Automated Planning for Prostate Stereotactic Body Radiation Therapy on the 1.5 T MR-Linac.

PURPOSE: Adaptive stereotactic body radiation therapy (SBRT) for prostate cancer (PC) by the 1.5 T MR-linac currently requires online planning by an expert user. A fully automated and user-independent solution to adaptive planning (mCycle) of PC-SBRT was compared with user's plans for the 1.5 T MR-linac. METHODS AND MATERIALS: Fifty adapted plans on daily magnetic resonance imaging scans for 10 patients with PC treated by 35 Gy (prescription dose [Dp]) in 5 fractions were reoptimized offline from scratch, both by an expert planner (manual) and by mCycle. Manual plans consisted of multicriterial optimization (MCO) of the fluence map plus manual tweaking in segmentation, whereas in mCycle plans, the objectives were sequentially optimized by MCO according to an a-priori assigned priority list. The main criteria for planning approval were a dose ≥95% of the Dp to at least 95% of the planning target volume (PTV), V33.2 (PTV) ≥ 95%, a dose less than the Dp to the hottest cubic centimeter (V35 ≤ 1 cm3) of rectum, bladder, penile bulb, and urethral planning risk volume (ie, urethra plus 3 mm isotropically), and V32 ≤ 5%, V28 ≤ 10%, and V18 ≤ 35% to the rectum. Such dose-volume metrics, plus some efficiency and deliverability metrics, were used for the comparison of mCycle versus manual plans. RESULTS: mCycle plans improved target dose coverage, with V33.2 (PTV) passing on average (±1 SD) from 95.7% (±1.0%) for manual plans to 97.5% (±1.3%) for mCycle plans (P < .001), and rectal dose sparing, with significantly reduced V32, V28, and V18 (P ≤ .004). Although at an equivalent number of segments, mCycle plans consumed moderately more monitor units (+17%) and delivery time (+9%) (P < .001), whereas they were generally faster (-19%) in terms of optimization times (P < .019). No significant differences were found for the passing rates of locally normalized γ (3 mm, 3%) (P = .059) and γ (2 mm, 2%) (P = .432) deliverability metrics. CONCLUSIONS: In the offline setting, mCycle proved to be a trustable solution for automated planning of PC-SBRT on the 1.5 T MR-linac. mCycle integration in the online workflow will free the user from the challenging online-optimization task.

Adaptive SBRT by 1.5 T MR-linac for prostate cancer: On the accuracy of dose delivery in view of the prolonged session time.

PURPOSE: Adaptive Stereotactic Body Radiotherapy (SBRT) of prostate cancer (PC) by online 1.5 T MRi-guidance prolongs session-time, due to contouring and planning tasks, thus increasing the risk of prostate motion. Hence, the interest to verify the adequacy of the delivered dose. MATERIAL AND METHODS: For twenty PC patients treated by 35 Gy (Dp) in five fractions, daily pre- and post- delivery MRi scans were respectively used for adapt-to-shape (ATS) optimization, and re-computation of the delivered irradiation (Drec). Two expansion recipes, from Clinical (CTV) to Planning target volume (PTV), which slightly differed in the posterior margin were used for groups I and II, of ten patients each. Plans had to assure D95% ≥ 95%Dp to PTV, and D1cc ≤ Dp to rectum, bladder, penile bulb, and urethral planning-risk-volume (urethral-PRV). The adequacy of the delivered dose was estimated by inter-fraction average (ifa) of dose-volume metrics computed from Drec. A cumulative dose (Dsum) was calculated from the five daily Drec deformed onto the simulation MRi. RESULTS: For each patient, CTV coverage resulted in D95% > 95%Dp when estimated as ifa by Drec. No significant difference for D95% and D99% metrics to CTV resulted between groups I and II. D1cc was < Dp for rectum, urethral-PRV, and penile bulb, whereas < 103.5%Dp for the bladder. Significant correlations resulted between metrics computed by Dsum and as ifa by Drec, by both linear-correlation analysis, and Receiver-Operating-Characteristic curve analysis. CONCLUSIONS: Our results for PC-SBRT confirm the adequacy of the delivered dose by ATS with 1.5 T MR-linac, and the consistency between dose-volume metrics computed by Drec and Dsum.

Retrospective measurements of thoron and radon by CDs/DVDs: a model approach.

An approach for retrospective measurements of thoron ((220)Rn) and radon ((222)Rn) by home-stored CDs/DVDs is proposed. It employs analysis of alpha tracks at two depths beneath the disk surface. The signal in the first one (69 µm) is due both to (220)Rn and (222)Rn, while the signal at the second (80 µm) is due only to (222)Rn. The second signal is used as to measure (222)Rn, as well as to determine and subtract 'the (222)Rn component' from the first signal. The remaining '(220)Rn component' is used to measure thoron. Numerical modelling is performed and the results show that simultaneous retrospective measurements of thoron and radon are possible over a wide range of environmental concentrations.

Modeling a 222Rn measurement technique based on absorption in polycarbonates and track-etch counting.

In the last decade the high radon absorption ability of some polycarbonates has been successfully used to measure Rn in air, water, and soil-gas. This report describes a theoretical model of the response of Rn detectors that are based on radon absorption and electrochemical etching of alpha-tracks in polycarbonates. The model is based on the volume distribution of the absorbed Rn and the response function of electrochemically etched Makrofol. Theoretical expression for the calibration factor is obtained, and comparison with experimental results is made. The comparison demonstrates very good correspondence between theoretical and experimental values. The conclusion is that this model can be useful in design and optimization of Rn measurements based on absorption in polycarbonates.