Radiation therapy has a pivotal role in the treatment of cancer. Approximately two-thirds of cancer patients need radiation therapy during the course of their disease. The delivery of radiotherapy has changed significantly over the last few decades. We have moved from conventional radiotherapy using rectangular fields to conformal radiation techniques like 3-Dimentional Conformal Radiotherapy (3D-CRT), Intensity Modulated Radiotherapy (IMRT), Image Guided Radiotherapy (IGRT), SRS, RapidArc, etc.
These changes in the radiation delivery have come about as a result of trying to improve the delivered dose to the tumour and reduced irradiation of the organ at risk. External beam radiotherapy involves the delivery of X-rays, gamma rays or particle radiation from linear accelerators or telecobalt machines.
Modern radiotherapy techniques
- IMRT: It optimises the delivery of radiation to irregularly shaped volumes. It produces concavities in the treatment volumes by sub-dividing the radiation beam into multiple component beams (beamlets), each of which may be modified individually. For example, when treating head and neck cancers, IMRT allows for a greater sparing of salivary glands, upper aero-digestive tract mucosa, optic nerves, and brain stem. This has the potential to decrease late toxicities like dryness of mouth, swallowing and speech difficulties. In gynaecological cancers, IMRT results in reducing acute and long term side effects like abdominal cramps, diarrhea, burning during urination etc.
- IGRT: Optimal IMRT delivery requires accurate image guidance. In tumours like lung cancer which exhibit large physiological motion, an extra margin around the target to account for organ motion may be quite large. Reduction in this margin allows for reduction in dose to normal tissues thus making possible higher dose delivery to the tumour. IGRT using inbuilt X-ray device, onboard CT scanners, and infra-red cameras helps in detecting and correcting the errors that occur during treatment delivery.
- PET-CT (MRI fusion for tumour delineation): CT scans are the standard imaging modality used in radiation treatment planning. However CT scan is inferior to MRI in delineating soft tissues, so a fusion of CT and MRI should be considered in tumours of brain, skull base, and prostate. Positron Emission Tomography (PET) scan enables biological imaging of tumours. PET-CT fusion helps in detecting microscopic disease which can be used for dose escalation and normal tissue sparing.
- Stereotactic Radiosurgery (SRS): SRS enables accurately conformed delivery of radiation in large fraction sizes, which also enables improved tumour control while limiting normal tissue toxicity. This technique is used in management of brain metastasis, acoustic neuromas, and Arterio-Venous Malformation (AVM). Extracranial body radiosurgery is also becoming popular in treatment of spine metastases, early stage lung cancer etc. Cyberknife is a linear accelerator mounted on a robotic arm that provides more than 1000 beam orientations. It has been used to treat tumours in lung, liver, and spine.
- RapidArc: This is a volumetric intensity-modulated arc therapy, whichm can deliver the required dose distribution with one or a few arcs. RapidArc combines high conformity with significantly shorter treatment durations, which reduces the risk of patient or tumour movement, and is more comfortable for the patient.
- Particle therapy: Charged particles like protons tend to deposit most of the energy in small areas. This has advantages in terms of normal tissue sparing and better dose homogeneity. Its current role lies in tumours close to the skull base, certain ocular tumours and pediatric patients. The last two decades have se en significant technological advances in radiation therapy in terms of more precise dose delivery and improved sparing of normal tissues. Clinical studies investigating these modalities are likely to further increase the efficacy of radiation in years to come.