# Target Volume Delineation: GTV, CTV, PTV Concepts (ICRU Reports)

## Overview

Accurate delineation of target volumes is the most critical physician-dependent step in radiation therapy planning. The International Commission on Radiation Units and Measurements (ICRU) has established a standardized framework through a series of reports that define treatment volumes using a nested volume system. This system places the Gross Tumor Volume (GTV) inside the Clinical Target Volume (CTV), which in turn is inside the Planning Target Volume (PTV). Each margin between these volumes accounts for a specific source of uncertainty. It is important to recognize that errors in target delineation cannot be corrected by any downstream processes; no amount of plan optimization, image guidance, or adaptive therapy can compensate for an initially incorrect target definition.

## ICRU Report Evolution

### ICRU 50 (1993)

The ICRU 50 report introduced the foundational framework of GTV, CTV, and PTV for three-dimensional conformal radiation therapy. It defined the treated volume and irradiated volume and established the principle that margins should account for both internal organ motion and external setup uncertainty.

### ICRU 62 (1999)

Building on the previous report, ICRU 62 introduced the concept of the Internal Margin (IM) to separately account for internal organ motion and deformation. This led to the definition of the Internal Target Volume (ITV), which equals the CTV plus the IM. The PTV was then defined as the ITV plus the Setup Margin (SM). This refinement allowed for a more rigorous analysis of the different sources of uncertainty in treatment planning.

### ICRU 83 (2010)

ICRU 83 addressed considerations specific to intensity-modulated radiation therapy (IMRT). It introduced dose reporting recommendations tailored for IMRT, including near-minimum dose (D98%), near-maximum dose (D2%), and median dose (D50%), moving away from point-based reporting. The report emphasized that the PTV is a geometric construct used for planning purposes and is not a biological volume. It also recommended reporting dose to the CTV whenever possible to better reflect the biological target.

### ICRU 91 (2021)

The most recent report, ICRU 91, updated recommendations for prescribing, recording, and reporting proton therapy. It addressed unique challenges such as range uncertainty, relative biological effectiveness (RBE), and robust optimization within the context of volume definitions. Additionally, it introduced concepts specific to particle therapy planning, reflecting advances in radiation oncology technology.

| ICRU Report | Year | Key Contributions |
|---|---|---|
| ICRU 50 | 1993 | Introduced GTV, CTV, PTV framework for 3D-CRT |
| ICRU 62 | 1999 | Added Internal Margin (IM) and ITV concepts; separated internal motion from setup error |
| ICRU 83 | 2010 | IMRT-specific reporting (D98%, D2%, D50%); PTV as geometric construct |
| ICRU 91 | 2021 | Proton therapy reporting; range uncertainty, RBE, robust optimization |

## Gross Tumor Volume (GTV)

### Definition

The Gross Tumor Volume (GTV) represents the gross demonstrable extent and location of the tumor. It corresponds to what can be seen, palpated, or imaged, and it contains the highest density of tumor cells.

### Delineation Principles

GTV delineation relies on all available imaging modalities, including CT, MRI, PET/CT, and clinical examination. Different imaging techniques may define different aspects of the GTV; for example, MRI is often superior for delineating soft tissue extent, while PET highlights metabolically active disease. Fusion of multimodality imaging enhances the accuracy of GTV definition. The GTV should be contoured on the imaging modality that provides the best soft tissue contrast for the specific site. In the postoperative setting, there is no GTV because the tumor has been removed; treatment targets the tumor bed, which corresponds to the CTV only.

### Common Pitfalls

Several pitfalls can affect GTV delineation. For lung tumors, contouring on lung window versus mediastinal window settings can change the apparent GTV boundary. Post-biopsy edema or atelectasis may be mistaken for tumor, leading to overestimation. Failure to incorporate all available imaging, such as neglecting to fuse diagnostic MRI for brain tumors, can reduce accuracy. Additionally, the selection of standardized uptake value (SUV) thresholds for PET-based GTV is not standardized and can significantly alter the volume delineated.

## Clinical Target Volume (CTV)

### Definition

The Clinical Target Volume (CTV) includes the GTV plus a margin to encompass subclinical microscopic disease that cannot be detected on imaging. It represents the tissue that must be treated to achieve the therapeutic goal. Unlike the GTV, the CTV is a clinical and biological concept that requires oncologic judgment.

