RT225 Special Procedures pt 4

Intensity Modulated Radiotherapy

The 3-D CRT technique which relies on multiple beams with optimized intensity modulated distributions is referred to as __

• Isocentric linac in conjunction wtih a MIMIC collimator

• Tomotherapy unit

• Isocentric linac in conjunction with a multileaf collimator

Three techniques are currently available for IMRT dose delivery:

Inverse Planning

Optimal modulated intensity for each beam is determined through the dose optimization process referred to as ___ incorporating dose criteria not only for the target volume but also for the neighboring organs at risk

Fluence files

___ generated by the inverse planning algorithm are transmitted electronically to the linear accelerator which by means of computer control delivers the intensity modulated beams

• Analytical Methods

• Iterative Methods

Inverse planning computational methods fall into two broad categories:

Analytical Methods

Use a back projection algorithm to arrive at the fluence distribution from the desired dose distribution

Iterative Methods

Minimize a cost function which quantitatively represents the deviation from the desired goal

3-D forward planning

Patient data required for inverse planning is similar to that required for ___

• 3-D anatomic image data (mainly CT and MRI)

• Image registration

• Image segmentation

• Definition of the PTV

Planning for IMRT treatment requires the following:

• Maximum dose

• Minimum dose

• Dose-volume histogram

For each PTV the user enters the desired criteria for the plan, such as:

• Desired limiting dose

• Dose-volume histogram

For each critical structure (organ at risk) the user enters the following criteria:

• Complexity of software used for inverse treatment planning

• Complexity of equipment used for IMRT dose delivery

• QA issues related to dose distribution optimization process and IMRT dose delivery process

Routine clinical application of IMRT is still hindered by several difficulties such as:

• Improve dose homogeneity inside target volume

• Increased speed and lesser complexity in finding optimized dose distribution

• Quantitative introduction of cost functions

• Adjustment of optimal treatment planning to actual dose delivery technique

Inverse planning techniques from IMRT advantages over standard forward planning:

Static MLC

This MLC approach is:

• Simple concept

• Slow dose delivery (5min/field)

• Hard on MLC hardware

Dynamic MLC

This MLC approach is:

• Not intuitive

• Fast dose delivery (1min/field)

• Easier on MLC hardware

1960s

Rudimentary IMRT treatments have been used clinically since the ___ with wedges or custom-made phyical compensators

1990s

Modern IMRT became possible during the latter part of the ___

• Target localization

• Treatment planning

• Dose delivery

Recent advances in high precision radiotherapy resulted from significant advances in all three areas of importance to radiotherapy:

Molecular Imaging

In the near future, ___ will allow determination of intra-tumoural volumes that require treatment with extra high doses as a result of tumour hypoxia and low intrinsic radiosensitivity

• Inter-fraction target movement

• Intra-fraction target movement

• Set-up errors

Accuracy of the actual dose delivery is still limited by the uncertainty in target position at the time of treatment as a result of:

Image Guided Radiotherapy

Imaging of patient anatomy on the treatment machine just prior to each daily dose fraction

Image Guided Radiotherapy

Provides an accurate knowledge of the target location on a daily basis and helps with the daily patient set-up on the therapy machine

Cone beam computed tomography

Enables visualization of the exact tumour location just prior to patient treatment on a linac

CBCT

Integrates CT imaging with an isocentric linac and involves an acquisition of multiple planar images about the patient in the treatment position on the linac table

Adaptive Radiotherapy

In this, the dose delivery for subsequent treatment fractions of a course of radiotherapy can be modified to compensate for inaccuracies in dose delivery that cannot be corrected simply by adjusting the patient’s position like in the IGRT

Respiratory motion

Has a significant effect on the dose delivery to targets in the chest and upper abdominal cavities and to compensate for these effects relatively large margins are added to clinical tumour volumes

Organ motion during treatment

The next big challenge in IGRT comes from the natural and unavoidable ___

4-D Imaging

The account for natural organ motion ___ technology was developed

4-D Imaging Technology

Allows viewing of volumetric CT images changing over the fourth dimension: time

Respiratory Gating System (RGS)

A special accessory added to a linac to compensate automatically and instantly for the effects of respiratory movement on external beam radiotherapy to the chest and upper abdomen