Dose fall-off patterns with volumetric modulated arc therapy and three-dimensional conformal radiotherapy including the “organ at risk” effect. Experience of linear accelerator-based frameless radiosurgery from a single institution

This study aimed to evaluate characteristics of dose fall-off pattern for linear accelerator based frameless stereotactic radiotherapy and radiosurgery using two different techniques, three-dimensional conformal radiotherapy and volumetric modulated arc therapy. The data from thirty patients who underwent frameless stereotactic radiotherapy/radiosurgery were considered for this analysis. These included 11 patients treated using three-dimensional conformal radiation and 19 patients treated with volumetric modulated arc therapy. Dose fall-offs in six predetermined directions from the edge of planning target volume and in direction of organs at risk were evaluated. These included determining dose fall-off (distance range) of 100%–80%, 100%–50%, 100%–20% from the treatment planning system. The first derivative (percentage of dose fall-off per millimetre) and second derivative (percentage of dose per square millimetre) of the dose fall-off was calculated. The dose fall-off is sharpest in the organ at risk direction if the organ at risk is hugging the planning target volume. However, for organs at risk separated from the planning target volume sharpest dose fall-off could be in either planning target volume superior or inferior direction. Three-dimensional conformal radiotherapy plans had a mean±SD of 14±2 beams (range: 11–17). Volumetric modulated arc therapy planning comprised of arc angles with sum±SD of 953±172.5° (range: 610°–1170°). Mean±SD for the single sharpest dose fall-off for all patients receiving three-dimensional conformal radiotherapy was 11.3±4.7%·mm−1, 7.8±4.8%·mm−1 and 7.1±5.6%·mm−1 for 100%–80%, 100%–50% and 100%–20% fall-off ranges respectively. For volumetric modulated arc therapy, the mean±SD of the single sharpest fall-off was 10.4±4.6%·mm−1, 7.8±3.0%·mm−1 and 7.3±5.0%·mm−1 for 100%–80%, 100%–50% and 100%–20% range. All organs at risk doses received doses within limits prescribed by AAPM-TG report 21. Mean dose to monitor units (MU) modulation factor MF=total MU deliveredprescription dose in cGy for three-dimensional conformal radiotherapy and volumetric modulated arc therapy were 1.97±1.0 MU·cGy−1 and 2.5±0.75 MU·cGy−1 respectively. Our study presents the dose fall-off patterns in context of frameless stereotactic radiation therapy. We have presented the dose fall-off data in the framework of three-dimensional conformal radiotherapy and volumetric modulated arc therapy in brain lesions. Cette étude visait à évaluer les caractér