May 8, 2012
Aperture boosts benefits of scanned protons
Proton therapy delivered using pencil-beam scanning (PBS) offers the potential to create highly conformal fields with reduced dose outside of the primary field. One downside of this scanning technique, however, is the larger lateral penumbra produced if the pencil beam is wide or the proton energy is small.
Placing a patient-specific aperture at the nozzle exit to remove large-angle scatter can reduce the penumbral width during PBS proton therapy. To investigate this idea further, a US-Australian research team has performed Monte Carlo simulations to assess the benefits of using such an aperture (Phys. Med. Biol. 57 2829).
"PBS has a number of potential advantages over double scattering, as demonstrated in several previous studies," said Stephen Dowdell, from Massachusetts General Hospital (MGH) and Harvard Medical School (Boston, MA). "However, the lack of material in the beamline in PBS means that any wide-angle scatter, for example from proton interactions with the ionization chambers, will potentially reach the patient."
Dowdell, and colleagues from the University of Wollongong, developed a Monte Carlo model of the PBS treatment head at MGH, based on an existing model of the double scattering (DS) system. The Monte Carlo code was validated against experimental data, and used to simulate PBS treatment of a clinical prostate case, with and without a patient-specific aperture. Results were then compared to DS-based simulations.
"The aperture simulated for PBS was identical to that used for the DS case," Dowdell noted. "There would be no additional cost or complexity compared to the apertures currently produced for DS treatments."
Dose values were calculated for depths of between 4.72 and 33.04 cm in Lucite, at distances of 2.5 to 50 cm from the lateral field edge. The simulated PBS field had a range of 22.8 g/cm2 and a lateral field size of 45.2 cm2. The researchers obtained the absorbed dose due to protons, and due to neutrons and photons created by beam scattering. The total absorbed dose was then converted to an equivalent dose, using appropriate weighting factors for the three particle types.
At a depth of 4.72 cm, the absorbed dose 2.5 cm from the field edge was approximately three times higher for PBS than DS. Adding the aperture to PBS reduced this by over an order of magnitude, to less than seen using DS. PBS improved when considering equivalent dose, due to the lower neutron component of the total dose. The benefit of the aperture in PBS diminished for lateral distances of greater than 7.5 cm.
At a depth of 18.8 cm (within the spread-out Bragg peak), adding the aperture to PBS sharpened the lateral penumbra and reduced doses close to the field edge. For example, at 2.5 cm from the field edge, the aperture reduced the absorbed dose in PBS by more than an order of magnitude. For lateral distances of more than 20 cm, dose curves were similar for PBS with and without the aperture. At distances greater than 10 cm from the field edge, the absorbed dose from DS was higher than that from PBS, highlighting the contribution of neutrons generated in the treatment head during DS delivery.
Finally, the researchers examined the situation distal to the spread-out Bragg peak (a depth of 28.32 cm), where there is zero dose contribution from primary protons and dose is entirely due to secondary particles. At all distances from the field edge, the increased neutron fluence seen in DS led to higher absorbed and equivalent doses than for either PBS technique. At this depth, the effect of the aperture in PBS was less prominent, causing just a slight reduction in absorbed and equivalent doses.
The authors concluded that the patient-specific aperture can reduce out-of-field doses and offers the potential to further improve the normal tissue sparing capabilities of PBS. They note that while proton interactions occurring within the aperture increase the neutron fluence compared to conventional PBS, the level of interactions is much lower for PBS than DS.
"In DS, the aperture is required to stop a large proportion of the beam, whilst in PBS it only interacts with particles that have been scattered through large angles. The vast majority of the beam in PBS will pass directly through the aperture," Dowdell explained. "There is a small increase in the neutron dose out-of-field, but this is offset by a reduction in proton dose, leading to a reduction in the total absorbed dose and equivalent dose when using an aperture in PBS."
Dowdell points out that there are several avenues for future research. "This study demonstrates the potential benefit of apertures in PBS; however we have only considered one case – a single clinical prostate field. Before a more concrete judgement of the efficacy of apertures in PBS can be made, different clinical cases need to be investigated," he told medicalphysicsweb.
• Related articles in PMB
Monte Carlo study of the potential reduction in out-of-field dose using a patient-specific aperture in pencil beam scanning proton therapy
Stephen J Dowdell et al Phys. Med. Biol. 57 2829
Golden beam data for proton pencil-beam scanning
Benjamin Clasie et al Phys. Med. Biol. 57 1147
Uncertainties and correction methods when modeling passive scattering proton therapy treatment heads with Monte Carlo
Bryan Bednarz et al Phys. Med. Biol. 56 2837
Experimental validation of a Monte Carlo proton therapy nozzle model incorporating magnetically steered protons
S W Peterson et al Phys. Med. Biol. 54 3217
About the author
Tami Freeman is editor of medicalphysicsweb.