What are the types of Intensity-Modulated Radiation Therapy (IMRT)?

Types of Intensity-Modulated Radiation Therapy (IMRT)

IMRT can be delivered through three main technical approaches: Simultaneous Integrated Boost (SIB), Sequential (SEQ), and delivery methods including step-and-shoot, sliding window, and Volumetric Modulated Arc Therapy (VMAT), with the choice depending on tumor location, fractionation schedule, and institutional expertise. 1, 2

Primary IMRT Fractionation Techniques

Simultaneous Integrated Boost (SIB)

• Uses differential "dose painting" throughout the entire treatment course, delivering 66-74 Gy to gross disease and 50-60 Gy to subclinical disease in each fraction 1
• Commonly applied with conventional fractionation (5 fractions/week) and accelerated schedules (6 fractions/week) 1
• Allows for dose escalation to high-risk areas while treating subclinical disease simultaneously in a single daily treatment 1

Sequential (SEQ) IMRT

• Delivers an initial lower-dose phase (weeks 1-5) followed by a high-dose boost volume phase (weeks 6-7) using 2-3 separate dose plans 1
• Commonly applied in standard fractionation and hyperfractionation schedules 1
• Provides flexibility in adjusting treatment volumes between phases based on tumor response 1

Concomitant Boost Accelerated

• Uses a "modified SEQ" approach delivering subclinical target doses once daily for 6 weeks, with a separate boost plan as a second daily fraction for the last 12 treatment days 1
• Allows for treatment intensification in the final phase without extending overall treatment time 1

IMRT Delivery Techniques

Fixed Gantry Methods

Step-and-Shoot (S&S)

• Delivers radiation with the gantry stationary at multiple fixed beam angles, with the multileaf collimator reshaping between segments 3, 4
• Considered easier to master with fewer variable parameters compared to rotational techniques 3
• Provides reliable dose distribution with well-established quality assurance protocols 3

Sliding Window (SW)

• Uses dynamic multileaf collimator movement while the beam is on at fixed gantry positions 3, 5
• May result in slightly higher doses to organs at risk compared to step-and-shoot, though differences are typically small 5
• Requires similar monitor units to step-and-shoot delivery 5

Rotational Techniques

Volumetric Modulated Arc Therapy (VMAT)

• The radiation source rotates around the patient in one or more arcs while continuously delivering radiation with dynamic multileaf collimator adjustments 2, 3
• Reduces treatment delivery time to less than 5 minutes and decreases monitor units by at least 40% compared to fixed gantry techniques 3, 5
• Provides superior high-dose sparing but increases low-dose bath to normal tissues 2
• For lung cancer specifically, decreases V20 dosimetric values to non-target lung, which is the primary predictor of fatal pneumonitis 2

Intensity Modulated Arc Therapy (IMAT)

• An earlier form of rotational IMRT that preceded VMAT development 3
• Less commonly used in current practice compared to VMAT 3

Clinical Application Considerations

Site-Specific Recommendations

• IMRT has proven benefit in reducing long-term toxicity for oropharyngeal, paranasal sinus, and nasopharyngeal cancers by reducing dose to salivary glands, temporal lobes, auditory structures, and optic structures 1
• Application to oral cavity, larynx, hypopharynx, and salivary glands is evolving and may be used at physician discretion 1
• For thymic malignancies, IMRT may further improve dose distribution and decrease normal tissue dose when ASTRO IMRT guidelines are strictly followed 1

Technical Selection Algorithm

• Choose VMAT when high-dose organ sparing is the priority, treatment efficiency is important, or contralateral lung sparing is critical 2
• Select step-and-shoot when institutional experience is limited with rotational techniques or when simpler quality assurance is preferred 3
• The experience of the treatment center with a given technique is most relevant with these complex treatment modalities 2

Critical Implementation Requirements

• CT-based planning is highly recommended with scans taken in treatment position 1
• Target delineation requires experience in imaging and thorough understanding of disease spread patterns 1
• Standards for target definition, dose specification, and normal tissue constraints continue to evolve 1
• Close interdisciplinary cooperation is critical for treatment planning, especially in postoperative settings or after induction chemotherapy 1

Common Pitfalls to Avoid

• VMAT is more difficult to master than step-and-shoot because of more variable parameters requiring careful quality control 3
• Avoid applying IMRT without adequate physics support and physician training in the specific technique 1
• Do not assume all IMRT delivery methods produce identical results—VMAT increases low-dose spread to normal tissues despite superior high-dose sparing 2
• Ensure proper quality controls are implemented for the specific delivery technique chosen, as requirements differ between fixed gantry and rotational methods 3