### Determining CTV Margins

CTV margins are determined based on knowledge of tumor biology, natural history, and patterns of spread. Factors influencing the margin around the GTV include the histologic type and grade, with high-grade tumors often requiring larger margins. The pattern of local invasion, such as perineural, perivascular, or submucosal spread, also guides margin size. Anatomic barriers to tumor spread, including fascia, bone, and meninges, must be respected. Surgical findings and pathological data further inform margin decisions. The CTV should not extend beyond anatomic boundaries that the tumor does not cross, such as intact bone cortex, uninvolved fascial planes, or pleural surfaces, and should avoid extending into air cavities or through intact anatomic barriers.

### Elective Nodal CTV

In many disease sites, the CTV includes regional lymph node stations at risk for subclinical involvement. The nodal CTV is based on known patterns of lymphatic drainage and the risk of occult nodal disease. Consensus contouring atlases exist for most major sites, including head and neck, breast, cervix, and rectum. The decision to include elective nodal volumes depends on the probability of subclinical involvement, generally using a threshold of greater than 5-10%.

### CTV in the Postoperative Setting

After complete surgical resection, no GTV exists. The CTV in this context encompasses the surgical bed plus areas at risk for residual microscopic disease. This delineation is guided by surgical and pathology reports, including margin status, extracapsular extension, perineural invasion, and lymphovascular invasion. Preoperative imaging is essential for defining the original tumor extent to inform postoperative CTV delineation.

## Planning Target Volume (PTV)

### Definition

The Planning Target Volume (PTV) is a geometric concept designed to ensure that the prescribed dose is delivered to the CTV despite geometric uncertainties. It is defined as the CTV plus margins accounting for setup error and internal motion, if not separately accounted for. The PTV ensures that the CTV is covered by the prescribed dose with an acceptable probability.

### Sources of Uncertainty Addressed by PTV

The PTV margin accounts for several sources of uncertainty. Setup error refers to day-to-day variations in patient positioning relative to the treatment beams, which include systematic errors (mean offsets) and random errors (daily variations around the mean). These errors are reduced by image guidance (IGRT) and immobilization devices. Internal organ motion involves physiological movement of the CTV relative to bony anatomy, such as respiratory motion, cardiac motion, peristalsis, and changes in bladder or rectal filling. Internal motion is addressed by the Internal Target Volume (ITV), gating, breath-hold techniques, or adaptive strategies. Delineation uncertainty, which arises from interobserver variability in CTV contouring, is a significant source of error but is not formally included in the margin recipe.

### Margin Recipes

The most widely used margin recipe is the van Herk formula, which calculates the PTV margin as 2.5 times the systematic error plus 0.7 times the random error. This formula ensures that 90% of the patient population receives at least 95% of the prescribed dose to the CTV. Systematic errors are more detrimental than random errors, which explains the heavier weighting in the formula. Institutional margin protocols should be based on measured setup uncertainties specific to the immobilization and IGRT strategy used. Typical PTV margins with daily cone-beam CT (CBCT) IGRT vary by site: brain stereotactic radiosurgery (SRS) uses 0-2 mm, head and neck treatments use 3-5 mm, lung stereotactic body radiation therapy (SBRT) uses 5 mm when using the ITV approach, prostate margins range from 3-7 mm depending on IGRT strategy and rectal spacer use, and pelvis treatments commonly use 5-10 mm.

| Site | Typical PTV Margin (with daily CBCT) | Notes |
|---|---|---|
| Brain SRS | 0–2 mm | Frameless mask with intrafraction monitoring |
| Head and neck | 3–5 mm | Thermoplastic mask immobilization |
| Lung SBRT | 5 mm (around ITV) | ITV accounts for respiratory motion |
| Prostate | 3–7 mm | Varies with IGRT frequency and rectal spacer |
| Pelvis | 5–10 mm | Depends on bladder/rectal filling consistency |

## Related Volumes

### Internal Target Volume (ITV)

The Internal Target Volume (ITV) is defined as the CTV plus an Internal Margin to account for organ motion. It is most commonly used for lung and liver targets subject to respiratory motion. The ITV is defined using four-dimensional CT (4D-CT) by taking the union of CTV positions across all respiratory phases. When the ITV is used, the PTV margin only accounts for setup error, as internal motion is already incorporated.

