Comp Plan: A computer program to generate dose and radiobiological metrics from dose-volume histogram files

Evaluating the relationship between contouring variability and modelled treatment outcome for prostate bed radiotherapy

Objectives.Contouring similarity metrics are often used in studies of inter-observer variation and automatic segmentation but do not provide an assessment of clinical impact. This study focused on post-prostatectomy radiotherapy and aimed to (1) identify if there is a relationship between variations in commonly used contouring similarity metrics and resulting dosimetry and (2) identify the variation in clinical target volume (CTV) contouring that significantly impacts dosimetry.Approach.The study retrospectively analysed CT scans of 10 patients from the TROG 08.03 RAVES trial. The CTV, rectum, and bladder were contoured independently by three experienced observers. Using these contours reference simultaneous truth and performance level estimation (STAPLE) volumes were established. Additional CTVs were generated using an atlas algorithm based on a single benchmark case with 42 manual contours. Volumetric-modulated arc therapy (VMAT) treatment plans were generated for the observer, atlas, and reference volumes. The dosimetry was evaluated using radiobiological metrics. Correlations between contouring similarity and dosimetry metrics were calculated using Spearman coefficient (Γ). To access impact of variations in planning target volume (PTV) margin, the STAPLE PTV was uniformly contracted and expanded, with plans created for each PTV volume. STAPLE dose-volume histograms (DVHs) were exported for plans generated based on the contracted/expanded volumes, and dose-volume metrics assessed.Mainresults. The study found no strong correlations between the considered similarity metrics and modelled outcomes. Moderate correlations (0.5 <Γ< 0.7) were observed for Dice similarity coefficient, Jaccard, and mean distance to agreement metrics and rectum toxicities. The observations of this study indicate a tendency for variations in CTV contraction/expansion below 5 mm to result in minor dosimetric impacts.Significance. Contouring similarity metrics must be used with caution when interpreting them as indicators of treatment plan variation. For post-prostatectomy VMAT patients, this work showed variations in contours with an expansion/contraction of less than 5 mm did not lead to notable dosimetric differences, this should be explored in a larger dataset to assess generalisability.

An investigation of the conformity, feasibility, and expected clinical benefits of multiparametric MRI-guided dose painting radiotherapy in glioblastoma

Background

New technologies developed to improve survival outcomes for glioblastoma (GBM) continue to have limited success. Recently, image-guided dose painting (DP) radiotherapy has emerged as a promising strategy to increase local control rates. In this study, we evaluate the practical application of a multiparametric MRI model of glioma infiltration for DP radiotherapy in GBM by measuring its conformity, feasibility, and expected clinical benefits against standard of care treatment.

Methods

Maps of tumor probability were generated from perfusion/diffusion MRI data from 17 GBM patients via a previously developed model of GBM infiltration. Prescriptions for DP were linearly derived from tumor probability maps and used to develop dose optimized treatment plans. Conformity of DP plans to dose prescriptions was measured via a quality factor. Feasibility of DP plans was evaluated by dose metrics to target volumes and critical brain structures. Expected clinical benefit of DP plans was assessed by tumor control probability. The DP plans were compared to standard radiotherapy plans.

Results

The conformity of the DP plans was >90%. Compared to the standard plans, DP (1) did not affect dose delivered to organs at risk; (2) increased mean and maximum dose and improved minimum dose coverage for the target volumes; (3) reduced minimum dose within the radiotherapy treatment margins; (4) improved local tumor control probability within the target volumes for all patients.

Conclusions

A multiparametric MRI model of GBM infiltration can enable conformal, feasible, and potentially beneficial dose painting radiotherapy plans.

Dose planning variations related to delineation variations in MRI-guided brachytherapy for locally advanced cervical cancer

PURPOSE

To examine the variability in prescribed dose due to contouring variations in intracavitary image-guided adaptive brachytherapy for cervical cancer. To identify correlations between dosimetric outcomes and delineation uncertainty metrics.

METHODS AND MATERIALS

A data set from an EMBRACE sub-study on contouring uncertainties was used, consisting of magnetic resonance images of six patients with cervical cancer delineated by 10 experienced observers (target volumes and organs at risk). Two gold standard contours were generated, an expert consensus and the simultaneous truth and performance level estimation. Plans were individually optimised to all of the contour sets (12 in total). Plans were applied to the gold standard contour sets, and dose volume histogram parameters including D₉₀, D₉₈ and D_2cm³ were determined. The variability between plans was assessed. Dose volume histogram parameters and delineation uncertainty metrics were correlated using the Spearman's non-parametric rank correlation.

