Ion recombination correction in carbon ion beams.

PURPOSE: In this work, ion recombination is studied as a function of energy and depth in carbon ion beams. METHODS: Measurements were performed in three different passively scattered carbon ion beams with energies of 62 MeV/n, 135 MeV/n, and 290 MeV/n using various types of plane-parallel ionization chambers. Experimental results were compared with two analytical models for initial recombination. One model is generally used for photon beams and the other model, developed by Jaffé, takes into account the ionization density along the ion track. An investigation was carried out to ascertain the effect on the ion recombination correction with varying ionization chamber orientation with respect to the direction of the ion tracks. The variation of the ion recombination correction factors as a function of depth was studied for a Markus ionization chamber in the 62 MeV/n nonmodulated carbon ion beam. This variation can be related to the depth distribution of linear energy transfer. RESULTS: Results show that the theory for photon beams is not applicable to carbon ion beams. On the other hand, by optimizing the value of the ionization density and the initial mean-square radius, good agreement is found between Jaffé's theory and the experimental results. As predicted by Jaffé's theory, the results confirm that ion recombination corrections strongly decrease with an increasing angle between the ion tracks and the electric field lines. For the Markus ionization chamber, the variation of the ion recombination correction factor with depth was modeled adequately by a sigmoid function, which is approximately constant in the plateau and strongly increasing in the Bragg peak region to values of up to 1.06. Except in the distal edge region, all experimental results are accurately described by Jaffé's theory. CONCLUSIONS: Experimental results confirm that ion recombination in the investigated carbon ion beams is dominated by initial recombination. Ion recombination corrections are found to be significant and cannot be neglected for reference dosimetry and for the determination of depth dose curves in carbon ion beams.

Under-response of a PTW-60019 microDiamond detector in the Bragg peak of a 62 MeV/n carbon ion beam.

To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freiburg microDiamond detector, its response was compared to the response of a plane-parallel Markus chamber in a 62 MeV/n mono-energetic carbon ion beam. Results obtained with two different experimental setups are in agreement. As recommended by IAEA TRS-398, the response of the Markus chamber was corrected for temperature, pressure, polarity effects and ion recombination. No correction was applied to the response of the microDiamond detector. The ratio of the response of the Markus chamber to the response of the microDiamond is close to unity in the plateau region. In the Bragg peak region, a significant increase of the ratio is observed, which increases to 1.2 in the distal edge region. Results indicate a correlation between the under-response of the microDiamond detector and high LET values. The combined relative standard uncertainty of the results is estimated to be 2.38% in the plateau region and 12% in the distal edge region. These values are dominated by the uncertainty of alignment in the non-uniform beam and the uncertainty of range determination.

Can treatment of pediatric Hodgkin's lymphoma be improved by PET imaging and proton therapy?

BACKGROUND AND PURPOSE: To explore a new positron emission tomography (PET)-based target concept for pediatric Hodgkin's lymphoma (PHL). PATIENTS AND METHODS: For 10 patients, the planning target volume PTV1 was based on initial CT tumor extension and PTV2 on anatomy-related PET-positive lymph node levels after chemotherapy. The treatment techniques investigated (prescribed dose 19.8 Gy) comprised opposed-field (2F), intensity-modulated photon (IMXT), and single-field (PS) proton techniques. Treatment concepts were compared concerning dose-volume histogram (DVH) parameters and organ-equivalent doses (OED). RESULTS: The median PTV1 and PTV2 were 902 ± 555 cm(3) and 281 ± 228 cm(3). When using PTV2 instead of PTV1 for all techniques, the D(2%) of the heart was reduced from 14 to 9 Gy and the D(mean) of the thyroid from 16.6 to 2.7 Gy. Low- (20%), median- (50%), and high-dose volumes (80%) were reduced by 60% for the heart and bones using PTV2. PS reduced the high-dose volume of the lungs and the heart by up to 60%. IMXT increased the low-dose volumes and OED. PTV2 reduced OED by 54 ± 10% for all organs at risk. CONCLUSION: PTV2 has a high impact on the treated volume and on sparing of organs at risk. The combination of an adaptive target volume definition with protons could contribute to future PHL treatment concepts.