In radiotherapy planning, the 'planning target volume' (PTV) is defined as the:
- A Gross tumour volume as visible on imaging
- B Volume of tissue receiving 50% of the prescribed dose
- C Clinical target volume plus a margin for set-up uncertainties and patient/organ motion ✓
- D Region of subclinical microscopic tumour extension beyond the GTV
Correct answer: C. Clinical target volume plus a margin for set-up uncertainties and patient/organ motion
Explanation
The ICRU (International Commission on Radiation Units) defines volumes hierarchically: GTV (gross tumour visible on imaging) → CTV (GTV + margin for subclinical spread) → PTV (CTV + setup/motion margins). The PTV is a geometrical construct accounting for patient positioning uncertainties and internal organ motion to ensure the CTV reliably receives the prescribed dose. It is not a biological volume itself. The 50% isodose line defines the isodose curve, not the PTV.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.