External beam radiotherapy

Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)


Definitive radiotherapy

Many cancers can be cured using radiation as the primary modality, with or without concurrent chemotherapy (See Table 8.3). Concurrent chemotherapy is chosen to increase radiosensitivity when the tumor is large, radioresistant, or near critical structures. Use of definitive radiation allows for organ and tissue preservation, particularly for head and neck and skin cancers, by replacing potentially disfiguring surgeries.

Adjuvant radiotherapy

Radiation is given after primary surgery (e.g., breast cancer, prostate cancer) or chemotherapy (e.g., lymphoma) to eradicate microscopic residual disease. The dose required to treat adjuvantly is less than that required in the definitive setting because the tumor burden is less.

Neoadjuvant radiotherapy

Radiation PRIOR to primary treatment is most often used to reduce tumor size before surgery and improve the chance of a complete resection. In situations where adjuvant radiation would be generally recommended, radiation before surgery can often be per formed with less toxicity, because smaller fields can be used and the tumor itself pushes normal tissue out of the radiation field.

Neoadjuvant radiation has been used with concurrent chemotherapy for GI malignancies and locally advanced breast cancers. Neoadjuvant radiotherapy is commonly used for sarcomas to reduce the need for amputation.

Palliative radiotherapy

radiation is an excellent modality for relief of symptoms caused by metastatic or locally progressive disease. Brain metastases are commonly treated with radiation, which does not have the same blood-brain barrier limitations as chemotherapy. It is also commonly used for painful bone metastases or lesions impinging on vital organ functions, such as bronchial obstruction.

Treatment planning

Beam dosimetry

Before clinical use, the physical proper ties of the radiation beam must first be determined. Proper ties include output over time, dose at different depths, beam symmetr y, and the penumbra (region of dose buildup beginning at the beam edge, typically 0.5 –1 cm). These factors are used to calculate irradiation time required for a specific dose given.

Patient positioning, immobilization, and localization

Customizing radiation treatment to a particular tumor requires that the tumor position be reliably reproduced each day of treatment. During the planning process, immobilization devices are used to reduce daily “setup error.” For treatment requiring ver y tight margins because of normal tissue constraints, stereotactic frames, implanted fiducial markers, or daily pretreatment imaging of the target (by ultrasound or CT) can be used to determine the exact location of the tumor before treatment.

Target definition

For super ficial tumors, the target can be defined clinically (i.e., skin cancers). For most cancers, however, imaging is used to ensure the tumor and areas at risk for microscopic spread are included in the radiation field. Imaging modalities can include:

  • Fluoroscopy: bony anatomical markers used, e.g., bone metastases
  • CT: structures “contoured” in planning system, e.g., lung, breast
  • MRI: improved anatomical definition, e.g., CNS, prostate, sarcoma
  • PET: improved staging, e.g., GI, lung

The radiation oncologist then defines organs to be avoided and several target volumes. This can be done in a planning software system or directly onto film:

  • GTV: gross tumor volume, clinically apparent disease
  • CTV: clinical target volume, GTV + expansion for microscopic disease based on known patterns of spread, often ~1 cm
  • PTV: planning treatment volume, CTV + expansion allowing for daily target motion and setup uncertainty. This is 0.5 –1 cm, depending on tumor location and immobilization techniques.

Field design

The direction and number of fields are chosen to optimize coverage of the target volume while ensuring that normal structures do not exceed dose tolerance limits. Multiple beam plans can be used to provide dose sculpting around the target and avoid normal structures. Increasing the number of fields increases the complexity of the calculation and can extend the daily treatment time. Field borders are chosen to include the PTV with additional consideration of the beam’s penumbra. Blocks are used to shape the beam and protect normal tissues.

Dose calculation

The complexity of planning calculations depends on the type of plan selected by the radiation oncologist and is guided by location of the tumor, nearby normal tissue tolerances, and the goal of treatment. Image-based planning creates a topographical map of dose, called an isodose plan, which shows coverage of the target and normal structures.

Intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) is a technology using non-uniform beam intensity within each field. Combined with multiple beam angles, the modulation of dose given within each field can sculpt dose around target and normal structures.

This technology increases the volume of tissue receiving a low dose, but it can dramatically reduce the volume of normal structures receiving a high dose of radiation. Treatment time is often increased because the beam is on for a longer period of time per field.

Quality assurance

If patient treatment is planned on a software system, fields are verified and marked on the patient before beginning treatment. Positioning is rechecked intermittently throughout the treatment to verif y correct positioning (called “portal imaging”). IMRT and stereotactic treatment require additional quality assurance per formed by medical physicists.

Oxford American Handbook of Oncology-Oxford University Press (2015) F8.2

Figure 8.2. Example of an IMRT plan for a left-sided orophar yngeal head and neck cancer. The primary tumor (red) is covered by the 70 Gy isodose line (green). Nodal areas at risk (orange) receive at least 50 Gy (brown line). Note significant sparing of the right parotid (green), with the 60 (blue), 35 (pink), and 21 Gy (light green) isodose lines curving to avoid the structure. Right parotid is contoured in yellow.

Table 8.3. Typical external beam radiation treatment for specific disease sites

Site Stage Treatment
Brain High-grade astrocytoma Surgery, then concurrent chemo-RT to 60 Gy
Head and neck Early stage N0 Surgery or RT to 64–70 Gy
Advanced stage Chemo-RT to 70 Gy or Surgery, then chemo-RT to 60–66 Gy
Breast Early stage (T1–2 N0) Modified radical mastectomy or lumpectomy + 60 Gy to breast. Hormonal and/or chemotherapy
Advanced stage (T3 or N+) Surgery. Chemotherapy. RT 60 Gy to breast/chest wall and regional lymph nodes
Lung Early stage (T1–2 N0) Surgery or RT 60 –70 Gy to tumor
Advanced stage (N2+) Preoperative chemotherapy ± 45 –50 Gy or definitive chemo-RT to 60 –70 Gy
Esophagus Preoperative chemo-RT to 50 Gy
Stomach Postoperative chemo-RT to 45 Gy
Pancreas Preoperative or definitive chemo-RT to 50 Gy
Rectum Preoperative chemo-RT to 50 Gy
Anus Chemo-RT to 54 Gy
Hodgkin lymphoma Stage I – II Chemotherapy then involved field RT 20 –30 Gy
Non-Hodgkin lymphoma Stage I – II Chemotherapy then involved field RT 30 Gy
Prostate Prostatectomy or brachytherapy or definitive radiation 74 –78 Gy ± androgen deprivation therapy

 

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