Accuracy in contouring of small and low contrast lesions: comparison between diagnostic quality computed tomography scanner and computed tomography simulation scanner-A phantom study

[Gross tumor volume (GTV) : Basics, methods, registration, limitations]

CLINICAL ISSUE

Gross tumor volume (GTV) denotes the macroscopic tumor which as the central target volume needs to be correctly identified for successful radiotherapy.

STANDARD RADIOLOGICAL METHODS AND METHODICAL INNOVATIONS

In precision radiotherapy, GTV is outlined on 3D tomographic images. The basis is computed tomography (CT), which is often supplemented by additional diagnostic information, e. g. magnetic resonance imaging (MRI) and positron emission tomography (PET). New developments like dual-energy CT, functional MRI and specific PET tracers facilitate a continuously better differentiation between tumor and surrounding normal tissue.

ACHIEVEMENTS

The concept of GTV is a central part of radiotherapy and the basis of radiation treatment planning. Studies regarding the interobserver variability are performed in order to analyze the impact of different imaging modalities, interventions and observer qualifications, and to deduce steps to constantly improve the practical realization. Each tumor entity presents specific challenges which are demonstrated here using examples.

Coverage-based treatment planning to accommodate delineation uncertainties in prostate cancer treatment

PURPOSE

To compare two coverage-based planning (CP) techniques with fixed margin-based (FM) planning for high-risk prostate cancer treatments, with the exclusive consideration of the dosimetric impact of delineation uncertainties of target structures and normal tissues.

METHODS

In this work, 19-patient data sets were involved. To estimate structure dose for each delineated contour under the influence of interobserver contour variability and CT image quality limitations, 1000 alternative structures were simulated by an average-surface-of-standard-deviation model, which utilized the patient-specific information of delineated structure and CT image contrast. An IMRT plan with zero planning-target-volume (PTV) margin on the delineated prostate and seminal vesicles [clinical-target-volume (CTV prostate) and CTVSV] was created and dose degradation due to contour variability was quantified by the dosimetric consequences of 1000 alternative structures. When D98 failed to achieve a 95% coverage probability objective D98,95 ≥ 78 Gy (CTV prostate) or D98,95 ≥ 66 Gy (CTVSV), replanning was performed using three planning techniques: (1) FM (PTV prostate margin = 4,5,6 mm and PTVSV margin = 4,5,7 mm for RL, PA, and SI directions, respectively), (2) CPOM which optimized uniform PTV margins for CTV prostate and CTVSV to meet the D98,95 objectives, and (3) CPCOP which directly optimized coverage-based objectives for all the structures. These plans were intercompared by computing percentile dose-volume histograms and tumor-control probability/normal tissue complication probability (TCP/NTCP) distributions.

RESULTS

Inherent contour variability resulted in unacceptable CTV coverage for the zero-PTV-margin plans for all patients. For plans designed to accommodate contour variability, 18/19 CP plans were most favored by achieving desirable D98,95 and TCP/NTCP values. The average improvement of probability of complication free control was 9.3% for CPCOP plans and 3.4% for CPOM plans.

CONCLUSIONS

When the delineation uncertainties need to be considered for prostate patients, CP techniques can produce more desirable plans than FM plans for most patients. The relative advantages between CPCOP and CPOM techniques are patient specific.