Survival curve behaviour and degree of correspondence between the linear-quadratic (LQ) model and experimental data in an extensive dose range for high dose rates were analysed. Detailed clonogenic assays with irradiation given in 0.5 Gy increments and a total dose range varying from 10.5 to 16 Gy were performed. The cell lines investigated were: CHOAA8 (Chinese hamster fibroblast cells), U373MG (human glioblastoma cells), CP3 and DU145 (human prostate carcinoma cell lines). The analyses were based on chi2-statistics and Monte Carlo simulation of the experiments. A decline of LQ fit quality at very low doses (<2 Gy) is observed. This result can be explained by the hypersensitive effect observed in CHOAA8, U373MG and DU145 data and an adaptive-type response in the CP3 cell line. A clear improvement of the fit is discerned by removing the low dose data points. The fit worsening at high doses also shows that LQ cannot explain this region. This shows that the LQ model fits better the middle dose region of the survival curve. The analysis conducted in our study reveals a dose dependency of the LQ fit in different cell lines.

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Irradiation of longitudinally adjacent PTVs with Helical TomoTherapy (HT) may be clinically necessary, for example in treating a recurrent PTV adjacent to a previously‐treated volume. In this work, the parameters which influence the cumulative dose distribution resulting from treating longitudinally adjacent PTVs are examined, including field width, pitch, and PTV location. In‐phantom dose distributions were calculated for various on‐ and off‐axis cylindrical PTVs and were verified by ion chamber and film measurement. Dose distributions were calculated to cover 95% of the PTV by the prescribed dose (DP) using 25 and 50 mm long HT fields with pitches of either 0.3 or 0.45. These dose distributions where then used to calculate the 3D dose distribution in the junction region between two PTVs. The best junction uniformity was obtained for fields of equal width, with larger fields providing better intra‐PTV dose homogeneity than smaller fields. Junctioning fields of different widths resulted in a much larger dose inhomogeneity, but this could be improved significantly by dividing the junction end of the PTV treated with the smaller field into multiple (up to 4) sub‐PTVs, with the prescribed dose in each sub‐PTV decreasing with proximity to the junction region. This provided a PTV matching with dose homogeneity similar to that achieved when junctioning two PTVs, both irradiated by the 50 mm field, and provided a distribution where 95% of the PTV received at least the prescribed dose, with maximum excursions from prescribed dose varying from −19% to +13%. We conclude that junctioning adjacent PTVs is possible. Treating longitudinally adjacent PTVs with different widths is a challenge, but dose uniformity is improved by breaking PTVs into multiple contiguous sub‐PTVs modified to feather (broaden) the effective junctioning region.PACS number: 87.55.D

This study demonstrated good-quality outcomes for RFA treatment of colorectal cancer liver metastases from a Canadian perspective and compared favorably with published studies.

Survival curve behaviour and degree of correspondence between Linear Quadratic model1 (LQ) and experimental data in an extensive dose range for high dose rate was analyzed. Detailed clonogenic assays with irradiation given in 0.5Gy increments and a total dose range varying from 10.5 and 16Gy were performed. The cell lines investigated were: CHOAA8, hamster fibroblast cells; U373MG, human glioblastoma cells; and human prostate carcinoma cell lines CP‐3, and DU‐145. The analyses were based on χ2 statistic and Monte Carlo simulation of the experiment. A decline of fit quality at very low doses (<2Gy) is observed. This result can be explained by the hypersensitive effect observed in CHOAA8 and U373MG data and an adaptive type response in CP3 cell line. A clear improvement is discerned at slightly higher doses. This could be a result of the linearity existent in the trend of survival curve at low doses, that will affect the total fit in a range from 0Gy to final dose in linear quadratic region. The impact of including low dose data is shown through α/β ratios showing relative differences of 42, 40 and 23% for CHOAA8, CP3 and U373MG. LQ model cannot explain survival at high doses. This is shown as a deterioration of goodness of fit for high doses. A comparison of Linear Quadratic Linear model2 (LQL) with the LQ in fitting the experimental data at high doses was also performed.

Although higher doses are obtained, 4DCT is a sufficiently accurate approach that could be used safely to account for breathing motion.

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