Abstract
The delineation of the target volume, i.e. the volume which should be irradiated with a therapeutic dose of irradiation, is of utmost importance in radiotherapy. Modern imaging techniques cannot be missed in this process. Diagnosticians and radiation oncologists therefore should understand each other’s needs and potential.
Keywords: Radiotherapy, treatment planning, target volumes, biological target volume
Staging for radiotherapy
Radiotherapy like surgery is a treatment for locoregional tumour growth. So, imaging modalities are extremely important for staging. Computerized tomography (CT), ultrasonography and magnetic resonance imaging (MRI) are used in the work-up of many tumours, e.g. in cervical cancer and head and neck tumours to detect lymph nodes, in rectal cancer to judge whether a free margin can be obtained to make the patient a suitable candidate for short term irradiation followed by surgery. New imaging modalities are also being introduced for staging such as fluorodeoxy glucose positron emission tomography (FDG-PET) scanning in lung cancer and MR lymphography with nanoparticles in prostate cancer.
Image guided radiotherapy volumes
Whereas surgery is tumour eradication under direct vision, radiotherapy is tumour eradication under indirect vision. So, imaging is of extreme importance for radiotherapy planning. In radiation oncology the target volume is that part of the body where a therapeutic dose of irradiation should be applied. This target volume is subdivided into several subsets: the gross tumour target volume (GTV), the clinical target volume (CTV) and the planning target volume (PTV). The GTV and CTV have a biological background. The GTV is what you can see, measure or palpate. The CTV is the suspected microscopic extension of the disease, and as in surgery, radiation oncologists also take a margin around the visible tumour. The GTV can include regional lymph nodes when microscopic spread to lymph nodes is expected; and when they should be treated electively, one could consider this as a nodal CTV. The PTV is a geometric and not a biological concept. This volume by two margins takes into account organ movement (internal margin) and positioning in accuracies and inaccuracies of the irradiation delivering equipment (set-up margin). The PTV is created to ensure that the therapeutic dose is indeed delivered to the CTV.
Although one could imagine that the delineation of the GTV is most simple, we have to realise that different imaging modalities ‘see’ tumours or organs differently, e.g. the size of the prostate is different on MRI than CT. What is the truth? Often radiation oncologists use both. Consequently image registration or image fusion has become of paramount importance. Like surgery, in radiotherapy we also do some harm to normal tissues. Healthy tissue is unavoidably irradiated. However, the radiation dose in these tissues should be kept below tolerance which means below the dose which creates clinically manifest severe damage. For this reason a fourth volume in the irradiated part of the body has to be taken into account: the volume around an organ at risk. This is the planning organ at risk volume (PRV). For example, when very high doses are given to the prostate, one has to identify the PRV of the rectum. With the use of so-called dose volume histograms, an impression can be obtained of the percentage of the rectum receiving a certain percentage of the total dose which correlates with the risk of complications.
Volume delineation
To define the target volumes many radiotherapy departments now use their own dedicated CT scanners. One has to realize that these scanners are different from diagnostic ones. They have a larger aperture to allow for scanning in treatment position and they have a flat table top. Also the scanning protocols can be different from the diagnostic ones. Planning scans are performed without the use of contrast so as not to disturb the physiological size and density of some of the organs. Scanning times might be different, e.g. slower to detect lung movements and also the slice distance may be different, usually 3 mm, to obtain a reliable digitally reconstructed radiograph (DRR). So, planning scans are not diagnostic scans and one should be careful about using them for diagnostic purposes. Furthermore, diagnostic scans are not planning scans and one should be careful about using them without certain precautions for treatment planning.
Intensive collaboration between the imaging departments and radiotherapy departments is necessary nowadays for radiation oncology. It would be advisable to collaborate with a dedicated diagnostician who understands the needs of the radiation oncologist. Proper diagnostic reports should contain the information of importance for the radiation oncologist, meaning that the slices on which the GTV can be seen should be mentioned in the diagnostic report. On the other hand, the radiation oncologist should increase his/her knowledge in diagnostic imaging by additional training as resident and as part of a CPD (continuing professional development) programme. Because different types of images are used and have to be matched with the planning CT slices, image registration has become an important part of the volume delineation process. Image registration can be performed by the use of anatomical landmarks but also with the use of gold markers which are used in prostate cancer GTV delineation.
Apart from the dedicated planning CT scanners, radiation oncologists also make use of a simulator consisting of a traditional X-ray apparatus, in some cases provided with a CT extension with which low quality CT slices can be made; these are sufficient for the contouring of the patient. Essential in all these simulation or localization procedures is that the images are acquired in relation to a three-dimensional (3-D) set of co-ordinates. Skin markers make it possible to position the patient in a reproducible manner, making it possible to deliver the treatment dose in a large number of fractions over a substantial overall treatment time, e.g. 35 fractions in 7 weeks. Dose plans finally are created on the basis of the delineated volumes in relation to the co-ordinates.
Biological image guided radiotherapy
The tumour cell density might not be the same throughout a tumour or an organ with a tumour. To date in the treatment of prostate cancer the whole organ is treated with a homogeneous dose while in many patients one could identify a dominant intratumoral lesion (DIL) with modern imaging techniques. It is logical to suppose that the irradiation dose in the DIL should be higher than in the rest of the prostrate and so a fifth volume can be introduced: ‘the biological target volume’ (BTV).
Similarly, areas with other biological properties could be identified such as hypoxic areas or rapidly proliferating areas in order to adapt the dose distribution accordingly. A new horizon is looming for all kinds of biological imaging using new isotopes for PET scanning and with the use of MR spectroscopy. Combined PET/CT scanners could facilitate the necessary image registration with reference to the co-ordinates. The developments in imaging techniques and radiotherapy equipment have written the history of radiotherapy in the past 30 years, from an uncontrolled inhomogeneous dose distribution via a controlled homogeneous distribution towards a controlled inhomogeneous dose distribution in biological image guided radiotherapy.
Target volume delineation on breast cancer and breast cancer treatment
The role of radiotherapy in breast cancer is as an adjuvant to surgery. Either the breast is removed by mastectomy or the tumour only as part of a breast conserving procedure. Consequently, the target volume only is a CTV. In breast conserving therapy the tumour bed after lumpectomy usually receives a higher dose. The visualization of this tumour bed is still not optimal and usually the only way is the use of radio-opaque clips which can interfere with follow-up MRI scans. Research to develop better methods is urgently required.
Conclusion
In the second half of the last century radiology in many countries split into radiotherapy and radiodiagnostics and imaging as independent specialties. Today the interaction of our disciplines is more intensive than ever which creates the need to understand each other’s needs and potential.