Dr Bibhash Chandra Goswami
The State Cancer Institute, Gauhati Medical College has commissioned TomoTherapy for treatment of cancer. This article is to give a short account of radiation treatment for cancer focusing on Helical TomoTherapy technology since this is a newer technology to be available in this part of our country. TomoTherapy literally means “slice treatment” and is a new form of radio therapy that modifies the design of a diagnostic CT (computerized tomography) scan into atreatment delivery machine, thereby combining the precision of CT imaging withthe radiation treatment. Unlike traditional radiation therapy systems with a slow-moving external gantry designed for positioning individual beams onto the tumour from a few different directions, TomoTherapy rapidly rotates the beam around the patient (and inside the housing of the unit), thus allowing the beam to enter the patient from many different angles in succession. The inclusion of CT imaging technology within the TomoTherapy unit allows precise localization of the target before and during treatment.
Radiotherapy is delivery of a carefully measured dose of ionizing radiation to a diseased area, mostly cancer. Radiation kills tumour cells and prevents abnormal cell growth. This method of cancer treatment is in use for over 100 years. Before 1970s, cancer bearing area was localized by clinical examination and basic imaging like x-rays. Obviously, a two dimensional (2D) image was available for a three dimensional (3D) tumour. With development seen in Imaging technology like Ultrasonography (USG), Computed Tomography (CT scan), Magnetic Resonance Imaging (MRI) and their various types, brought a new era of 3D imaging and precise tumor localization. However, these are anatomical or structural information and a new era of Biological or Molecular imaging emerged with invention of Positron Emission Tomography (PET scan) and hybrid forms like PET-CT, Single Photon Emission Computed Tomography fused with CT (SPECT-CT), PET-MRI (PET fused with Magnetic Resonance Imaging) etc. With combination of these anatomical and physiological imaging, precise tumour mapping became available opening a new chapter of cancer management where cancer bearing areas could be more precisely identified and treated sparing nearby critical and/or vital organs for accurate delivery of radiation therapy. Simultaneous progress in computer technology has contributed transforming conventional radiotherapy to a conformal one. Although number of conformal techniques are available, Intensity Modulated Radiotherapy (IMRT) and subsequently Image Guided Radiotherapy (IGRT) have emerged as efficient methods. In IMRT, different doses can be prescribed to target volumes as per need, that is called a dose painting, protecting nearby vital structures. In IGRT, the imaging facility inside the treatment room is used to match with the pre-treatment planning scans to assure daily patient set-up accuracy.
Last two decades have seen development of innovative IMRT techniques, one of these being TomoTherapy. InTomoTherapy, perfectly modulated radiation is delivered when simultaneously CT images are acquired for set-up verification. The patient is translated either step-wise (Serial TomoTherapy) or continuously as we see in a CT scanner (Helical TomoTherapy). Helical technology is widely accepted. Helical TomoTherapy was proposed by Mackie and subsequently commercial manufacturing started at Madison, USA and the latest model named Radixact is currently available.The 6MV linear accelerator machine with a Helical CT scan facility deliver IMRT with image guidance by comparing and correcting treatment position to that of prescription CT planning images enabling precise target localization just before and during treatment.
Helical TomoTherapy has many novels.This is the only machine available today to deliver helical treatment with rotational IMRT. Another feature is the ability to treat a large area ( Magna field ) for example brain and spinal cord or a total bone-marrow irradiation required for some haematologic conditions. A linear accelerator has the largest field size of 40cm whereas TomoTherapy can treat upto 160cm at a time. Cancer is known for metastases meaning disease spread to multiple sites. In TomoTherapy multiple sites can be simultaneously treated without changing the machine position or patient couch thereby ensuring precise treatment in a shorter time. Another beauty of this machine is the capability to track the tumour during treatment and this is helpful when the tumour is in an organ having internal motion example being liver, lung, pancreas, prostate etc. Motion correction therefore ensures perfect treatment delivery.
Radiosurgery (SRS) was originally envisioned to treat intracranial lesions by deliveringa high dose of ionizing radiation in a single treatment session using multiplebeams precisely collimated to a target inside the cranium. Advances in both imaging and computer technologies resulted in wider applications of radiosurgery. There are now a variety of different radiosurgery and stereotacticradiotherapy techniques available for intracranial and extracranial use and TomoTherapy is an advanced machine suitable for such treatment.