Arc Therapy Quality Assurance Research Articles

Interpreting the patterns of local failure following postoperative volumetric-modulated arctherapy in oral cavity and oropharynx cancers: Impact of the different methods of analysis

PurposeIntensity-modulated radiation therapy or volumetric-modulated arctherapy is nowadays the recommended radiation technique for the treatment of head and neck cancers. However, by providing a significant dose gradient between target volumes and organs at risk, there is a risk of target missing and thus recurrence in case of inadequate delineation. It is therefore necessary to determine the origin of these recurrences to improve clinical practice. Over the past years, different methods have been described for the analysis of recurrences. Using the patterns of failure of patients with oral cavity and oropharynx carcinoma, treated with postoperative volumetric-modulated arctherapy in our institution, the purpose of this work was to analyse the sites of local recurrences and to evaluate the disparity in the classification of recurrences when different methods were used. Material and methodsBetween 2011 and 2019, 167 patients who underwent postoperative volumetric-modulated arctherapy for oral cavity or oropharyngeal cancers were included (60 and 40 % respectively). Two or three dose levels were prescribed (54Gy, 59.4/60Gy±66Gy). Local recurrence occurred in 17 patients (10.2 %). We assessed the patterns of local recurrences according to four methods: 1/ volume-based method using the volume overlap between the recurrence volume and initial target volumes; 2/ volume-based method of overlap between the recurrence volume and the 95 % treatment isodose; 3/ point-based method using the position of the barycentre of the recurrence volume; 4/ combined centroid method classifying recurrences according to both the initial target volumes and dose distribution. Each case was reviewed to make a clinical judgment on these classifications and assessed them as “appropriate”, “possible”, or “inappropriate”. ResultsFor the volume-based method using overlap between the recurrence volume and the initial clinical target volume, this classification was clinically judged as inappropriate in 11 out of 17 cases (65 %). For the volume-based method using overlap between the recurrence volume and the 95 % prescribed isodose, this classification was clinically judged as appropriate in 15 out of 17 cases (88 %). For the point-based method, this classification was clinically judged as appropriate in 14 out of 17 cases (82 %). Thirteen out of 17 local recurrences had the same classification between this point-based method and the volume-based method of overlap between the recurrence volume and the 95 % prescribed isodose. For the combined centroid method, among 17 local recurrences nine were classified as type A, two as type B, two as type C, three as type D and one as type E. This classification was clinically judged as appropriate in 15 out of 17 cases (88 %). Only five out of 17 of the local recurrences were classified the same way according to the four different methods (29 %). ConclusionRecurrences that are “marginal” or “outfield” represent a major challenge for intensity-modulated radiation therapy/volumetric-modulated arctherapy quality assurance and improvement of delineation recommendations. To date, there are no published methods that give complete satisfaction.

EP-1471: Volumetric modulated arc therapy quality assurance: Characterisation and evaluation of Octavius 4D system

EP-1471: Volumetric modulated arc therapy quality assurance: Characterisation and evaluation of Octavius 4D system

MO‐FF‐A1‐02: Calibration of a Novel 4D Diode Array for IMRT and VMAT QA

Purpose: To develop calibration procedures for a novel 4D diode array for IMRT as well as arc therapy quality assurance (QA). Method and Materials: A novel 4D diode array (ArcCHECK) was designed for rotational therapy QA. Its cylindrical and isotropic design presents a consistent detector image in beam's eye view for all gantry angles. Signals were acquired every 50 ms, allowing for real‐time per‐beam QA as well as measuring composite dose distributions. An efficient calibration procedure was developed to obtain diode sensitivity and directional response dependence with one full gantry rotation. In this process, each diode was in turn irradiated under the same beam condition on beam central axis. The directional response was obtained using TPS‐calculated beam profiles, which were validated in rectangular geometry. A real time algorithm to derive beam angle based on beam edge back projections was developed to apply the corresponding directional response factors. Its clinical application for IMRT QA was demonstrated with 17 IMRT head and neck beams delivered with planned beam angles. Measured dose distribution was compared to TPS calculation in three dimensions with %diff and distance‐to‐agreement analysis. Its feasibility for VMAT QA was investigated with a prostate conformal arc plan. Results: For the IMRT beams, average passing rate was 94.3%±2.1% and 99.6%±0.8% with 1%/2mm and 1%/3mm, respectively. For the conformal arc plan, the passing rate was 97.7% with 1%/2mm and 100% with 1%/3mm. Conclusion: An efficient calibration procedure was developed to obtain both the diode sensitivity and directional response dependence. Real‐time gantry angles were derived accurately based on beam edge back projections, which were used to apply directional response factors. Excellent agreement with TPS calculation was achieved for both IMRT and VMAT plans. ArcCHECK is an efficient and valuable tool for both IMRT and VMAT QA.Research sponsored by Sun Nuclear Corporation.