RT225 Special Procedures pt 3

Intraoperative Radiotherapy (IORT)

A special radiotherapeutic technique that delivers in a single session a radiation dose of the order of 10-20 Gy to a surgically exposed internal organ, tumour, or tumor bed

Surgery and Radiotherapy

IORT combines two conventional modalities of cancer treatment: __ and __

• Surgeon

• Radiation oncologist

• Medical physicist

• Anaestesiologist

• Nurse

• Pathologist

• Radiation Therapist

IORT team consists of:

• Orthovoltage x-rays

• Megavoltage electrons

• High dose rate brachytherapy with iridium-192 source

Three beams are used for IORT:

Chemotherapy, external beam radiotherapy

IORT is often applied as part of a treatment protocol that includes other modalities such as __ and __

Shrink, Simpler

The initial treatment attempts to ___, making the subsequent surgical resection ___

Significant Morbidity

When a surgical resection of a residual tumour mass is finally attempted, it may happen that not all the tumour can be removed without ___

Gastrointestinal Cancers

The largest clinical experience with IORT has been with ___ in adults

Retroperitoneum

Historically, tissues within the ___, including the pancreas, rectum ,and stomach have been most commonly treated with IORT, and, on a smaller scale, bladder, breast and gynecological malignancies

Large radiation dose

To improve local and regional control, a ___ is delivered during the surgical procedure

Decrease normal tissue toxicity

Main biologic advantage of the IORT is its ability to ___ by displacing or shielding sensitive structures from the radiation beam

A single session

Main disadvantage of IORT is the dose delivery in ___

Operating Room, Treatment Room

IORT procedure requires an __ for the surgical procedure, and a __ for delivery of dose

• Define the target area

• Shield tissues outside the target area from radiation

• Keep sensitive tissues from the target area during irradiation

Applicators are important for three reasons:

Applicator System

Once a radiation modality and the location in which the treatment unit is to be installed are selected, an __ must be obtained

• Orthovoltage x-rays

• Megavoltage electron beams

• High dose rate (HDR) iridium-192 brachytherapy sources

Three modalities that may be used to deliver IORT:

Radiation parameters, radiation modality

Before clinical implementation of an IORT program, an extensive characterization of ___ of the ___ muse be carried out and documented

Dosimetry data

Must be summarized so that it can be quickly understood and readily used

Electron beams

Most IORT programs today are based on ___ produced by megavoltage linacs, since they provide several advantages over x-rays for the purpose of IORT

Conformal Radiotherapy

Shapes the prescription dose volume to the planning target volume (PTV) while at the same time keeping the dose to specified organs at risk below their tolerance dose

Three-dimensional conformal radiotherapy (3-D CRT)

Is based on 3-D anatomic information and the use of dose distributions that conform tightly to the target volume

Tumouricidal dose

3-D CRT must provide a ___ to the tumour and a limited dose to critical organs

• Maximized tumour control probability (TCP)

• Minimized normal tissue complication probability (NTCP)

Conformal dose delivery has two main objectives:

• Anatomical Imaging

• Functional Imaging

Conformal radiotherapy target localization is achieved through:

• Forward treatment planning

• Inverse treatment planning

Conformal radiotherapy treatment planning is achieved through:

Forward treatment planning

Techniques which design uniform intensity beams shaped to the geometrical projection of the target

Inverse treatment planning

Techniques which, in addition to beam shaping, use intensity modulated beams to improve target dose homogeneity and to spare organs at risk

Multileaf Collimators

Modern dose delivery techniques range from the use of standard regular and uniform coplanar beams to intensity modulated non-coplanar beams produced with ____

• Acquisition of anatomic information in the form of transverse (axial) images

• Determination of the planning target volume (PTV) by the radiation oncologist by contouring the PTV on each individual axial image (segmentation process)

• Design of radiation fields using the beam’s-eye-view option in the treatment planning software

• Optimization of the treatment plan through the design of optimal field sizes, beam directions, beam energies, etc.

Treatment planning process for 3-D CRT consists of 4 steps:

20-60 pairs, tungsten

Modern linacs are equipped with MLCs that incorporate from __ to __ pairs of narrow, closely abutting __ leaves

5 and 10mm

In an MLC each tungsten leaf is projecting a typical width of between ___ and ___ at the linac isocentre

microMLCs

MLCs projecting leaf widths less than 5mm at the isocentre are referred to as __

• Shaping irregular fields of less than 10cm in maximal field dimension, such as head and neck fields

• Irregular fields with less than 3cm in largest dimension that are used in radiosurgery

microMLCs are used for:

17 g/cm³ to 18.5 g/cm³

Density of tungsten alloy leaves is __ to __

6cm to 7.5cm

Thickness of the leaves along the beam direction is __ to __

2%, 1%

Primary transmission through tungsten leaves is less than __, in comparison with about __ for linac jaws

1mm

Each leaf of a MLC is individually motorized and computer controlled, allowing positioning with an accuracy better than __ and the generation of irregular radiation fields, shaped to conform to the beam’s eye view (BEV) target cross section

DC motor

Each MLC leaf is driven by a separate, miniature __

Uniform Intensity Across the field

Radiation beams in standard external beam radiotherapy including 3-D conformal radiotherapy usually have a __, fulfilling the linac flatness and symmetry specifications

• Compensating for contour irregularities

• Compensating for tissue inhomogeneities

• Compensating for highly irregular target volumes

• Sparing organs at risk located in the vicinity of target volume

Non-uniform beam intensities (intensity modulation) can be used to improve dose distributions by: