3.3 ICRU Target Volume Definitions (GTV, CTV, PTV, PRV, OAR)

Standardized Target Volumes: ICRU Reports 50, 62, and 83

\nTo standardize prescription patterns, treatment reporting, and inter-institutional clinical trials, the International Commission on Radiation Units and Measurements established explicit definitions for target and normal tissue volumes in ICRU Report 50 (1993), ICRU Report 62 (1999), and ICRU Report 83 (2010) for IMRT. \nRadiation oncology plans do not simply target the visible tumor; they construct nested geometric expansions designed to capture microscopic disease, physiological motion, and daily positioning errors.

   +-------------------------------------------------------------+
   |  Planning Target Volume (PTV)                               |
   |   +-----------------------------------------------------+   |
   |   |  Internal Target Volume (ITV)                       |   |
   |   |   +---------------------------------------------+   |   |
   |   |   |  Clinical Target Volume (CTV)               |   |   |
   |   |   |   +-------------------------------------+   |   |   |
   |   |   |   |  Gross Tumor Volume (GTV)           |   |   |   |
   |   |   |   |   (Visible / Palpable Disease)      |   |   |   |
   |   |   |   +-------------------------------------+   |   |   |
   |   |   +---------------------------------------------+   |   |
   |   +-----------------------------------------------------+   |
   +-------------------------------------------------------------+

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Detailed ICRU Target Definitions

1. Gross Tumor Volume (GTV)

\nThe Gross Tumor Volume (GTV) is the demonstrable extent and location of the malignant growth. It consists of primary disease, metastatic regional lymph nodes, or distant metastases identifiable via physical examination or imaging modalities (CT, MRI, PET, ultrasound).

• Note: In post-operative patients who have undergone complete surgical resection, GTV = 0 (no visible tumor remains).

2. Clinical Target Volume (CTV)

\nThe Clinical Target Volume (CTV) is a tissue volume containing the GTV plus an anatomical expansion to encompass subclinical microscopic malignant disease. Microscopic spread cannot be imaged directly but is known to exist based on pathological patterns.

• Expansion Example: For a high-grade glioma, the CTV equals GTV + 1.5 cm to 2.0 cm margin for infiltrating tumor cells. For prostate adenocarcinoma, CTV includes the prostate gland plus the proximal seminal vesicles and pelvic lymph node chains.

3. Internal Target Volume (ITV)

\nIntroduced in ICRU Report 62, the Internal Target Volume (ITV) consists of the CTV plus an Internal Margin (IM). The internal margin compensates for expected physiological variations in target size, shape, and position within the patient during treatment (e.g., respiration, bowel peristalsis, rectal gas, bladder filling).

$\text{ITV} = \text{CTV} + \text{Internal Margin (IM)}$

4. Planning Target Volume (PTV)

\nThe Planning Target Volume (PTV) is a geometric concept designed to ensure that the prescribed radiation dose is delivered to the CTV/ITV despite physical variations. The PTV adds a Setup Margin (SM) to account for external positioning uncertainties, patient translation, linac calibration drift, and mechanical laser tolerances.

$\text{PTV} = \text{ITV} + \text{Setup Margin (SM)} = \text{CTV} + \text{IM} + \text{SM}$ \nVan Herk's margin formula is frequently used in clinical physics to compute optimal PTV margins based on systematic ($\Sigma$) and random ($\sigma$) setup uncertainties:

$\text{PTV Margin} = 2.5\Sigma + 0.7\sigma$

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Normal Tissue Volumes: OAR, PRV, Treated, and Irradiated Volumes

Organ at Risk (OAR)

\nAn Organ at Risk (OAR) is a normal tissue organ whose radiation sensitivity may significantly influence treatment planning or dose prescription. OARs are classified by radiobiological architecture:

• Serial Organs: Function is lost if a single subunit is disabled (e.g., spinal cord, brainstem, optic nerve). Max dose ($D_{\max}$) is critical.
• Parallel Organs: Function is maintained if a critical volume fraction remains undamaged (e.g., lung, liver, parotid glands). Mean dose ($D_{\text{mean}}$) or volume constraints ($V_{20}$) are critical.

Planning Organ at Risk Volume (PRV)

\nAnalogous to the PTV for target volumes, a Planning Organ at Risk Volume (PRV) adds a safety margin around an OAR to account for setup variability and internal organ movement. For example, a 3 mm PRV expansion around the spinal cord ensures that positioning shifts do not push the high-dose PTV gradient into the actual spinal cord tissue.

Treated Volume and Irradiated Volume

• Treated Volume: The volume of tissue enclosed by an isodose surface (e.g., 95% isodose line) selected as appropriate to achieve the therapeutic purpose.
• Irradiated Volume: The volume of tissue receiving a dose that is significant in relation to normal tissue tolerance (typically the 50% isodose volume).

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ICRU 83 Dose Prescription and Reporting Metrics

\nIn modern 3D-CRT and IMRT planning, ICRU 83 mandates reporting specific dose-volume parameters from the Cumulative Dose-Volume Histogram (DVH):

• $D_{50%}$ (Median Dose): Dose received by 50% of the target volume. Represents the mean prescription dose.
• $D_{98%}$ (Near-Minimum Dose): Dose received by 98% of the PTV. Defines target coverage adequacy.
• $D_{2%}$ (Near-Maximum Dose): Dose received by 2% of the PTV. Defines hot spots within the target volume.
• Conformity Index (CI): Measures how closely the prescription isodose volume matches the PTV contour.