Dose Plan Quality Research Articles

The effect of tumor shape irregularity on Gamma Knife treatment plan quality and treatment outcome: an analysis of 234 vestibular schwannomas

The primary aim of Gamma Knife (GK) radiosurgery is to deliver high-dose radiation precisely to a target while conforming to the target shape. In this study, the effects of tumor shape irregularity (TSI) on GK dose-plan quality and treatment outcomes were analyzed in 234 vestibular schwannomas. TSI was quantified using seven different metrics including volumetric index of sphericity (VioS). GK treatment plans were created on a single GK-Perfexion/ICON platform. The plan quality was measured using selectivity index (SI), gradient index (GI), Paddick’s conformity index (PCI), and efficiency index (EI). Correlation and linear regression analyses were conducted between shape irregularity features and dose plan indices. Machine learning was employed to identify the shape feature that predicted dose plan quality most effectively. The treatment outcome analysis including tumor growth control and serviceable hearing preservation at 2 years, were conducted using Cox regression analyses. All TSI features correlated significantly with the dose plan indices (P < 0.0012). With increasing tumor volume, vestibular schwannomas became more spherical (P < 0.05) and the dose plan indices varied significantly between tumor volume subgroups (P < 0.001 and P < 0.01). VioS was the most effective predictor of GK indices (P < 0.001) and we obtained 89.36% accuracy (79.17% sensitivity and 100% specificity) for predicting PCI. Our results indicated that TSI had significant effects on the plan quality however did not adversely affect treatment outcomes.

Effect of fluence smoothening in intensity modulated radiotherapy planning and delivery

Objective: In Intensity Modulated Radiotherapy (IMRT) planning, optimization is a computational problem, potentially susceptible to noise and artifacts (high frequency spatial fluctuations) producing sharp fluence peaks and valleys in millimetric spatial scale. A solution to this problem is to smooth the beam profiles. Methods and Materials: In the Eclipse TPS, fluence smoothing is achieved within the objective function of the inverse treatment planning systems. Plans were developed for a 6 MV photon beam from a Varian Clinac-DHX equipped with a 120 leaves MLC. Total of 160 dose plans were compared and fed into the analysis process. The dose plan quality has been analysed in terms of Statistical computation by means of two-sided paired t-test between two smoothening levels (s25 and s75) in terms of Homogeneity-Index, Conformity-Index, Target dose coverage and OAR dose differences in terms of max, min and mean doses. Results: From our present study on the influence of smoothening of fluence levels in IMRT plans results, there was a reduction in total MU’s with no significant statistical variation in terms of mean differences of HI Index, CI index, PTV coverages, OAR doses. Moreover, the reduction in MU’s will help the less head leakage dose hence the lower whole-body dose, which will help the patient to reduce the chances for secondary malignancies. Conclusion: Hence, we conclude that, higher fluence smoothening levels with acceptable difference in target coverage and minimum variation of OAR should be selected for the execution.

EP-1936: Dose plan quality in the DBCG HYPO trial: an evaluation based on all treatment plans in the study

EP-1936: Dose plan quality in the DBCG HYPO trial: an evaluation based on all treatment plans in the study

TU-AB-201-05: Automatic Adaptive Per-Operative Re-Planning for HDR Prostate Brachytherapy - a Simulation Study On Errors in Needle Positioning

Purpose: To develop adaptive planning with feedback for MRI-guided focal HDR prostate brachytherapy with a single divergent needle robotic implant device. After each needle insertion, the dwell positions for that needle are calculated and the positioning of remaining needles and dosimetry are both updated based on MR imaging. Methods: Errors in needle positioning may occur due to inaccurate needle insertion (caused by e.g. the needle’s bending) and unpredictable changes in patient anatomy. Consequently, the dose plan quality might dramatically decrease compared to the preplan. In this study, a procedure was developed to re-optimize, after each needle insertion, the remaining needle angulations, source positions and dwell times in order to obtain an optimal coverage (D95% PTV>19 Gy) without exceeding the constraints of the organs at risk (OAR) (D10% urethra<21 Gy, D1cc bladder<12 Gy and D1cc rectum<12 Gy). Complete HDR procedures with 6 needle insertions were simulated for a patient MR-image set with PTV, prostate, urethra, bladder and rectum delineated. Random angulation errors, modeled by a Gaussian distribution (standard deviation of 3 mm at the needle’s tip), were generated for each needle insertion. We compared the final dose parameters for the situations (I) without re-optimization and (II) with the automatic feedback. Results: The computation time of replanning was below 100 seconds on a current desk computer. For the patient tested, a clinically acceptable dose plan was achieved while applying the automatic feedback (median(range) in Gy, D95% PTV: 19.9(19.3–20.3), D10% urethra: 13.4(11.9–18.0), D1cc rectum: 11.0(10.7–11.6), D1cc bladder: 4.9(3.6–6.8)). This was not the case without re-optimization (median(range) in Gy, D95% PTV: 19.4(14.9–21.3), D10% urethra: 12.6(11.0–15.7), D1cc rectum: 10.9(8.9–14.1), D1cc bladder: 4.8(4.4–5.2)). Conclusion: An automatic guidance strategy for HDR prostate brachytherapy was developed to compensate errors in needle positioning and improve the dose distribution. Without re-optimization, target coverage and OAR constraints may not be achieved. M. Borot de Battisti is funded by Philips Medical Systems Nederland B.V.; M. Moerland is principal investigator on a contract funded by Philips Medical Systems Nederland B.V.; G. Hautvast and D. Binnekamp are full-time employees of Philips Medical Systems Nederland B.V.

