Clinical target volume

Clinical target volume (CTV) contains the GTV when present and/or subclinical microscopic disease that has to be eradicated to cure the tumour. CTV definition is based on histological examination of post mortem or surgical specimens assessing extent of tumour cell spread around the gross GTV, as described by Holland et al. (1985) for breast cancer. The GTV-CTV margin is also derived from biological characteristics of the tumour, local recurrence patterns and experience of the radiation oncologist. A CTV containing a primary tumour may lie in continuity with a nodal GTV/CTV to create a CTV-TN (e.g. tonsillar tumour and ipsilateral cervical nodes). When a potentially involved adjacent lymph node which may require elective irradiation lies at a distance from the primary tumour, separate CTV-T and CTV-N are used (Fig. 2.2), e.g. an anal tumour and the inguinal nodes. CTV can be denoted by the dose level prescribed, as for example, CTV-T50 for a particular CTV given 50Gy. For treatment of breast cancer, three CTVs may be used for an individual patient: CTV-T50 (50Gy is prescribed to the whole breast); CTV-T66 (66Gy to the tumour bed); and CTV-N50 (50Gy to regional lymph nodes). Variation in CTV delineation by the clinician (‘doctor’s delineation error’) is the greatest geometrical uncertainty in the whole treatment process. Studies comparing outlining by radiologists and oncologists have shown a significant inter-observer variability for both the GTV and/or CTV at a variety of tumour sites. This is greater than any intra-observer variation. Published results for nasopharynx, brain, lung, prostate, medulloblastoma and breast all show significant discrepancies in the volumes outlined by different clinicians. Improvements can be made with training in radiological anatomy which enables clinicians to distinguish blood vessels from lymph nodes and to identify structures accurately on computed tomography (CT) and magnetic resonance imaging (MRI). Joint outlining by radiologists and oncologists can improve consistency and ensure accurate interpretation of imaging of the GTV. Consensus guidelines such as those for defining CTV for head and neck nodes (Gregoire et al. 2000) and pelvic nodes (Taylor et al. 2005) have improved CTV delineation greatly. Protocols for outlining GTV and CTV at all tumour sites are needed and suggestions are made in each individual chapter.

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