Slice-based plan evaluation methods for three dimensional conformal radiotherapy treatment planning

Voxel based evaluation of sequential radiotherapy treatment plans with different dose fractionation schemes

OBJECTIVE

This study presents a methodology for voxel-based evaluation of two phase sequential radiotherapy treatment plans having conventional dose scheme in the first phase and subsequent hypofractionation dose scheme in the second phase based upon different priority [planning target volume (PTV), clinical target volume (CTV) and organs at risk (OAR)] of display modes.

METHODS

A case of carcinoma prostate was selected for demonstration. Varian Eclipse treatment planning system (TPS) was used for contouring and planning. In the first phase, a dose of 52 Gy in 26 fractions to the PTV and in the second phase, a dose of 19.5 Gy in 3 fractions to the PTV Boost was planned on the same CT data set. Both the plans (Phase 1 and Phase 2) were exported and processed using "Voxel-based radiobiology display (VRb) tool". Plan Sum for Biologically effective dose (BED)-Cube and equivalent dose of 2Gy (EQD₂)-Cube was reconstructed using a combination of linear quadratic (LQ) and linear quadratic-linear (LQ-L) radiobiological models. Tumor control probability (TCP) and normal tissue complication probability (NTCP) for different target volumes and organs were also calculated using EQD₂-volume histograms of the Plan Sum.

RESULTS

An in-house graphical user interface (GUI) is developed to present the qualitative and quantitative evaluation of the multiphase treatment plans with different display modes and dose regimens. The voxel based TCP obtained for the combined target volume was 90.56%. NTCP for the bladder and rectum was calculated from the Plan Sum histograms and found to be 0.33% and ~0.0% respectively.

CONCLUSION

The proposed methodology using the VRb tool offers superior plan evaluation for multiphase sequential radiotherapy treatment plans over the existing methods.

ADVANCES IN KNOWLEDGE

PTV, CTV and OAR priority based display modes in VRb tool offers better understanding of radiobiological evaluation of sequential radiotherapy treatment plans.

On the use of AAA and AcurosXB algorithms for three different stereotactic ablative body radiotherapy (SABR) techniques: Volumetric modulated arc therapy (VMAT), intensity modulated radiation therapy (IMRT) and 3D conformal radiotherapy (3D-CRT)

Aim

The purpose of this study was to investigate the dosimetric characteristics of three stereotactic ablative body radiotherapy (SABR) techniques using the anisotropic analytical algorithm (AAA) and Acuros XB algorithm. The SABR techniques include coplanar volumetric modulated arc therapy (C-VMAT), non-coplanar intensity modulated radiation therapy (NC-IMRT) and non-coplanar three-dimensional conformal radiotherapy (NC-3D CRT).

Background

SABR is a special type of radiotherapy where a high dose of radiation is delivered over a short time. The treatment outcome and accuracy of the dose delivered to cancer patients highly depend on the dose calculation algorithm and treatment technique.

Materials and methods

Twelve lung cancer patients underwent 4D CT scanning, and three different treatment plans were generated: C-VMAT, NC-IMRT, NC-3D CRT. Dose calculation was performed using the AAA and Acuros XB algorithm. The dosimetric indices, such as conformity index (CI), homogeneity index, dose fall-off index, doses received by organs at risk and planning target volume, were used to compare the plans. The accuracy of AAA and Acuros XB (AXB) algorithms for the lung was validated against measured dose on a CIRS thorax phantom.

Results

The CIs for C-VMAT, NC-IMRT and NC-3D CRT were 1.21, 1.28 and 1.38 for the AAA, respectively, and 1.17, 1.26 and 1.36 for the Acuros XB algorithm, respectively. The overall dose computed by AcurosXB algorithm was close to the measured dose when compared to the AAA algorithm. The overall dose computed by the AcurosXB algorithm was close to the measured dose when compared to the AAA algorithm.

Conclusion

This study showed that the treatment planning results obtained using the Acuros XB algorithm was better than those using the AAA algorithm in SABR lung radiotherapy.

A simple plan evaluation index based on the dose to critical structures in radiotherapy

The dose to critical structures plays a very important role in treatment plan evaluation and forms a major challenging parameter in radiotherapy treatment planning. In this study, a simple index, Plan Normal tissue complication Index (PNI) has been proposed for treatment plan evaluation based on the dose to surrounding critical structures. To demonstrate the proposed index, four different critical treatment sites that include the prostate, upper abdominal cancer, lung, and head and neck were selected for this study. A software progam (PNIcalc) has been developed to compute the PNI from the exported dose-volume histogram data and from the tissue tolerance data published by Emami et al. and Kehwar et al. The software also shows the parameters that exceed the threshold limits of dose-volume parameters presented in the QUANTEC recommendations (2010). In all the studied cases, PNI gave an overall picture of the dose received by the critical structures and also indicate the fractional volume exceeding the tolerance limit. The proposed index, PNI gives a quick comparison and selection of treatment plans that result in reduced dose to the critical structures. It can be used as an additional tool for routine treatment plan evaluation in external beam radiotherapy.