on the importance of target size for selective boosting of hypoxic tumor subvolumina
On the importance of choice of target size for selective boosting of hypoxic tumor subvolumina in carbon ion therapy Purpose: Functional imaging methods in radiotherapy are maturing and can to some extent uncover radio resistant structures found within a tumour entity. Selective boost of identified hypoxic subvolumina with high-LET radiation may in principle lead to increased tumour control due to LET-dependence of the oxygen enhancement ratio (OER). Here, we demonstrate the importance of minimizing field sizes in order not to dilute the dose average LET by quantifying the OER as a function of PTV size and PTV position. Methods: Several treatment plans are produced with TRiP, using a 256x256x256 mm<sup>3</sup> water phantom and SOBP optimization on physical dose. Box formed PTV volumes between 0.15 - 1010 cm<sup>3</sup>, and PTV positions ranging from 3 cm to 200 cm depth (relative to the surface where the beam enters) are examined. For each plan the minimum, maximum and the dose averaged LET of the PTV is calculated. The numbers are translated to OER using several sets of data found in literature for various cell lines. Results: We find a strong dependence of the dose average LET and OER as a function of PTV volume. Dose averaged LET varies between 41 keV/µm (1010 cm<sup>3</sup>) and 110 keV/µm (0.15 cm<sup>3</sup>). The respective OERs range from 2.2 to 1.3 (Barendsen et al. 1966) and from 2.9 to 1.8 (Furusawa et al. 2000, both HSG and V79 data). Varying the position of the PTV for a 127.5 cm<sup>3</sup> volume we find 52 keV/µm at 30 cm depth and 60 keV/µm at 200 cm depth. Conclusion: In order to maximize effects of high-LET radiation, fields should be kept small and confined to hypoxic structures of the tumour entity. Deep seated tumours may additionally reduce the possible achievable effect. All cell lines investigated here did not reach an OER of 1, even for the smaller structures, which may indicate that the achievable dose average LET of carbon ions is too low, and heavier ions than carbon may be considered for functional LET-painting.