engleski

Clinically significant differences in dose distributions calculated for patients with nasopharyngeal cancer using dose-to-water and dose-to-medium reporting modes

Introduction Implementing advanced radiotherapy techniques largely affects the outcome and quality of life for nasopharyngeal cancer patients. Due to irregularly shaped target volumes and their proximity to organs at risk, treatment planning and dose delivery represents a challenge for all professionals involved in the process of radiotherapy treatment of the head and neck region. Development of advanced dose calculation algorithms for radiotherapy treatment planning based on Monte Carlo simulation allows reporting external beam photon doses in terms of dose-to-medium (Dm), as well as dose-to-water (Dw). This study was conducted with the goal of determining the effect of dose-to-water and dose-to-medium reporting modes, both built in Elekta Monaco TPS, on Intensity Modulated Radiotherapy (IMRT) dose distributions for nasopharyngeal cancer patients. Methods and materials The analysis of dose distributions calculated utilizing both reporting modes for 13 patients with nasopharyngeal cancer was performed retrospectively, with the research being conducted in University Hospital Rijeka. Treatment consisted of multiple target dose levels for dose prescription, namely, simultaneous integrated boost technique. The dose prescribed to the visible disease volume totaled 70Gy delivered in 35 fractions. A 6MV photon beam and a multileaf collimator with 160 leafs was used for IMRT planning, with plans being normalized to 100% coverage of the planning target volume with 95% of the prescribed dose. Dose volume constraints were allocated in accordance to international standards. The comparison of dose distributions was evaluated by considering the quantities important for respective volumes of interest. Heterogeneity index (HI) and conformity index (CI) methodology, as well as the maximum dose (D2), was used for target volumes. Maximum dose (D2), mean dose and dose volume concepts were used concerning specific organs at risk. Results For all target volumes (GTV, CTVHR, CTVIR, CTVLR, PTVHR, PTVIR and PTVLR) and most of organs at risk (medulla, chiasm, eye lens, eye nerves, brain stem, parotids, retina and thyroid gland) the differences between two reporting modes were shown to be statistically insignificant. The significant difference, which might impact the treatment outcome, was observed for two bony structured organs at risk: mandible and cochlea. The analysis for these organs at risk suggested dose-to-water being higher than dose-to-medium with mean difference 9, 91% (p=0, 000009) for the mandible volume covered with total dose of 70Gy. Following, the difference in mean dose was observed to be 8, 74% (p=0, 037) and 6, 87% (p=0, 029) for left and right cochlea, respectively. Conclusion In this study, no statistically significant difference between reporting modes when considering target volumes is shown. The same trend also applies for most of the organs at risk. However, a significant difference in dose distribution was observed in mandible, which is bony structured, and cochlea, which is surrounded by a bone labyrinth. Our results suggest that caution has to be taken when interpreting the calculated dose data, as a possibility of late complications in bony structures exists. It is important to be aware of the fact that doses to high density volumes calculated according to dose-to-water approach could be up to 15% higher when compared to dose-to-medium concept.

dose-to-water ; dose-to-medium ; IMRT ; nasopharyngeal cancer

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