A research team headed up at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, has carried out a detailed investigation of the out-of-field dose profiles for photon and charged-particle therapies. In particular, they considered whether particle therapy can reduce the risk of secondary malignancies by minimizing healthy tissue dose (Phys. Med. Biol. 57 5059).
"The advantageous depth-dose curve of ions leads to better sparing of normal tissue on the proximal and distal sides of the target," said GSI researcher Robert Kaderka. "This advantage is clear close to the target, but few studies have investigated the low-dose region far out-of-field."
Dose profiles
The researchers used a diamond detector to measure the lateral dose fall-off in an irradiated water phantom. "The experimental set-up was designed to achieve a systematic comparison of different radiation types and fill the lack of data currently available in literature," Kaderka explained.
They first measured dose profiles for square fields of 6, 18 and 25 MV photons. Higher beam energies led to an increased out-of-field dose, particularly near the field edge, although the overall influence of beam energy on peripheral dose profile was not large. Comparisons of 5×5 and 10×10 cm fields of 18 MV photons revealed that the out-of-field dose increased with increasing field size, attributed to increased scattering from the treatment site.
Next, the researchers examined the influence of primary beam energy on the dose profiles of scanned protons and carbon ions, measuring lateral dose distributions for energies corresponding to 50, 125 and 200 mm range in water. At all energies, the dose profile for carbon ions exhibited a sharper fall-off close to the target than seen for protons. Far out-of-field, the dose deposited for low-energy carbon ions was lower than for protons, but as the carbon-ion energy increased, the associated peripheral dose reached one order of magnitude higher than for protons.
Dose profiles for passively scattered protons were not correlated with field size, while scanned protons exhibited a slightly increased dose far from the target at larger field sizes. In contrast, the dose dependence on field size was strong for carbon ions, with a larger field size increasing the out-of-field dose by up to one order of magnitude. This is attributed to the greater number of primary ions required to irradiate a larger field increasing the yield of secondary particles.
Comparisons of passive and scanned protons showed that the former is characterized by a sharper penumbra. Further from the target, the dose profile of the passively modulated beam levels off, while that of the scanned beam decreases to a lower value in the far-out-of-field region.
Independent of radiation type and delivery technique, the lateral dose dropped to 10% of the target dose within 10 mm from the field edge. At increasing distances, however, the photon dose was significantly (up to 400 times) greater than the dose from charged particles. The authors note that this emphasizes the clear supremacy of charged particles over X-rays in sparing healthy tissue. Among charged particles, scanned carbon ions had the steepest dose gradient near to the field edge, while scanned protons delivered the lowest peripheral dose at larger distances.
Neutron study
In addition to the scattered primary beam, high-energy photon irradiation and passively scattered particles can also lead to unwanted neutron production. Thus the researchers also used the water phantom to study the neutron yield. "Neutrons are biologically very harmful, as they can penetrate deep in the patient depositing dose far away from the target. Thus, they represent a major contributor to the risk of developing secondary malignancy following radiotherapy," Kaderka noted.
The researchers used a bubble detector spectrometer to record the complete photoneutron energy spectrum for 18 MV photon irradiation (this detector type cannot be used with charged particles). Out-of-field spectra showed a peak around 1 MeV, while the in-field neutron distribution exhibited a broader, higher-energy peak. The neutron flux at 100 mm water depth was attenuated by more than one order of magnitude compared with the surface flux.
The out-of-field neutron dose was independent of the distance to the target, suggesting that the accelerator head is the main neutron source rather than the patient, and implying a need for shielding materials. "Our measurements show that a patient is immersed in an almost isotropic neutron bath in high-energy photon therapy," said Kaderka. "The water phantom results also show the strong neutron moderation in water, implying that a new generation of linacs could be equipped with a layer of water to greatly reduce the out-of-field dose from neutrons."
Finally, the researchers used thermoluminescence detectors to assess the fluence of secondary thermal neutrons. "We measured thermal neutrons as a first approach for understanding and comparing neutron production mechanisms in different types of radiotherapy," Kaderka told medicalphysicsweb. "While this only represents a small fraction of the neutron spectrum – especially for charged particles – some basic hints can be obtained from the results."
The highest thermal neutron yields were seen for high-energy photons and passively delivered protons, with a constant value over all distances from the target. Kaderka notes that paediatric patients and those with cancer-prone syndromes may particularly benefit from treatment with charged particles, especially if scanning is used to reduce unwanted neutron irradiation.
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