Radiation Therapy

Radiation therapy is the delivery of a controlled and measured dose of ionizing radiation to the lung cancer.  There are two approaches to the delivery of radiation therapy:

·      External Beam Radiation Therapy:  

The radiation is directed into the tumor from a source that is outside the patient’s body.

Various types of radiation therapy can be used.  These include X-rays (photons) or particle therapy (neutrons, electrons, protons).

·      Brachytherapy:

The radiation is delivered to the tumor from a radioactive source(s) that is placed into the patient’s body.


MODERN EXTERNAL BEAM RADIOTHERAPY 

DNA Strand Breaks Rizwan Nurani

DNA Strand breaks in cancer cells induced by radiation

In the last twenty years there have been rapid advances in the role of radiation oncology in the treatment of lung cancer.  The vast majority of lung cancers treated with radiation therapy are treated with an external beam approach.  A machine called the medical linear accelerator is used to generate high energy x-rays.  The x-rays are then directed through the head of the machine to the cancer target.  When the x-rays reach the target they create DNA strand breaks within the cancer cells.  Cancer cells have a reduced ability to repair this damage and thus the damaged DNA causes the death of the cancer cell.  Adjacent normal cells that are in the path of the radiation can also experience breaks in their DNA.  Normal cells have a more robust DNA repair capability and the vast majority of this damage is repaired.   To take advantage of the different repair capabilities of normal and cancer cells, radiation therapy is broken down into smaller daily doses of treatment.  With the smaller dose, normal cells can more completely repair their DNA in between the daily treatments and thus keep the likelihood of long term side-effects low.  It is this phenomenon that underpins the traditional approach of using radiation therapy daily for a course of five to seven weeks.

 

Three Dimensional Conformal Radiation Therapy (3DCRT)

Multileaf collimator Rizwan Nurani

 In an effort to minimize the side-effects of radiation therapy, the radiation beam can be shaped by lead blocks called Multi-Leaf Collimators (MLCs) mounted in the head of the linear accelerator.  This allows the x-ray beam to be shaped to match the target and thus keep the volume of normal cells exposed to a meaningful dose of radiation low.  Computers are used to generate a three dimensional representation of the cancer target and surrounding normal structures.  By examining these structures in all three dimensions, Dr. Rizwan Nurani can determine which directions allow for the best approach path of the radiation beam.  Once all the beam paths are chosen, the cumulative dose to every point in the patients is calculated and optimized. 

Intensity Modulated Radiation Therapy (IMRT)

 

IMRT takes three dimensional conformal radiation therapy to an additional level.  All the steps described above are first performed.  Computers are then used to determine the optimal position of each lead block (MLCs) at each point in time.  Thus the MLCs move during the delivery of the radiation.  This allows a different dose of radiation to be delivered to each point in three-dimensional space within the radiation beam.  IMRT is equivalent to painting with radiation in three dimensions. 

IMRT is very helpful in areas that need the radiation dose to fall of quickly, for example when the tumor is right next to a critical normal structure that has a low tolerance for radiation dose.  In lung cancer this situation can arise when the cancer is close to the spinal cord, food pipe, or chest wall. An additional advantage of intensity modulated radiation therapy is that it can make sure that a uniform dose of radiation is deposited even if the tissue surrounding the lung cancer target is very uneven.



Image Guided Radiation Therapy (IGRT)

Cyberknife IGRT Rizwan Nurani

Dual Room Mounted X-Ray Imaging

 IGRT uses a diagnostic x-ray source built into the Linear accelerator to check the positioning of the tumor immediately before delivering the treatment.  Fine adjustments can be made to the position of the tumor to make sure that it is in the right position during the delivery of the treatment.  With older techniques, Radiation Oncologists built safety margins around the area being treated as the position of the cancer can be slightly different on a daily basis.  This safety margin allowed for the tumor to get the full dose of radiation but also resulted in the unnecessary radiation of a larger volume of normal structures. Since IGRT allows for daily adjustment to the position of the target, the safety margins can be greatly reduced.  Reduced safety margins results in less normal tissue radiation and less side-effects. 

 IGRT has been particularly important in the era of IMRT.  IMRT allows radiation oncologists like Dr. Rizwan Nurani to deliver the treatment is a very precise manner.  In this setting, it is critical that the lung cancer target is localized just prior to the delivery of the treatment.  With the highly conformal nature of IMRT, a small movement of the target could result in missing a part of the target. IGRT ensures the accuracy of the precise radiation therapy performed with IMRT techniques.

Volumetric Modulated Arc Therapy (VMAT)

 Traditional Intensity Modulated Radiation Therapy (IMRT) required the head of the linear accelerator to move to different pre-specified positions.  Once the position coordinates were confirmed, the MLCs movement pattern for that position were then activated.  Finally the radiation beam was turned on to deliver the pre-programmed units of radiation in a uniform manner.  At the completion of this, the beam would then be turned off and the head of the linear accelerator would move to the next position and the whole process would be repeated for each position.  For a typical treatment that required five to nine position changes, with five to nine beam start and stop actions, the total treatment time would approach twenty to thirty minutes.  Long treatment times are problematic as the patient suffers from the discomfort of having to lie still and there is a higher chance that the patient or cancer would move during the treatment period.  So even if IGRT was used at the start of the treatment to align the target, the treatment may not be accurate towards the end of the treatment period.

Using a combination of faster computer processing power, faster and more precise multi-leaf collimators and sophisticated software , it is now possible to have the head of the gantry rotate around the patient at a variable speed while the MLCs are moving to their pre-programmed positions and having the radiation beam turned on during the entire process.  In this manner, radiation oncologists like Dr. Nurani can plan for the treatment delivery to be completed within two minutes decreasing the likelihood of patient discomfort and target movement.

Tumor motion management

 Lung Cancers move with the breathing of the patient.  It is important to account for this movement so that the target is not missed.  With older techniques, a safety margin of normal tissue was treated around the target to make sure that breathing motion did not result in the cancer moving out of the treated envelope.  With newer techniques described below, the position of the cancer can be accounted for and the delivery of the radiation matched to this.  Smaller safety margins can then be used resulting in less normal tissue being treated and less side effects. Motion management techniques include:

Abdominal compression

            A device that applies pressure to the abdomen decreases the depth of the patients breathing and thus decreases the amount of tumor movement.  The decreased tumor movement is then measured during the planning phase and a smaller individualized safety margin can be used.

Gating

            Gating involves matching the position of the tumor, to the patient’s breath cycle and only treating at a specified part of the breath cycle. 

fiberoptic light diodes rizwan nurani

Optical Sources that are placed and move with the chest wall during breathing

             Infrared or other light sources are placed on the chest wall of the patient.  A camera mounted in the room tracks the patients breathing phase by monitoring the rise and fall of the light sources on the chest wall.  As the infrared sources are being tracked, the IGRT source is used to take x-ray pictures of the tumor at the same time.  The x-ray pictures are then used to generate internal pictures of the tumor and the internal position of the tumor is matched to the position of the external infrared sources.  This allows a model of internal tumor co-ordinates to be matched to the breathing cycle.  The radiation beam is then only turned on at a particular phase in the breathing cycle when the tumor position moves into the chosen treatment co-ordinates window.

© Rizwan Nurani 2012