Maria Do Carmo Lopes

This manuscript is dedicated to the analysis of the dosimetric parameters of a linear accelerator, using a device that, in reality, is quite old. Currently, there are devices on the market for this purpose with much greater versatility and equipped w...

Radiotherapy (RT) has seen considerable changes in the last decades, offering an increased range of treatment modalities to cancer patients. Volumetric Modulated Arc Therapy (VMAT), one of the most efficient RT arc techniques, particularly with respect to dose delivery time, has recently considered noncoplanar arc trajectories while irradiating the patient, thanks to technological advances in the most recent generation of RT systems. In a preliminary work we have proposed a two-step optimization approach for noncoplanar arc trajectory optimization (ATO). In this paper, treatment plans considering 5-, 7-, 9-, 11- and 21-beam ensembles in the first step (optimal selection of noncoplanar irradiation directions) of the proposed approach are compared to assess the trade-offs between treatment quality and computational time required to obtain a noncoplanar VMAT plan. The different strategies were tested resorting to a head-and-neck tumor case already treated at the Portuguese Institute of Oncology of Coimbra (IPOC). Results obtained presented similar treatment quality for the different strategies. However, strategies that consider a reduced number of beams in the first step clearly outperform the other strategies in terms of computational times. Results show that for a similar tumor coverage, treatment plans with optimal beam irradiation directions obtained an enhanced organ sparing.

Background: This work aims at clinically validating a graphical tool developed for treatment plan assessment, named SPIDERplan, by comparing the plan choices based on its scoring with the radiation oncologists (RO) clinical preferences. Methods: SPIDERplan validation was performed for nasopharynx pathology in two steps. In the first step, three ROs from three Portuguese radiotherapy departments were asked to blindly evaluate and rank the dose distributions of twenty pairs of treatment plans. For plan ranking, the best plan from each pair was selected. For plan evaluation, the qualitative classification of ‘Good’, ‘Admissible with minor deviations’ and ‘Not Admissible’ were assigned to each plan. In the second step, SPIDERplan was applied to the same twenty patient cases. The tool was configured for two sets of structures groups: the local clinical set and the groups of structures suggested in international guidelines for nasopharynx cancer. Group weights, quantifying the importance of each group and incorporated in SPIDERplan, were defined according to RO clinical preferences and determined automatically by applying a mixed linear programming model for implicit elicitation of preferences. Intra- and inter-rater ROs plan selection and evaluation were assessed using Brennan-Prediger kappa coefficient. Results: Two-thirds of the plans were qualitatively evaluated by the ROs as ‘Good’. Concerning intra- and inter-rater variabilities of plan selection, fair agreements were obtained for most of the ROs. For plan evaluation, substantial agreements were verified in most cases. The choice of the best plan made by SPIDERplan was identical for all sets of groups and, in most cases, agreed with RO plan selection. Differences between RO choice and SPIDERplan analysis only occurred in cases for which the score differences between the plans was very low. A score difference threshold of 0.005 was defined as the value below which two plans are considered of equivalent quality. Conclusion: Generally, SPIDERplan response successfully reproduced the ROs plan selection. SPIDERplan assessment performance can represent clinical preferences based either on manual or automatic group weight assignment. For nasopharynx cases, SPIDERplan was robust in terms of the definitions of structure groups, being able to support different configurations without losing accuracy.

Purpose: To apply the recent code of practice from the IAEA/AAPM, TRS 483, to helical tomotherapy (HT) for reference and relative dosimetry obtaining correction factors for the Exradin A1SL ionization chamber.Methods: The beam quality correction factor for the A1SL chamber was obtained through three different approaches following TRS 483 concepts and compared with published values. The determination of the reference absolute dose for the machine‐specific reference (msr) field was complemented with relative dosimetry through the determination of output factors of small fields using different detectors. The response of A1SL was compared with correction‐free film results and corrected output factors of other detectors.Results: A weighted mean beam quality correction factor of 0.9945± 0.0073 was obtained for the A1SL chamber which is in agreement with values reported in the literature. Output factors obtained with different detectors were in agreement, given the uncertainty level. Considering the film output factors as free of corrections, the average value for A1SL output factors corrections was 1.000 ± 0.007.Conclusions: The beam quality correction factors for the A1SL chamber obtained through the three different pathways recommended by TRS 483 agreed with each other and also with published values. The measurements from the A1SL chamber normalized to the msr field in HT can be taken as output factors for small clinical field sizes without further corrections.