Target and organ dose estimation from intensity modulated head and neck radiation therapy using 3 deformable image registration algorithms

Radiation therapy margin reduction for patients with localized prostate cancer: A prospective study of the dosimetric impact and quality of life

OBJECTIVES

To investigate the impact of reducing Clinical Target Volume (CTV) to Planning Target Volume (PTV) margins on delivered radiation therapy (RT) dose and patient reported quality-of-life (QOL) for patients with localized prostate cancer.

METHODS

Twenty patients were included in a single institution IRB-approved prospective study. Nine were planned with reduced margins (4 mm at prostate/rectum interface, 5 mm elsewhere), and 11 with standard margins (6/10 mm). Cumulative delivered dose was calculated using deformable dose accumulation. Each daily CBCT dataset was deformed to the planning CT (pCT), dose was computed, and accumulated on the resampled pCT using a parameter-optimized, B-spline algorithm (Elastix, ITK/VTK). EPIC-26 patient reported QOL was prospectively collected pre-treatment, post-treatment, and at 2-, 6-, 12-, 18-, 24-, 36-, 48-, and 60-month follow-ups. Post -RT QOL scores were baseline corrected and standardized to a [0-100] scale using EPIC-26 methodology. Correlations between QOL scores and dosimetric parameters were investigated, and the overall QOL differences between the two groups (QOLMargin-reduced -QOLcontrol ) were calculated.

RESULTS

The median QOL follow-up length for the 20 patients was 48 months. Difference between delivered dose and planned dose did not reach statistical significance (p > 0.1) for both targets and organs at risk between the two groups. At 4 years post-RT, standardized mean QOLMargin-reduced -QOLcontrol were improved for Urinary Incontinence, Urinary Irritative/Obstructive, Bowel, and Sexual EPIC domains by 3.5, 14.8, 10.2, and 16.1, respectively (higher values better). The control group showed larger PTV/rectum and PTV/bladder intersection volumes (7.2 ± 5.8, 18.2 ± 8.1 cc) than the margin-reduced group (2.6 ± 1.8, 12.5 ± 8.3 cc), though the dose to these intersection volumes did not reach statistical significance (p > 0.1) between the groups. PTV/rectum intersection volume showed a moderate correlation (r = -0.56, p < 0.05) to Bowel EPIC domain.

CONCLUSIONS

Results of this prospective study showed that margin-reduced group exhibited clinically meaningful improvement of QOL without compromising the target dose coverage.

Comparisons of normal tissue complication probability models derived from planned and delivered dose for head and neck cancer patients

BACKGROUND AND PURPOSE

Normal tissue complication probability (NTCP) models are typically derived from the planned dose distribution, which can deviate from the delivered dose due to anatomical day-to-day variations. The aim of this study was to compare NTCP models derived from the planned and the delivered dose for head and neck cancer (HNC) patients.

MATERIAL AND METHOD

322 HNC patients who received radiotherapy with daily CBCT guidance were included in this retrospective study. The delivered dose was estimated by deformably accumulating dose from daily CBCT to planning anatomy. We used a Lyman-Kutcher-Burman NTCP model, to relate the equivalent uniform dose (EUD) of organs at risk (OAR) with oral mucositis, xerostomia and dysphagia respectively. We compared the model parameters and performances.

RESULTS

The median differences between planned and delivered EUD to the OARs were significantly larger for patients with toxicity than without for acute dysphagia (≥G2 and ≥G3) and late dysphagia (≥G3) (p < 0.05). Those differences resulted in small differences in steepness and agreement to the data between delivered- and planned-fitted NTCP curves, and the differences were not significant. The differences in AUC were less than 0.01.

CONCLUSION

Differences between delivered and planned dose did not lead to significant differences in NTCP curves. The additional clinical relevance of NTCP models using accumulated dose for oral mucositis, xerostomia and dysphagia in HNC radiotherapy is likely to be limited.

Mixed-beam approach for high-risk prostate cancer: Carbon-ion boost followed by photon intensity-modulated radiotherapy. Dosimetric and geometric evaluations (AIRC IG-14300)

BACKGROUND AND PURPOSE

The aim was to evaluate dosimetric uncertainties of a mixed beam approach for patients with high-risk prostate cancer (PCa). The treatment consists of a carbon ion radiotherapy (CIRT) boost followed by whole-pelvis intensity-modulated RT (IMRT).

MATERIALS AND METHODS

Patients were treated with a CIRT boost of 16.6 Gy/4 fractions followed by whole-pelvis IMRT of 50 Gy/25 fractions, with consequent long term androgen deprivation therapy. Deformable computed tomography image registration (DIR) was performed and corresponding doses were used for plan sum. A comparative IMRT photon plan was obtained as whole-pelvis IMRT of 50 Gy/25 fractions followed by a boost of 28 Gy/14 fractions. DIR performances were evaluated through structure-related and image characteristics parameters.

RESULTS

Until now, five patients out of ten total enrolled ended the treatment. Dosimetric parameters were lower in CIRT + IMRT than IMRT-only plans for all organs at risk (OARs) except femoral heads. Regarding DIR evaluation, femoral heads were the less deformed OAR. Penile bulb, bladder and anal canal showed intermediate deformation. Rectum was the most deformed. DIR algorithms were patient (P)-dependent, as performances were the highest for P3 and P4, intermediate for P2 and P5, and the lowest for P1.