### Treated Volume

The treated volume is the volume enclosed by the prescription isodose surface. Ideally, it conforms closely to the PTV, but it may extend beyond it or result in undercoverage depending on the dose distribution.

### Irradiated Volume

The irradiated volume refers to the volume of tissue receiving a dose considered significant in relation to normal tissue tolerance. It is typically larger than the treated volume due to dose falloff gradients.

### Organs at Risk (OARs)

Organs at risk are normal tissues whose radiation sensitivity influences treatment planning or dose prescription. Serial organs, such as the spinal cord and brainstem, depend on the integrity of all subunits for function, whereas parallel organs, such as the lung and liver, maintain function until a threshold volume is damaged. The Planning Organ at Risk Volume (PRV) is defined as the OAR plus a margin for uncertainty, analogous to the PTV concept applied to OARs.

<image>A nested volume diagram showing the concentric relationship between GTV (innermost, solid tumor), CTV (GTV plus microscopic extension margin), ITV (CTV plus internal motion margin), and PTV (ITV/CTV plus setup margin). Each margin layer is color-coded with labels indicating the source of uncertainty it addresses. Arrows show that the GTV-to-CTV margin is a clinical/biological decision while the CTV-to-PTV margin is a physical/geometric calculation.</image>

<image>An axial CT slice of a head and neck cancer case showing the GTV (contoured in red on the primary tumor), CTV (blue, including the GTV with a margin that respects anatomic barriers such as bone and air), and PTV (green, uniform expansion around CTV). Adjacent panels show the same slice with the elective nodal CTV contoured based on consensus guidelines, covering levels II-IV lymph node stations. Labels identify each volume and the rationale for the margin at each interface.</image>

<image>A schematic illustration of the van Herk margin recipe concept. The top panel shows a population of patients with systematic setup errors (each patient has a different mean offset) and random errors (daily variation around the mean). The bottom panel shows how the PTV margin formula (2.5 Sigma + 0.7 sigma) ensures that the minimum dose to the CTV exceeds 95% of the prescription for 90% of the population. Arrows connect the statistical concepts to the margin calculation.</image>

## Key Clinical Pearls

The Clinical Target Volume (CTV) is the most important contour to draw because it embodies oncologic judgment about where disease exists and where it might extend; all other volumes are geometric and physical constructs. Interobserver variability in target delineation represents the largest single source of uncertainty in radiation therapy, often exceeding setup error. Therefore, participation in peer review contouring sessions and use of consensus atlases are essential. Planning Target Volume (PTV) margins should never be applied uniformly without understanding the specific immobilization and image guidance strategy; for example, a 7 mm margin appropriate with weekly port films may be excessive with daily CBCT, unnecessarily increasing normal tissue dose. In IMRT and VMAT planning, steep dose gradients at the PTV edge mean that small errors in PTV definition have larger dosimetric consequences than in 3D conformal radiation therapy, making accurate delineation even more critical. The PTV is not a biological volume and should not be used for dose reporting or outcome analysis; instead, CTV coverage metrics such as D95, D98, and V95 should be reported whenever possible. When contouring the CTV, always ask whether you would take the same margin surgically; CTV margins should reflect oncologic reality rather than arbitrary geometric expansions.

## References

- ICRU Report 50. "Prescribing, Recording, and Reporting Photon Beam Therapy." 1993.  
- ICRU Report 62. "Prescribing, Recording, and Reporting Photon Beam Therapy (Supplement to ICRU Report 50)." 1999.  
- ICRU Report 83. "Prescribing, Recording, and Reporting Photon-Beam Intensity-Modulated Radiation Therapy (IMRT)." 2010.  
- ICRU Report 91. "Prescribing, Recording, and Reporting of Proton-Beam Therapy." 2021.  
- van Herk M. "Errors and margins in radiotherapy." *Semin Radiat Oncol*. 2004;14(1):52-64.  
- Vinod SK et al. "Uncertainties in volume delineation in radiation oncology: a systematic review and recommendations for future studies." *Radiother Oncol*. 2016;121(2):169-179.