LUNG CARCINOMA-TARGET VOLUME DELINEATION

CT-based planning and motion management

• Computed tomography (CT)-based planning using conformal techniques and respiratory motion management is the standard of care in the treatment of both non–small cell lung cancer (NSCLC) and small cell lung cancer (SCLC).
• Common external beam techniques include:
  • Three-dimensional conformal radiation therapy (3D-CRT)
  • Intensity-modulated radiation therapy (IMRT)
  • Stereotactic body radiotherapy (SBRT)
• Each approach uses multiple beam angles and can vary in dose conformality; all require accurate delineation of target volumes, normal structures, and organs-at-risk (OARs) and careful evaluation of dose-volume histograms during planning.
• Understanding at-risk nodal levels of the mediastinum is essential; consensus atlases such as those developed at the University of Michigan provide guidance.

Simulation and motion assessment

• Simulation: Assessment of respiratory motion, correct patient positioning, and appropriate immobilization during radiation simulation are vital to radiation planning.
• Patients should ideally be simulated with their arms above their head to maximize the number of beam arrangements that can be used.
• A four-dimensional (4D) simulation should be performed to assess for internal motion whenever possible.

Organs at risk and additional structures

• In addition to the target volume, the following OARs should be contoured when in proximity to the treatment field:
  • Heart
  • Lungs
  • Spinal cord
  • Esophagus
  • Chest wall
  • Great vessels
  • Proximal bronchial tree (PBT)
  • Brachial plexus for superiorly located tumors or high paratracheal/supraclavicular lymph node involvement
• Available atlases should be used for accurate contouring of these structures.
• The liver should be contoured for right lower lobe tumors located close to the diaphragm.
• For low-lying left lower lobe tumors or left pleural tumors, the spleen may receive significant radiation and should be contoured.
• Consensus guidelines for contouring the brachial plexus are available and should be referenced as needed.

Field design and elective nodal irradiation

• For both NSCLC and SCLC in the setting of gross disease, an involved-field approach is widely accepted.
• This is based on prior publications demonstrating a low rate of failure in elective nodal regions and a randomized trial showing improved outcomes with involved-field versus elective nodal approaches.
• Therefore, elective nodal regions should generally not be routinely covered if the treating physician is confident in the understanding of the sites of active disease.

Target volume definition and expansion strategy

• Physicians should delineate targets using a combination of:
  • Physical examination
  • Contrast-enhanced CT scan
  • Positron emission tomography (PET)
  • Evaluation of the mediastinum with mediastinoscopy or endobronchial ultrasound (EBUS), when appropriate
• The gross target volume (GTV) is defined as macroscopic disease.
• There are two principal approaches for expanding GTV to subsequent target volumes:
  • Approach 1: Delineate the GTV and then assess for internal motion, creating an internal GTV (iGTV). The iGTV is expanded to create the internal CTV (iCTV), which is further expanded to yield the planning target volume (PTV).
  • Approach 2: Expand the GTV to a CTV margin to encompass microscopic disease, then expand to an internal target volume (ITV) to account for internal motion, followed by expansion to the PTV for daily variations in patient position and movement.
• The second approach is also used in the postoperative setting, where there may be no residual GTV or iGTV.

Typical margins and SBRT considerations

• For early-stage NSCLC treated with SBRT, standard treatment margins from the iGTV to the iCTV are often on the order of 0–0.2 cm.
• The proximal bronchial tree (PBT) should be consistently contoured for SBRT. The PBT includes:
  • The distal 2 cm of the trachea
  • The carina
  • The right and left mainstem bronchi
  • The lobar bronchi
• An area 2 cm beyond the PBT radially is sometimes defined as the “no-fly zone (NFZ)” and is important when considering SBRT fractionation for central lesions.
• Doses for SBRT are variable depending on tumor location and extent; a key criterion is achieving a biologically effective dose (BED) of >100 Gy.

Margins for locally advanced NSCLC and SCLC

• For locally advanced NSCLC (stage II–III), margins from the iGTV to the iCTV of 0.5–0.8 cm are typically used, based on prior histologic analyses.
• PTV margins depend on setup error and motion management:
  • Often 1.0–1.5 cm if there is no assessment of internal motion or no daily image-guided radiation therapy (IGRT).
  • Typically 0.5–1.0 cm if either 4D CT planning or CBCT / kV imaging is used daily.
• For SCLC, a standard GTV-to-CTV margin has not been well defined; margins of 0.5–1.0 cm are acceptable, often including the ipsilateral hilum.
• CTV-to-PTV margins for SCLC usually follow similar guidelines as for NSCLC, depending on motion assessment and IGRT technique.

