11.02 - ICRU Reports 50 and 62

ICRU report 83, which discusses volumes and guidelines when performing IMRT, advances some of these concepts further

The International Committee on Radiation Units and Measurements (ICRU) provides numerous guidelines for working with radiation. The ICRU reports 50 and 62 provide specific information for guiding radiation treatment. These recommendations suggest various volumes which are useful for planning radiation treatments, as well as a reference point where the dose should be calculated.

GTV - Gross Tumour Volume

The GTV is the volume that contains the visible or clinically detectable tumour. This may be on clinical examination or on imaging. This is the smallest of all volumes and is not present in every plan (eg. adjuvant radiotherapy following excision of the main tumour mass).

CTV - Clinical Target Volume

The CTV is the volume which has been determined to require radiation treatment. This includes the GTV as well as areas of clinical risk – such as lymph node groups or the region about the GTV that may have microscopic involvement. The CTV should be included in every plan.

ITV - Internal Target Volume

The ITV includes a margin to account for physiological patient movements that are unable to be accounted for during treatment. This may include movement of the gut, beating of the heart or respiration. The margin required is known as the internal margin (IM) and may vary in height, breadth and depth based on the location within the body.
The ITV is a newer concept that attempts to divide treatment inaccuracies into internal patient factors and external factors. If a method to reduce the effect of internal movements is used (eg. respirator gating) then the ITV can be substantially reduced.

PTV - Planning Target Volume

The PTV is an expansion from the ITV to account for external treatment inaccuracies. These may vary based on the department and the treatment site – for instance a treatment inaccuracy of 7 mm for body treatments and 3 mm for head and neck treatments. This distance is the external margin (EM). Improving the external factors which lead to treatment inaccuracies may reduce the external margin and allow for smaller PTV expansions.

OAR - Organs At Risk

Organs at risk are volumes placed on organs which are susceptible to radiation. They place constraints on the beam arrangement and dose that may be delivered. OARs may have different radiation tolerances based on the tissue involved.

ICRU Reference Point

The ICRU recommends reporting the dose at a single point within the PTV. The point should be clinically relevant, easily defined, and placed in a region of uniform dose (away from steep dose gradients or inhomogeneities if possible). The point should be at the center of the PTV and at the intersection of the beam axes if possible.