What the clinician wants to know: radiation oncology perspective

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 radiotherapyRadiotherapy 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 volumesWhereas 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 ...