A simple method of evaluating margin-growing accuracy in image-guided radiation therapy

Dynamic Changes in Bladder Morphology Over Time in Cervical Cancer Patients

Objectives:

Continuous surveillance of bladder volume (BV) is beneficial during the treatment of various urogenital diseases because the bladder is always changing its position, size and even shape at different filling phases. For this purpose, we quantified the motion of the urinary bladder.

Methods:

Daily ultrasound measurements and weekly cone-beam computed tomography scans were obtained from 89 patients in the supine position. BV, bladder centroid positions, and triaxial lengths in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions were compared across different time points.

Results:

BV linearly increased over time, and the mean urinary filling rate (v_tot) was correlated with the patients’ age and water consumption. The greatest bladder centroid motion occurred longitudinally, with less movement observed laterally. The maximum bladder centroid movement was 18.8 ± 2.2 mm inferiorly and 1.8 ± 0.9 mm posteriorly for every 10% decrease in BV. The rates of changes in triaxial lengths differed across the 4 filling phases. The rate was the largest at a BV range of 10-80 mL, especially in the LR direction, with values of 5.9 ± 1.0, 3.6 ± 1.0, and 3.9 ± 1.0 mm per every 10-mL BV increase for LR, AP, and SI, respectively. With bladder filling (<80 mL), the maximum increase in triaxial length was observed in the SI direction and the rates of all changes considerably decreased, especially at BV > 600 mL.

Conclusion:

The v_tot could be used to evaluate the temporal changes in the bladder. The spatial changes should be assessed according to different filling phases based on the centroid position and triaxial lengths.

Impact of CT slice thickness on volume and dose evaluation during thoracic cancer radiotherapy

Introduction

Accurate delineation of targets and organs at risk (OAR) is required to ensure treatment efficacy and minimize risk of normal tissue toxicity with radiotherapy. Therefore, we evaluated the impacts of computed tomography (CT) slice thickness and reconstruction methods on the volume and dose evaluations of targets and OAR.

Patients and methods

Eleven CT datasets from patients with thoracic cancer were included. 3D images with a slice thickness of 2 mm (2–CT) were created automatically. Images of other slice thickness (4–CT, 6–CT, 8–CT, 10–CT) were reconstructed manually by the selected 2D images using two methods; internal tumor information and external CT Reference markers. Structures and plans on 2–CT images, as a reference data, were copied to the reconstructed images.

Results

The maximum error of volume was 84.6% for the smallest target in 10–CT, and the maximum error (≥20 cm³) was 10.1%, 14.8% for the two reconstruction methods, internal tumor information and external CT Reference, respectively. Changes in conformity index for a target of <20 cm³ were 5.4% and 17.5% in 8–CT. Changes on V₃₀ and V₄₀ of the heart were considerable. In the internal tumor information method, volumes of hearts decreased by 3.2% in 6–CT, while V₃₀ and V₄₀ increased by 18.4% and 46.6%.

Conclusion

The image reconstruction method by internal tumor information was less affected by slice thickness than the image reconstruction method by external CT Reference markers. This study suggested that before positioning scanning, the largest section through the target should be determined and the optimal slice thickness should be estimated.

Dependence of volume calculation and margin growth accuracy on treatment planning systems for stereotactic radiosurgery

OBJECTIVE

Uncertainties in radiotherapy target structures are partly dependent on differences between volume calculation and margin growing methods in treatment planning systems (TPS). These uncertainties are exacerbated with very small structures such as those common in stereotactic radiosurgery.

METHODS

Data from a national commissioning programme for SRS was used to assess variation in reported volumes for six benchmark cases, including malignant and benign indications. Reported volumes were compared both with and without any margins added according to local practice.

RESULTS

137 plans were submitted, with a total of 311 structures and covering seven TPS. For volumes < 1 cm³ agreement was within 0.05 cm³, and for volumes > 1 cm³ agreement was within 5%. Systematic differences were seen between TPS, partly because of different methods for calculating the end slice volume. About one third of structures had a margin added, of 1-2 mm. Most TPS over-grew the volumes, compared to the approximation of a perfect sphere, especially Pinnacle and Eclipse.

CONCLUSION

Differences between volume calculation methods may lead to 5-10% variation in reported volumes from different TPS. This should be taken into account when comparing multicentre studies, and it is recommended that a minimum volume of 0.05 cm³ be used for any near-point doses to allow more consistent comparisons. When margins are added to small structures, there may be up to 40% difference to nominal margin size. Such differences are still small compared to interobserver variation in delineation. Advances in knowledge: This study quantifies the potential uncertainties in clinical volume calculation and margin growth with small radiosurgical targets.