MO‐D‐M100J‐01: Dose Painting With Intensity Modulated Proton Therapy and Intensity Modulated X‐Ray Therapy: A Comparison

Purpose: To compare intensity modulated proton therapy (IMPT) versus intensity modulated x‐ray therapy (IMXT) for the delivery of nonuniform dose prescriptions based on hypoxia‐imaging, so‐called dose painting. Materials and Methods: IMXT delivered with helical tomotherapy (HT) was compared to IMPT delivered with spot scanning (SS) and distal gradient tracking (DGT). The novel DGT method places beam spots where dose prescription gradients occur along the pencil beam axis. Fundamental dosimetric properties of each modality were assessed by creating optimized plans for 144 variations of a cylindrical phantom with six boost regions embedded inside a base tumor region. Clinical cases with biologically conformal dose prescriptions based on PET with the ‐ATSM hypoxia imaging radiopharmaceutical were planned. The effects on the nonuniform dose distribution of delivering IMPT on a 180° arc versus equi‐spaced beams spread over 360° were investigated. Results: Phantom studies showed that nonuniform dose plan quality for tomotherapy, SS, and DGT, was similar, but DGT plans were most sensitive to phantom size and boost region proximity. IMPT reduced normal tissue integral dose by a mean factor of around two relative to IMXT. Clinical dose deviations from the prescription were comparable for all modalities, but arc IMPT deliveries markedly reduced normal tissue dose and improved critical structure sparing without compromising the dose distribution in the tumor. Conclusions: In the target volume, IMXT and IMPT deliver comparable nonuniform dose distributions. IMPT offers improved integral normal tissue dose and sparing of critical structures over IMXT, as was the case for uniform dose deliveries. DGT reduces required beam spots by a factor of about three relative to SS. IMPT dose painting will require similar management of intrafractional patient motion as for IMXT, with the additional consideration of proton spot placement uncertainty.TR Mackie has a conflict of interest due to financial interest in TomoTherapy, Inc.

Shielding strategies for Gamma Knife surgery of pituitary adenomas

The relative performances of two plugging strategies commonly used for pituitary adenoma dose plans were evaluated in terms of factors that influence dose plan quality. Dose plans and clinical treatment data were obtained in 108 patients treated with the Model C Gamma Knife at the University of Virginia. These data were analyzed to determine factors (including plugging strategy) influencing the quality of the dose plans in terms of beam time, conformity, dose to the optic apparatus, and plugging burden. For both secretory and nonsecretory adenomas, beam time (p(secretory) < 0.001, p(nonsecretory) = 0.015) and plugging burden (p(secretory) = 0.007, p(nonsecretory) = 0.038) were reduced when using the customized plugging strategy. The choice of plugging strategy was found to play no significant role in conformity or dose to the optic apparatus. Other factors found to play a significant role in adenoma dose plan quality included tumor volume, prescription dose, and distance from the target to the optic pathways. While both plugging strategies were effective at providing the required protection to the optic pathways, the authors found that the customized plugging strategy provided more efficient performance in pituitary adenoma treatments.

Improved -Knife Radiosurgery Using Combinations of Different-Sized Collimators

The authors present a new method for using the Leksell γ knife. To obtain isodose curves that are best suited to the target volumes, they propose, for a given shot, combining on the same helmet collimators of different sizes (4, 8, 14, and 18 mm), as well as plugs. The method for optimizing the positions of the collimators and of the plugs on the helmets is described. Tests were carried out on fictitious volumes to compare conventional treatment plans with those obtained after optimization. The collimator combination method demonstrates an increase in dose plan quality as measured by the conventional dose quality indexes found in the literature, as well as savings of irradiation and patient positioning time. The authors demonstrate that it is thus possible to achieve a considerable improvement in γ-knife performance, without changing its main technical characteristics.