RESULTS

There is a dosimetric variability between observers, patients and the gold standard contour used for analysis. Approximately 3 Gy D₉₀ EQD2₁₀ variability (SD) was observed for the CTVHR and 1.2-3.6 Gy D_2cm³ EQD2₃ for the organs at risk. The maximum geometric dimensions of the delineations are most commonly correlated with dosimetry changes. Although the correlations are similar across gold standards, the direction of these correlations differs, indicating that the dosimetric outcomes are dependent on the contour that the plan is optimised to.

CONCLUSION

This study highlights the dosimetric differences interobserver uncertainty in contouring can have for cervical cancer brachytherapy. The importance of carefully choosing a gold standard from which to benchmark is reiterated.

The impact of contour variation on tumour control probability in anal cancer

BACKGROUND

While intensity modulated radiotherapy (IMRT) has been widely adopted for the treatment of anal cancer (AC), the added contour complexity poses potential risks. This study investigates the impact of contour variation on tumour control probability (TCP) when using IMRT for AC.

METHODS

Nine Australian centres contoured a single computed tomography dataset of a patient with AC. The same optimised template-based IMRT planning protocol was applied to each contour set to generate nine representative treatment plans and their corresponding dose volume histograms. A geometric analysis was performed on all contours. The TCP was calculated for each plan using the linear quadratic and logitEUD model.

RESULTS

The median concordance index (CI) for the bladder, head and neck of femur, bone marrow, small bowel and external genitalia was 0.94, 0.88, 0.84, 0.65 and 0.65, respectively. The median CI for the involved nodal, primary tumour and elective clinical target volumes were 0.85, 0.77 and 0.71, respectively. Across the nine plans, the TCP was not significantly different. Variation in TCP between plans increased as tumour cell load increased or radiation dose decreased.

CONCLUSIONS

When using IMRT for AC, contour variations generated from a common protocol within the limits of minor deviations do not appear to have a significant impact on TCP. Contouring variations may be more critical with increasing tumour cell load or reducing radiotherapy dose.

Use of planning metrics software for automated feedback to radiotherapy students

Background and purpose

Pre-registration teaching of radiotherapy planning in a non-clinical setting should allow students the opportunity to develop clinical decision-making skills. Students frequently struggle with their ability to prioritise and optimise multiple objectives when producing a clinically acceptable plan. Emerging software applications providing quantitative assessment of plan quality are designed for clinical use but may have value for teaching these skills. This project aimed to evaluate the potential value of automated feedback to second year BSc (Hons) Radiotherapy students.

Materials and methods

All 26 students studying a pre-registration radiotherapy planning module were provided with automated prediction of relative feasibility for left lung tumour planning targets by planning metrics software. Students were also provided with interim quantitative reports during the development of their plan. Student perceptions of the software were gathered using an anonymous questionnaire. Independent blinded marking of plans was performed after module completion and analysed for correlation with software-assigned marks.

Results

In total, 25 plans were utilised for marking comparison and 16 students submitted feedback relating to the software. Overall, student feedback was positive regarding the software. A ‘strong’ Spearman’s rank-order correlation (rs=0·7165) was evident between human and computer marks (p=0·000055).

Conclusions

Automated software is capable of providing useful feedback to students as a teaching aid, in particular with regard to relative feasibility of goals. The strong correlation between human and computer marks suggests a role in benchmarking or moderation; however, the narrow scope of assessment parameters suggests value as an adjunct and not a replacement to human marking.

Results of the Australasian (Trans-Tasman Oncology Group) radiotherapy benchmarking exercise in preparation for participation in the PORTEC-3 trial

INTRODUCTION

Protocol deviations in Randomised Controlled Trials have been found to result in a significant decrease in survival and local control. In some cases, the magnitude of the detrimental effect can be larger than the anticipated benefits of the interventions involved. The implementation of appropriate quality assurance of radiotherapy measures for clinical trials has been found to result in fewer deviations from protocol. This paper reports on a benchmarking study conducted in preparation for the PORTEC-3 trial in Australasia.

METHODS

A benchmarking CT dataset was sent to each of the Australasian investigators, it was requested they contour and plan the case according to trial protocol using local treatment planning systems. These data was then sent back to Trans-Tasman Oncology Group for collation and analysis.

