Implementing Plan of the Day for Cervical Cancer: A Comparison of Target Volume Generation Methods.

Purpose: Owing to substantial interfraction motion in cervical cancer, plan-of-the-day (PotD) adaptive radiation therapy may be of benefit to patients. Implementation is limited by uncertainty over how to generate the planning target volumes (PTVs). We compared published methods on our own patients. Methods and Materials: Forty patients each had 3 planning scans with variable bladder filling and daily cone beam computed tomographies (cone beam CTs) during radiation therapy; 5 to 11 cone beam CTs were selected to represent interfraction motion. Clinical target volumes (CTVs) and organs at risk were contoured following EMBRACE-II guidelines. A literature search identified 30 adaptive and nonadaptive solutions to PTV generation, which we applied to our patients. PTV sizes and mean coverage of the daily CTV were determined. For 11 patients, the clinically implemented, subjectively edited plan library was also investigated. Results: Eleven studies assessed 15 PotD strategies against nonadaptive comparators on a median of 14 patients (range, 9-23). Some PotD approaches applied margin recipes to the CTV on each planning scan, some modeled the CTV against bladder volume, and others applied incremental isotropic margins to the CTV with a single planning scan. Generally, coverage improved as PTV size increased. The fixed isotropic margin required to provide 100% coverage of all patients was 44 mm, with a mean PTV size of 3316 cm3. The PotD strategy with the best coverage was a 2-plan library formed by modeling the CTV against bladder volume with extrapolation; it provided 98% mean coverage with 1419-cm3 mean PTV size. A 3-plan library consisting of the CTV on each planning scan with 10-mm margin provided 96% mean coverage with 1346-cm3 mean PTV size. The clinically implemented solution that employed subjective extrapolation had mean 100% coverage and 1282-cm3 PTV size on the 11-patient subset. Coverage provided by the best nonadaptive strategies was not statistically superior to the best PotD strategy (P = .13), but PTVs were larger (P = .02). Conclusions: We identified a modeled 2-plan method and a simple 3-plan method, both of which provided excellent coverage with small PTVs compared with nonadaptive strategies.

Developing and validating a simple urethra surrogate model to facilitate dosimetric analysis to predict genitourinary toxicity.

Purpose: The urethra is a critical structure in prostate radiotherapy planning; however, it is impossible to visualise on CT. We developed a surrogate urethra model (SUM) for CT-only planning workflow and tested its geometric and dosimetric performance against the MRI-delineated urethra (MDU). Methods: The SUM was compared against 34 different MDUs (within the treatment PTV) in patients treated with 36.25Gy (PTV)/40Gy (CTV) in 5 fractions as part of the PACE-B trial. To assess the surrogate's geometric performance, the Dice similarity coefficient (DSC), Hausdorff distance (HD), mean distance to agreement (MDTA) and the percentage of MDU outside the surrogate (UOS) were calculated. To evaluate the dosimetric performance, a paired t-test was used to calculate the mean of differences between the MDU and SUM for the D99, D98, D50, D2 and D1. The D(n) is the dose (Gy) to n% of the urethra. Results: The median results showed low agreement on DSC (0.32; IQR 0.21-0.41), but low distance to agreement, as would be expected for a small structure (HD 8.4mm (IQR 7.1-10.1mm), MDTA 2.4mm (IQR, 2.2mm-3.2mm)). The UOS was 30% (IQR, 18-54%), indicating nearly a third of the urethra lay outside of the surrogate. However, when comparing urethral dose between the MDU and SUM, the mean of differences for D99, D98 and D95 were 0.12Gy (p=0.57), 0.09Gy (p=0.61), and 0.11Gy (p=0.46) respectively. The mean of differences between the D50, D2 and D1 were 0.08Gy (p=0.04), 0.09Gy (p=0.02) and 0.1Gy (p=0.01) respectively, indicating good dosimetric agreement between MDU and SUM. Conclusion: While there were geometric differences between the MDU and SUM, there was no clinically significant difference between urethral dose-volume parameters. This surrogate model could be validated in a larger cohort and then used to estimate the urethral dose on CT planning scans in those without an MRI planning scan or urinary catheter.

Measurement and Incorporation of Laryngeal Motion Using cine-MRI on an MR-Linear Accelerator to Generate Radiation Therapy Plans for Early-stage Squamous Cell Cancers of the Glottis.

Purpose: Swallow-related motion of the larynx is most significant in the cranio-caudal directions and of` short duration. Conventional target definition for radical radiation therapy includes coverage of the whole larynx. This study longitudinally examined respiration- and swallow-related laryngeal motions using cine-magnetic resonance imaging. We further analyzed the dosimetry to organs at risk by comparing 3D-conformal radiation therapy (3D-CRT), volumetric modulated arc therapy (VMAT), and intensity modulated radiation therapy (IMRT) techniques. Methods: Fifteen patients with T1-2 N0 glottic squamous cell carcinomas were prospectively recruited for up to 3 cine-MRI scans on the Elekta Unity MR-Linear accelerator, at the beginning, middle, and end of a course of radical radiation therapy. Swallow frequency and motion of the hyoid bone, cricoid and thyroid cartilages, and vocal cords were recorded during swallow and rest. Adapted treatment volumes consisted of gross tumor volume + 0.5-1 cm to a clinical target volume with an additional internal target volume (ITV) for personalized resting-motion. Swallow-related motion was deemed infrequent and was not accounted for in the ITV. We compared radiation therapy plans for 3D-CRT (whole larynx), VMAT (whole larynx), and VMAT and IMRT (ITV for resting motion). Results: Resting- and swallow-related motions were most prominent in the cranio-caudal plane. There were no significant changes in the magnitude of motion over the course of radiation therapy. There was a trend of a progressive reduction in the frequency of swallow. Treatment of partial larynx volumes with intensity modulated methods significantly reduced the dose to carotid arteries, compared with treatment of whole larynx volumes. Robustness analysis demonstrated that when accounting for intrafraction swallow, the total dose delivered to the ITV/planning target volume was maintained at above 95%. Conclusions: Swallow-related motions are infrequent and accounting for resting motion in an ITV is sufficient. VMAT/IMRT techniques that treat more conformal targets can significantly spare critical organs at risk such as the carotid arteries and thyroid gland, potentially reducing the risk of carotid artery stenosis-related complications and other long-term complications.