnostic purposes started since then. After the discovery of radium by Marie-Curie and radioactivity by Henry-Becquerel, the earliest documented case of tumour cure was reported as early as 1902. The earliest X-ray equipment with coolidge tubes for therapeutic applications had X-ray production of 50Kv-80Kv and could treat only very superficial malignancies. To achieve a curable dose for tumours at a depth, multiple fields needed to be applied from different angles over the patient.
It was during the Second World War that magnetron was used for the air warfare. Magnetron is a device used for production of microwaves over which the electrons can be accelerated to produce high energy deep penetrating X-rays. The peaceful use of this nuclear energy was harnessed in the production of initial linear accelerators to treat deep seated cancer. From the era of reaching a target at depth in 60s, we have evolved to the era of targeting the deep targets precisely, accurately with reduced safety margins in today’s world with the introduction of Image Guided Radiotherapy (IGRT).
The question for a radiation oncologist is not whether he/she can localise the tumours with a 100 per cent accuracy on images, rather it is whether the tumor can be placed at the reference centre of radiation beam with >95 per cent accuracy on all days of the treatment?
External immobilisation can be achieved with plastic moulds, body frames or head fixation devices to restrain the patient from moving during radiation treatment on a daily basis. However, the internal organ motion especially in the thorax, abdomen and pelvis due to respiratory, cardiac motion or bowel/bladder filling is a source of deviation from the anticipated fixed geometry placement of tumor. This change in tumor position can vary not only on day to day (inter fraction) but also on a given day (intra fraction).
Targeting the Tumor
With the advent of intensity modulated radiotherapy (IMRT) radiation oncologists have been able to conform high dose regions with avoidance of critical organs. However, they are still limited in targeting the moving tumours in the region of lungs, liver, stomach, prostate etc. Besides this, the dynamic reduction in tumour during a seven-week course of radiation induces changes in the planned radiation portals, maybe exposing critical organs to high dose regions! Gated radiotherapy is one of the solutions to overcome this problem. Gated radiotherapy refers to acquiring planning CT-images at a particular breathing phase in a respiration correlated or 4-dimensional (4D) CT-scan and then gating the linear accelerator to irradiate the tumour in that phase. Since the direct localisation of a tumour mass in real time is difficult, if not impossible, various surrogates are used to derive the tumour position during treatment. The two forms of surrogates used at present are internal or implanted fiducial markers and external markers placed on the surface of patient’s abdomen.
Internal Gating: This is used with a real-time tracking system using four sets of diagnostic X-ray camera systems mounted on the floor of linac room and four image intensifiers mounted on the ceiling opposite the X-ray tubes. With this system, the fiducial makers placed at the tumour site can be directly tracked fluoroscopically at a video frame rate. The linear accelerator is gated to irradiate the tumor only when the marker is within the internal gating window. The size of the gating window is set at ± 1 to ± 3 mm. Preferably, 1.5-2.0 mm diameter gold seeds are inserted either bronchoscopically for lung tumors, or under fluoroscopy or ultrasound guidance for prostate tumours. For head and neck cancers, vertebral bony landmark is used as an internal marker.
External Gating: It involves placement of a light weight plastic block with passive infrared reflective markers on patient’s anterior abdominal wall. The surrogate marker of abdominal wall motion is monitored by a charge coupled device video camera mounted on the treatment room wall. This real-time position management (RPM) system permits both amplitude and phase gating for lung cancers.
The regularity of breathing waveform is checked by a filter and it disables the beam in case the breathing waveform becomes irregular by patient’s cough or motion. The major advantages of external gating are its non-invasiveness and no radiation dose from imaging. However, tracking the external marker is not equal to tracking the tumour and may cause error if trusted blindly. Another system available is the use of an abdominal belt with a tracking device to monitor and gate radiation beam.
Clinical implementation of tracking devices using kilovoltage or megavoltage CT-scanners has given a new dimension in targeting radiation to tumours. Whether this will translate in to a higher tumour dose with resultant higher control rates and reduction of normal tissue complications remains to be seen. The change in plans midway through the treatment following the decrease in tumour size viz adaptive radiotherapy (ART) using the available imaging data may become a real possibility in near future. However, a word of caution, the tighter we make the margins around the tumours, the higher may be the costs we pay in case of imprecise delivery. ‘The climb to a steep hill is slow at first’; should be the maxim in implementing finer technologies and due caution should be exercised so that we do not lose the benefits that radiation imparts to locoregional control in oncology.
At Artemis Health Institute, Gurgaon, an Elekta Synergy Linear Accelerator has been installed in the department of Radiation Oncology. Elekta Synergy offers the benefits of a digital liner accelerator with IGRT through volume imaging system. Elekta Linac has an MLCi head with two opposing sets of 40 ‘leaves’ and two thin back-up diaphragm occupying the standard X and Y diaphragms as in a standard head.
The advantage of IGRT over traditional forms of radiotherapy is manifold. The chances of recurrence of cancer are reduced as the detection of the exact tumour location and size and the precise method of destroying the cancer cell leaves very little room for the cancer to manifest itself again. It also leads to a significant reduction in the margin of healthy cells affected by the radiation. Because of reduced side effects, patient compliance is also much more with this kind of treatment. A large number of patients undergo complete treatment because of lower levels of discomfort and distress; hence success rates with IGRT treatment are much higher. Artemis Health Institute, Gurgaon is the first hospital in the entire Northern India to offer IGRT to its patients.
The writer is Consultant Radiation Oncologist Artemis Health Institute Gurgaon E-mail: firstname.lastname@example.org