Postoperative planning considerations

• In the postoperative scenario for NSCLC, there is no clear consensus on the extent of target delineation.
• Historically, large fields included the tumor bed, involved lymph node levels, bilateral mediastinum, ipsilateral bronchial stump, and supraclavicular lymph nodes for superiorly located tumors.
• With CT planning and comprehensive mediastinal lymph node dissection, many institutions now use more limited fields, focusing on:
  • Involved lymph node regions
  • The ipsilateral bronchial stump
  • Potential inclusion of one nodal level above and one below the involved region
• A CTV (no GTV present), an internal target volume (ITV) when motion is relevant, and a PTV with an ITV-to-PTV expansion of approximately 0.5 cm are commonly defined, depending on available IGRT techniques.

Dose guidance

• Standard treatment doses for NSCLC and SCLC across different scenarios are summarized in Table 13.1 (see next accordion).
• Dose constraints depend on total dose and number of fractions; further guidance is provided in QUANTEC (Quantitative Analyses of Normal Tissue Effects in the Clinic).

Appropriate radiation treatment regimens for lung cancer

Lung malignancy scenario	Accepted treatment doses
NSCLC, stage I stereotactic body radiation therapy (SBRT) — peripheral	Variable — commonly used regimens include 54 Gy in 18 Gy fractions, 48 Gy in 12 Gy fractions, 50 Gy in four fractions, and 50 Gy in five fractions.
NSCLC, stage I SBRT — central	Variable — examples include 50 Gy in five fractions, 70 Gy in ten fractions, and 60 Gy in eight fractions (more fractionated schedules used to reduce toxicity in central lesions).
NSCLC, stage II–III standard fractionation	60 Gy in 2 Gy fractions daily (conventionally fractionated chemoradiation standard).
Postoperative NSCLC	50–54 Gy in 1.8–2.0 Gy fractions — R0 resection 54–60 Gy in 1.8–2.0 Gy fractions — R1 resection 60 Gy in 2.0 Gy fractions (consider concurrent chemotherapy) — R2 resection
SCLC, limited stage	45 Gy in 1.5 Gy fractions twice daily (BID) with chemotherapy OR 66–70 Gy in 2.0 Gy fractions daily
SCLC, extensive stage (consolidative thoracic radiation)	30–45 Gy in 3.0 Gy fractions for consolidative chest radiation after systemic therapy.

Postoperative and SCLC-specific planning notes

• Postoperative NSCLC:
  • No universal consensus on field design; large historical fields have largely been replaced by limited fields focusing on involved lymph node regions and bronchial stump.
  • CTV, ITV, and PTV are defined with approximately 0.5 cm ITV-to-PTV margins depending on IGRT capabilities.
• SCLC:
  • Standard GTV-to-CTV margin is not well defined; margins of 0.5–1.0 cm are acceptable, often including the ipsilateral hilum.
  • Standard doses for limited-stage disease are 45 Gy in 1.5 Gy BID or 66–70 Gy in 2 Gy daily fractions.
  • For extensive-stage disease, consolidation thoracic doses generally range from 30 to 45 Gy in ten or more fractions.

Patterns of nodal spread

The table below summarizes the approximate frequency of involvement of individual mediastinal and hilar nodal stations according to the lobe involved by the primary lung tumor. Values are expressed as percentages of patients with nodal disease at that station.

Involved node station	Right Upper (n=45)	Right Middle / Lower (n=36)	Left Upper (n=35)	Left Lower (n=8)
Upper mediastinum (1)	9%	3%	0%	0%
Paratracheal (2)	40%	31%	3%	0%
Prevascular, retrotracheal, pretracheal (3)	73%	47%	29%	0%
Lower paratracheal (4)	36%	28%	17%	13%
Subaortic (5)	–	–	71%	13%
Para-aortic (6)	–	–	43%	25%
Subcarinal (7)	36%	69%	20%	38%
Paraesophageal (8)	9%	11%	3%	50%
Pulmonary ligament (9)	2%	6%	6%	13%

How to use this information in planning

• Recognize that patterns of nodal spread differ based on tumor lobe, which can guide selective inclusion of nodal stations when designing involved nodal fields.
• Right upper lobe tumors frequently involve paratracheal and prevascular/retrotracheal nodes (stations 2–3); left upper lobe tumors have high rates of subaortic (5) and para-aortic (6) involvement.
• Lower lobe tumors, particularly left lower lobe, show higher involvement of paraesophageal (8), subcarinal (7), and pulmonary ligament (9) stations.
• These patterns support targeted coverage rather than elective irradiation of all mediastinal stations in many patients.