Prostate Cancer: Radiotherapy Comparison
Proton Beam versus Non-Proton Techniques
Donald B. Fuller, MD, Reza Shirazi, MD, EE
(December 10, 2013)
It has been suggested that proton beam radiotherapy is "better" than other forms of radiotherapy for prostate cancer. We will break down the individual components and evaluate the strength of this assertion. Our comparison applies to treatment of a typical healthy patient with early disease.
Clinical Factors |
Proton |
IMRT/IGRT |
CyberKnife SBRT |
Significance |
Disease-free survival |
High |
High |
High |
No proven superiority with any of these methods. |
Rectal injury rate |
Low |
Low |
Low |
No proven superiority with any of these methods. |
Incontinence rate |
Low |
Low |
Low |
No proven superiority with any of these methods |
Erectile Dysfunction (ED) rate |
Variable |
Variable |
Variable |
No proven superiority with any of these methods - Risk is more dependent on age, medicines, pre-existing ED. |
Technical Factors |
Proton |
IMRT/IGRT |
CyberKnife SBRT |
Significance |
Maximum radiation dose ("Dmax") exposure to bladder, rectum, neurovascular bundles |
Comparable |
Comparable |
Lower |
Due to the continuous tracking feature of CyberKnife SBRT, smaller margins and sharper dose gradients against the prostate are possible. Dmax levels are comparable with proton beam versus IGRT/IMRT. |
Image-guidance (IGRT) |
Least developed |
More developed |
Most developed |
This is a very complex topic. The CyberKnife possesses the most advanced image guidance system and is the only device capable of tracking and treating a target with sub-millimeter accuracy. Calypso-guided IMRT also tracks the prostate at least as accurately as the CyberKnife (sub-millimeter accuracy with prostate location updated ten times a second), but does not have the same automated corrective feedback loop for beam aiming that CyberKnife has, and does not correct for rotational target motion as CyberKnife does. Even so, Calypso IMRT is considered accurate to within +/- 2 millimeters. Sophisticated image guidance is also used with standard IGRT/IMRT, also sub-millimeter accurate at the beginning of each treatment, though not continuously updated as with CyberKnife. Image-guidance is more problematic with proton beam, due to the potential to shift the Bragg Peak* off the target and uncertainty of the exact beam strength at the tail end of Bragg Peak. As a result, treatment margins with proton beam are typically larger and patient needs to be treated with a rectal balloon daily to compensate for the uncertainty of set up. The larger necessary treatment margins and need to have a rectal balloon placed daily potentially negate any other “physics” advantage attributed to proton beam radiotherapy for prostate cancer. *The Bragg Peak is the physics phenomenon that allows protons to "start and stop" depositing higher dose radiation before and after passing through the target region. |
Radiation dose exposure to "far away" tissues - e.g - subcutaneous tissues, the parts of the bladder and rectum that are not right next to the prostate - also known as "The Integral Dose" |
Lower |
Higher |
Higher |
This is the major superiority of proton beam radiotherapy. It is of greatest potential benefit in pediatric radiotherapy cases, as low dose radiation exposure to "far away tissues" may increase the risk of radiation-induced malignancy years later and children are most susceptible to this. Although radiation-induced malignancy is also a potential issue in adults, they are comparatively far less sensitive to radiation-induced malignancy. Bladder and rectal complications are more related to the high dose regions that occur next to the prostate, which are not reduced with protons. |
Practical Factors |
Proton |
IMRT/IGRT |
CyberKnife SBRT |
Significance |
Cost |
Highest |
Intermediate |
Lowest |
All forms of radiotherapy are expensive, with protons being the highest. |
Treatment course length |
4-10 weeks |
4-10 weeks |
1 week |
CyberKnife SBRT is the most efficient method |
Summary:
There is no clear superiority to proton beam radiotherapy in the treatment of prostate cancer, particularly considering the cost. The strongest clinical case for protons is likely for pediatric malignancy.
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