CONCLUSIONS

CIRT allows better OARs sparing while increasing the efficacy due to the higher radio-biological effect of carbon ions. However, a mixed beam approach could introduce DIR problems in multi-centric treatments with different operative protocols. The development of this prospective trial will lead to more mature data concerning the clinical impact of implementing DIR procedures in dose accumulation applications for high-risk PCa treatments.

Deformable Registration for Dose Accumulation

As deformable image registration makes its way into the clinical routine, the summation of doses from fractionated treatment regimens to evaluate cumulative doses to targets and healthy tissues is also becoming a frequently utilized tool in the context of image-guided adaptive radiotherapy. Accounting for daily geometric changes using deformable image registration and dose accumulation potentially enables a better understanding of dose-volume-effect relationships, with the goal of translation of this knowledge to personalization of treatment, to further enhance treatment outcomes. Treatment adaptation involving image deformation requires patient-specific quality assurance of the image registration and dose accumulation processes, to ensure that uncertainties in the 3D dose distributions are identified and appreciated from a clinical relevance perspective. While much research has been devoted to identifying and managing the uncertainties associated with deformable image registration and dose accumulation approaches, there are still many unanswered questions. Here, we provide a review of current deformable image registration and dose accumulation techniques, and related clinical application. We also discuss salient issues that need to be deliberated when applying deformable algorithms for dose mapping and accumulation in the context of adaptive radiotherapy and response assessment.

The impact of robustness of deformable image registration on contour propagation and dose accumulation for head and neck adaptive radiotherapy

Deformable image registration (DIR) is the key process for contour propagation and dose accumulation in adaptive radiation therapy (ART). However, currently, ART suffers from a lack of understanding of “robustness” of the process involving the image contour based on DIR and subsequent dose variations caused by algorithm itself and the presetting parameters. The purpose of this research is to evaluate the DIR caused variations for contour propagation and dose accumulation during ART using the RayStation treatment planning system. Ten head and neck cancer patients were selected for retrospective studies. Contours were performed by a single radiation oncologist and new treatment plans were generated on the weekly CT scans for all patients. For each DIR process, four deformation vector fields (DVFs) were generated to propagate contours and accumulate weekly dose by the following algorithms: (a) ANACONDA with simple presetting parameters, (b) ANACONDA with detailed presetting parameters, (c) MORFEUS with simple presetting parameters, and (d) MORFEUS with detailed presetting parameters. The geometric evaluation considered DICE coefficient and Hausdorff distance. The dosimetric evaluation included D₉₅, D_max, D_mean, D_min, and Homogeneity Index. For geometric evaluation, the DICE coefficient variations of the GTV were found to be 0.78 ± 0.11, 0.96 ± 0.02, 0.64 ± 0.15, and 0.91 ± 0.03 for simple ANACONDA, detailed ANACONDA, simple MORFEUS, and detailed MORFEUS, respectively. For dosimetric evaluation, the corresponding Homogeneity Index variations were found to be 0.137 ± 0.115, 0.006 ± 0.032, 0.197 ± 0.096, and 0.006 ± 0.033, respectively. The coherent geometric and dosimetric variations also consisted in large organs and small organs. Overall, the results demonstrated that the contour propagation and dose accumulation in clinical ART were influenced by the DIR algorithm, and to a greater extent by the presetting parameters. A quality assurance procedure should be established for the proper use of a commercial DIR for adaptive radiation therapy.

Representing the dosimetric impact of deformable image registration errors

Deformable image registration (DIR) is emerging as a tool in radiation therapy for calculating the cumulative dose distribution across multiple fractions of treatment. Unfortunately, due to the variable nature of DIR algorithms and dependence of performance on image quality, registration errors can result in dose accumulation errors. In this study, landmarked images were used to characterize the DIR error throughout an image space and determine its impact on dosimetric analysis. Ten thoracic 4DCT images with 300 landmarks per image study matching the end-inspiration and end-expiration phases were obtained from 'dir-labs'. DIR was performed using commercial software MIM Maestro. The range of dose uncertainty (RDU) was calculated at each landmark pair as the maximum and minimum of the doses within a sphere around the landmark in the end-expiration phase. The radius of the sphere was defined by a measure of DIR error which included either the actual DIR error, mean DIR error per study, constant errors of 2 or 5 mm, inverse consistency error, transitivity error or the distance discordance metric (DDM). The RDUs were evaluated using the magnitude of dose uncertainty (MDU) and inclusion rate (IR) of actual error lying within the predicted RDU. The RDU was calculated for 300 landmark pairs on each 4DCT study for all measures of DIR error. The most representative RDU was determined using the actual DIR error with a MDU of 2.5 Gy and IR of 97%. Across all other measures of DIR error, the DDM was most predictive with a MDU of 2.5 Gy and IR of 86%, closest to the actual DIR error. The proposed method represents the range of dosimetric uncertainty of DIR error using either landmarks at specific voxels or measures of registration accuracy throughout the volume.