RESULTS

Thirty three investigators from eighteen institutions across Australia and New Zealand took part in the study. The mean clinical target volume (CTV) volume was 383.4 (228.5-497.8) cm(3) and the mean dose to a reference gold standard CTV was 48.8 (46.4-50.3) Gy.

CONCLUSIONS

Although there were some large differences in the contouring of the CTV and its constituent parts, these did not translate into large variations in dosimetry. Where individual investigators had deviations from the trial contouring protocol, feedback was provided. The results of this study will be used to compare with the international study QA for the PORTEC-3 trial.

Utilising pseudo-CT data for dose calculation and plan optimization in adaptive radiotherapy

To quantify the dose calculation error and resulting optimization uncertainty caused by performing inverse treatment planning on inaccurate electron density data (pseudo-CT) as needed for adaptive radiotherapy and Magnetic Resonance Imaging (MRI) based treatment planning. Planning Computer Tomography (CT) data from 10 cervix cancer patients was used to generate 4 pseudo-CT data sets. Each pseudo-CT was created based on an available method of assigning electron density to an anatomic image. An inversely modulated radiotherapy (IMRT) plan was developed on each planning CT. The dose calculation error caused by each pseudo-CT data set was quantified by comparing the dose calculated each pseudo-CT data set with that calculated on the original planning CT for the same IMRT plan. The optimization uncertainty introduced by the dose calculation error was quantified by re-optimizing the same optimization parameters on each pseudo-CT data set and comparing against the original planning CT. Dose differences were quantified by assessing the Equivalent Uniform Dose (EUD) for targets and relevant organs at risk. Across all pseudo-CT data sets and all organs, the absolute mean dose calculation error was 0.2 Gy, and was within 2 % of the prescription dose in 98.5 % of cases. Then absolute mean optimisation error was 0.3 Gy EUD, indicating that that inverse optimisation is impacted by the dose calculation error. However, the additional uncertainty introduced to plan optimisation is small compared the sources of variation which already exist. Use of inaccurate electron density data for inverse treatment planning results in a dose calculation error, which in turn introduces additional uncertainty into the plan optimization process. In this study, we showed that both of these effects are clinically acceptable for cervix cancer patients using four different pseudo-CT data sets. Dose calculation and inverse optimization on pseudo-CT is feasible for this patient cohort.

Comparison of Oncentra® Brachy IPSA and graphical optimisation techniques: a case study of HDR brachytherapy head and neck and prostate plans

There are a number of different dwell positions and time optimisation options available in the Oncentra® Brachy (Elekta Brachytherapy Solutions, Veenendaal, The Netherlands) brachytherapy treatment planning system. The purpose of this case study was to compare graphical (GRO) and inverse planning by simulated annealing (IPSA) optimisation techniques for interstitial head and neck (HN) and prostate plans considering dosimetry, modelled radiobiology outcome and planning time. Four retrospective brachytherapy patients were chosen for this study, two recurrent HN and two prostatic boosts. Manual GRO and IPSA plans were generated for each patient. Plans were compared using dose–volume histograms (DVH) and dose coverage metrics including; conformity index (CI), homogeneity index (HI) and conformity number (CN). Logit and relative seriality models were used to calculate tumour control probability (TCP) and normal tissue complication probability (NTCP). Approximate planning time was also recorded. There was no significant difference between GRO and IPSA in terms of dose metrics with mean CI of 1.30 and 1.57 (P > 0.05) respectively. IPSA achieved an average HN TCP of 0.32 versus 0.12 for GRO while for prostate there was no significant difference. Mean GRO planning times were greater than 75 min while average IPSA planning times were less than 10 min. Planning times for IPSA were greatly reduced compared to GRO and plans were dosimetrically similar. For this reason, IPSA makes for a useful planning tool in HN and prostate brachytherapy.

Endorectal balloons in the post prostatectomy setting: do gains in stability lead to more predictable dosimetry?

PURPOSE

To perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients.

METHOD AND MATERIALS

Ten retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups.

RESULTS

The difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15-0.58 cm and 0.15-0.59 cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D95% (mean ± SD) for the non-ERB group was 2.1 ± 6.0 Gy and -0.04 ± 0.20 Gy. While for the ERB group the difference in D95% was 8.7 ± 12.6 Gy and 0.003 ± 0.104 Gy.

CONCLUSIONS

The use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed.