Volume 36, Issue 6, June 2009
Index of content:
- Industrial Physics Forum: Joint Imaging/Therapy Symposium: Ballroom C
Challenges of Post‐treatment and Real‐time Imaging of Dose Deposition During Proton Therapy
MO‐D‐BRC‐01: Introduction: Challenges of Post‐Treatment and Real‐Time Imaging of Dose Deposition in Proton Therapy36(2009); http://dx.doi.org/10.1118/1.3182220View Description Hide Description
The finite range of protons is the primary physical advantage leading to the potentially greatly improved dose distributions in protonradiation therapy. The other side of the coin is that it takes extra care to make sure that the protons actually stop at the right location. Fortunately, proton therapy provides unique opportunities to visualize interactions of the therapeutic beam with the patient. These interactions are more or less closely related to the delivered dose distribution, and can therefore be used as a dose surrogate for treatment verification. In this symposium we will highlight three examples of such interaction mechanisms, as well as their potential to provide clinically useful information about the in‐vivo dose distribution either post‐treatment or even in real‐time. Some of the methods are applicable to photon therapy as well.
In this short introduction we will discuss various reasons that lead to uncertainties in the proton range and will show estimates of the magnitude of the range uncertainties. We will then go over techniques to deal with range uncertainties in treatment planning, and will finally discuss what could be gained if we could reduce or eliminate range uncertainties.
1. Name three reasons for range uncertainties in proton therapy.
2. Quantify the magnitude of range uncertainties.
3. Name at least one method that reduces the impact of range uncertainties.
MO‐D‐BRC‐02: Clinical Significance of In‐Vivo Proton Range Detection and Potential of MRI Scanning After Proton Therapy36(2009); http://dx.doi.org/10.1118/1.3182221View Description Hide Description
Highly conformal radiation therapy has the potential advantage of limiting radiation exposure to normal tissue and improving the therapeutic ratio of radiation therapy. However, with improving technologies, much greater emphasis has been placed on the accuracy of both treatment delivery and localization. Shrinking PTV margins based on our confidence in the perceived accuracy of our treatment delivery runs the risk of geographic miss and subsequent marginal recurrences. Proton therapy is particularly prone to this risk. Range uncertainties exist at the distal edge of the Bragg peak. Also, proton range and depth of penetration is highly dependent on tissue density. Hence, for moving targets with changing densities within the field, like at the lung bases or dome of the liver, there is greater uncertainty in the dose that was actually delivered to the tumor and normal tissue. For these reasons, verification of dose delivered in patients receiving proton therapy is of critical importance.
Post‐treatment imaging may be a useful way to verify proton range in‐vivo. Radiation may produce changes in imaging characteristics within organs that may be used to verify that the target has been adequately treated. In this lecture we will review some of the issues that complicate proton planning. Furthermore, we will demonstrate the utility of MRI scanning after proton therapy to document target coverage for both static and dynamic tumor systems.
1. Discuss the clinical data regarding marginal misses with highly conformal therapy
2. Demonstrate the complexities of 4‐dimensional proton planning for moving targets, using liver as a model
3. Demonstrate the utility of post‐treatment MRI scanning as validation of the 4D‐proton treatment planning process and delivery for liver, as well as other clinical situations.
36(2009); http://dx.doi.org/10.1118/1.3182222View Description Hide Description
Full clinical exploitation of the superior dose conformality promised by ion beam therapy could greatly benefit from in‐vivo and non‐invasive verification of the dose delivery, to monitor safe application of the planned treatment and enable adaptive strategies in case of deviation between actual and intended irradiation. Indeed, detection of the surrogate transient pattern of tissue +‐activation (mainly 15O and 11C with half‐lives of 2 and 20 min, respectively) induced by therapeutic proton irradiation was envisaged since the late 1970s as a potential mean to visualize the treatment delivered to the patient. However, about 30 years elapsed before Positron‐Emission‐Tomography (PET) verification of proton therapy could be investigated in thorough clinical trials. This was mainly due to 1) the technical challenges for realization of dedicated in‐beam PET scanners integrated into clinical treatment units and 2) the methodological limitations (especially from co‐registration issues) of post‐radiation imaging using commercial standalone PET devices.
The recent advent of commercial combined PET/CT (Computed‐Tomography) scanners helped overcoming the major drawbacks of post‐radiation PETimaging alone, due to the availability of the additional CT information for co‐registration with the planning CT. To date, first clinical trials have been reported for about 50 patients scanned with PET/CT for up to 30 min starting up to 20 min after proton irradiation at the Massachusetts General Hospital (MGH), Boston, the National Cancer Center (NCC) of Kashiwa, Japan, and the University of Florida, Jacksonville. This presentation will give emphasis to the initial clinical experience from MGH. This is so far the only proton center addressing the feasibility of range verification by comparing the PET/CT measurement with a detailed Monte Carlo modeling of the expected activation, as done for in‐beam PET of >400 carbon ion patients at the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt.
Despite the promising results mainly achieved for head‐and‐neck tumour cases, post‐radiation PET/CT imaging suffers from several limitations including the loss/degradation of the activity signal due to physical decay and biological washout in the time elapsed between irradiation and imaging, the need of repositioning the patient at the imaging site and the internal organ motion during the prolonged PET scan. Improved performances of PET applications are expected for shorter delay times between irradiation and imaging. Therefore, recent developments are exploring the clinical advantages of novel in‐beam and in‐room PET detectors at NCC and MGH, respectively.
The so far accumulated/ongoing clinical experiences in proton therapy are restricted to passive beam delivery. However, as shown for 12C therapy and proton phantom studies with stationary targets, PET verification is also applicable to the emerging scanned ion beam techniques. These are very sensitive to organ motion. Therefore, this presentation will also address the potential benefits of time‐resolved imaging, as supported by first phantom experiments for 4D in‐beam PET verification of carbonion beam tracking at GSI.
1. Review the motivation and principles of PET(/CT) verification of proton therapy
2. Underline merits and limitations of the current clinical implementations of post‐radiation imaging
3. Provide an overview on the latest/ongoing developments aiming at improved clinical performances
36(2009); http://dx.doi.org/10.1118/1.3182223View Description Hide Description
The proton beam radiotherapy has a merit in that the dosedeposit falls off at the end of the beam range. However, mismatch of the falloff location with the prescribed dose boundary can cause adverse effects. Measurement of gamma‐rays emitted by nuclear reactions of the incident proton beam with target materials is the only method to verify the dose distribution in situ. We have investigated two different methods of measuring the prompt gamma distribution: 1) multi‐layer collimation method, which measures the 1D distribution of gamma production at the target, to assure the endpoint of the dosedeposition, 2) electron‐tracking Compton camera, which can reconstruct gamma trajectories, to verify the dose distribution. The collimation method can be effectively used when the background neutron flux is low. Hence, it can be more readily applicable to the scanning method in terms of the therapy beam delivery compared to the scattering method. Secondly, the Compton camera is a powerful tool to image the prompt gamma distribution. A preliminary experiment has been carried out with a water phantom at the beam energy of 150 MeV, and images of prompt gamma generation are attained. The clinical application could be considered with further improvement of detection efficiency.
This lecture will review the uses of multi‐layer shielding and the Compton camera in the measurement of prompt gamma‐rays emitted from the target to verify the dosedeposition during proton therapy.
1. Understand the physics of dosedeposition by the proton therapy
2. Understand multi‐layer shielding techniques to measure the prompt gamma distribution for the verification of the protondose.
3. Understand the use of an electron‐tracking Compton camera for imaging the prompt gamma generation by a proton beam.