Evaluation of dosimetric misrepresentations from 3D conventional planning of liver SBRT using 4D deformable dose integration

Abstract

The purpose of this study is to evaluate dosimetric errors in 3D conventional planning of stereotactic body radiotherapy (SBRT) by using a 4D deformable image registration (DIR)‐based dose‐warping and integration technique. Respiratory‐correlated 4D CT image sets with 10 phases were acquired for four consecutive patients with five liver tumors. Average intensity projection (AIP) images were used to generate 3D conventional plans of SBRT. Quasi‐4D path‐integrated dose accumulation was performed over all 10 phases using dose‐warping techniques based on DIR. This result was compared to the conventional plan in order to evaluate the appropriateness of 3D (static) dose calculations. In addition, we consider whether organ dose metrics derived from contours defined on the average intensity projection (AIP), or on a reference phase, provide the better approximation of the 4D values. The impact of using fewer (<10) phases was also explored. The AIP‐based 3D planning approach overestimated doses to targets by 1.4% to 8.7% (mean 4.2%) and underestimated dose to normal liver by up to 8% (mean −5.5%; range −2.3% to −8.0%), compared to the 4D methodology. The homogeneity of the dose distribution was overestimated when using conventional 3D calculations by up to 24%. OAR doses estimated by 3D planning were, on average, within 10% of the 4D calculations; however, differences of up to 100% were observed. Four‐dimensional dose calculation using 3 phases gave a reasonable approximation of that calculated from the full 10 phases for all patients, which is potentially useful from a workload perspective. 4D evaluation showed that conventional 3D planning on an AIP can significantly overestimate target dose (ITV and GTV+5mm), underestimate normal liver dose, and overestimate dose homogeneity. Implementing nonadaptive quasi‐4D dose calculation can highlight the potential limitation of 3D conventional SBRT planning and the resultant misrepresentations of dose in some regions affected by motion and deformation. Where the 4D approach is unavailable, contouring on the full expiration phase may yield more accurate dose calculations, most relevant in the case of the healthy liver, but the absolute dose differences are in general small for the other healthy organs. The technique has the potential to quantify under‐ and over‐dosage and improve treatment plan evaluation, retrospective plan analysis, and clinical outcome correlation.PACS numbers: 87.55.‐x, 87.55.D‐, 87.55.de, 87.55.dk, 87.55.Qr, 87.57.nj

Development of an Advanced Deformable Phantom to Analyze Dose Differences due to Respiratory Motion

“…9, and this process is similar to that from previous studies. 6,12) Fig. 10 shows the 3D dose distribution, 4D dose distribution, and the distribution of difference between the 3D and 4D dose, and this difference mainly occurred in the SI direction.…”

“…The tendency of this difference was similar to that reported by a previous study using patient cases. 12) In further study, the effects of the factors that influence the difference between the 3D and 4D dose calculations, such as tumor size and motion, will be quantitatively analyzed using this phantom. Ultimately, the phantom could contribute to the discrimination of patients who would benefit from the 4D dose by estimating the condition of significant difference between the 3D dose and 4D dose according to the tumor size and motion.…”

“…8,12) In particular, in lung cancer patients, Valdes et al 8) expected that small tumor with large motion will show significant difference. Estimating the condition of factors such as tumor size and motion underlying this significant difference is important because the 4D dose does not always provide significant advantage than the 3D dose to all patients, despite reflecting the dosimetric impact of the respiratory motion.…”

“…4) Currently, a 4D dose calculation, which could reflect the dosimetric impact of respiratory motion and estimate a more realistic delivered dose than a 3D dose, can be performed using the 4DCT data and a deformable image registration (DIR), and various studies related to the 4D dose calculation have been conducted. [5][6][7][8][9][10][11][12][13] Guckenberger et al 10) compared the 3D and 4D dose in terms of a biological effective dose (BED) in seven patients.…”

The Importance of Quasi-4D Path-Integrated Dose Accumulation for More Accurate Risk Estimation in Stereotactic Liver Radiotherapy

“…13 This deformable image registration-based approach revealed that conventional 3-dimensional (3D) planning approaches can significantly underestimate the dose to the planning target volume (PTV) in liver SBRT. 14 The purpose of this study is to investigate the impact of deformation on untargeted healthy organs and evaluate corresponding risk estimates in liver SBRT. Specifically, we demonstrate a methodology for assessing the risk of detriment using conventional 3D and 4D dose-warping approaches in terms of normal tissue complication probabilities 15,16 and latent risks of radiocarcinogenesis and cardiac complications.…”