Defining the tumour and target volumes for radiotherapy

Abstract

Radiotherapy is a localised treatment. The definition of tumour and target volumes for radiotherapy is vital to its successful execution. This requires the best possible characterisation of the location and extent of tumour. Diagnostic imaging, including help and advice from diagnostic specialists, is therefore essential for radiotherapy planning. There are three main volumes in radiotherapy planning. The first is the position and extent of gross tumour, i.e. what can be seen, palpated or imaged; this is known as the gross tumour volume (GTV). Developments in imaging have contributed to the definition of the GTV. The second volume contains the GTV, plus a margin for sub-clinical disease spread which therefore cannot be fully imaged; this is known as the clinical target volume (CTV). It is the most difficult because it cannot be accurately defined for an individual patient, but future developments in imaging, especially towards the molecular level, should allow more specific delineation of the CTV. The CTV is important because this volume must be adequately treated to achieve cure. The third volume, the planning target volume (PTV), allows for uncertainties in planning or treatment delivery. It is a geometric concept designed to ensure that the radiotherapy dose is actually delivered to the CTV. Radiotherapy planning must always consider critical normal tissue structures, known as organs at risk (ORs). In some specific circumstances, it is necessary to add a margin analogous to the PTV margin around an OR to ensure that the organ cannot receive a higher-than-safe dose; this gives a planning organ at risk volume. This applies to an organ such as the spinal cord, where damage to a small amount of normal tissue would produce a severe clinical manifestation. The concepts of GTV, CTV and PTV have been enormously helpful in developing modern radiotherapy. Attention to detail in radiotherapy planning is vital, and does affect outcomes: 'the devil is in the detail'. Radiotherapy planning is also dependent on high quality imaging, and the better the imaging the better will be the outcomes from radiotherapy.

Figure 1

Diagrammatic plot of tumour control probability (TCP) or normal tissue complication probability (NTCP) vs. radiotherapy dose. Sparing normal tissues shifts the NTCP curve to the right (B to C), allowing a lower incidence of normal tissue damage for the same dose (dose 1) or the same level of NTCP for a higher dose (dose 2). This is the basis for an improvement in the therapeutic ratio. This benefit can be the result of any measure which reduces the normal tissue dose, for example including better target imaging, conformal radiotherapy, and improved patient immobilisation.

Figure 2

The shape of the treatment volume from two techniques of radiotherapy planning. On the left is a cuboidal shape based on old-fashioned ‘square’ planning from orthogonal radiographs; on the right is a spherical shape produced from conformal planning. The two volumes are designed to treat the same tumour target, but the sphere is half the volume of the cube.

Figure 3

Diagram to illustrate the main radiotherapy planning volumes, taken from ICRU Report 50.

Figure 4

Planning volumes for a patient with WHO Grade 4 glioma (glioblastoma). (a) Planning CT showing contrast-enhancing tumour. (b) The GTV is the visible tumour. (c) A margin for microscopic spread has been added to make the CTV; the margin is the same in all directions except that it is restricted by the skull. (d) The PTV has been added outside the CTV to account for uncertainties in planning and execution of treatment; this extends beyond the inner table of the skull.

Figure 5

CT planning scan for a patient with a soft tissue sarcoma of the (anatomical) posterior compartment of the thigh. The tumour has been resected so no GTV exists. (a) The CTV is shown, restricted anteriorly by the femur and intermuscular septa. (b) The PTV has been added outside the CTV and in places extends beyond the outside of the patient.

Figure 6

Example of the value of image co-registration for radiotherapy planning, which allows planning based directly on the MRI data. (a) Radiotherapy planning CT scan showing right acoustic schwannoma. (b) Diagnostic MRI, which shows the schwannoma clearly. (c) Electronic co-registration of the diagnostic MRI with the planning CT within the radiotherapy planning system. The intersection point can be moved as necessary.