The impact of different optimization strategies on the agreement between planned and delivered doses during volumetric modulated arc therapy for total marrow irradiation.

AIM OF THE STUDY: To evaluate the agreement between planned and delivered doses and its potential correlation with the plans' complexity subjected to dosimetric verification. MATERIAL AND METHODS: Four isocentre volumetric modulated arc therapy for total marrow irradiation plans optimized simultaneously with (P1) and without (P2) MU reduction were evaluated dosimetrically by γ method performed in a global mode for 4 combinations of γ-index criteria (2%/2 mm, 2%/3 mm, 3%/2 mm, and 3%/3 mm). The evaluation was conducted for 4 regions (head and neck, chest, abdomen and upper pelvis, and lower pelvis and thighs) that were determined geometrically by the isocentres. The Wilcoxon test was used to detect significant differences between γ passing rate (GPR) analysis results for the P1 and P2 plans. The Pearson correlation was used to check the relationship between GPR and the plans' complexity. RESULTS: Except for the head and neck region, the P2 plans had better GPRs than the P1 plans. Only for hard combinations of γ-index criteria (i.e. 2%/3 mm, 2%/2 mm) were the GPRs differences between P1 and P2 clinically meaningful, and they were detected in the chest, abdomen and upper pelvis, and lower pelvis and thighs regions. The highest correlations between GPR and the indices describing the plans' complexity were found for the chest region. No correlation was found for the head and neck region. CONCLUSIONS: The P2 plans showed better agreement between planned and delivered doses compared to the P1 plans. The GPR and the plans' complexity depend on the anatomy region and are most important for the chest region.

Effect of isocenter deviation on volume modulated arc therapy plan results and gamma passing rate in the treatment of cervical cancer.

BACKGROUND: Isocenter deviation, often induced by small displacements of both the device and the patient, is a common error seen in radiotherapy. In this study, we investigated the impact of isocenter deviation on the results of the volumetric modulated arc therapy (VMAT) plan and dosimetric verification gamma passing rate in the treatment of cervical cancer. METHODS: The clinical data of 15 patients with cervical cancer who were treated with VMAT were retrospectively collected and analyzed. In this study, the isocenter site modification method was adopted. The VMAT plan with isocenter deviation adjustment was set as the experimental group, while the original plan was set as the control group. The impact of isocenter deviation on the results of the VMAT plan and dosimetric verification gamma passing rate was analyzed. Applying gamma analysis with different test criterions, the impact of isocenter deviation on the gamma passing rates was evaluated, and the sensitivity of different test criterions in identifying isocenter deviation was also analyzed. RESULTS: There was a significant difference in the average dose in the target area between experimental group and control group (P<0.05). In organs at risk (OAR) terms, isocenter deviations also caused significant differences in dose parameters between the two groups. Except that there was no significant difference in the rectal V40 between two groups when the isocenter deviation was greater than 3 mm in the y axis direction. With the increase in the isocenter deviation, there was a trend towards decreased gamma passing rates with different analysis criterions in the experimental group. The 2 mm/2% standard had the highest sensitivity for identifying isocenter deviation. CONCLUSIONS: Isocenter deviation has significant effects on the results of the volume rotation intensity modulation plan and dosimetric verification gamma passing rates in the treatment of cervical cancer. When the isocenter deviation was less than 3 mm, a higher gamma passing rate (>90%) could also be obtained under the condition of the 3 mm/3% test criterions. It is recommended that the 2 mm/2% test standard should be utilized in clinical practice.

Preliminary clinical study on brass compensator-based intensity-modulated radiation therapy.

OBJECTIVE: The objective of this study is to preliminarily evaluate the feasibility of brass compensator-based intensity-modulated radiation therapy (CB-IMRT). MATERIALS AND METHODS: Ten patients (three cases of nasopharyngeal cancer, four of esophageal cancer, and three of rectal cancer) who underwent an IMRT treatment planning were selected for this study. The transmission coefficient of brass plates with different thicknesses was measured under a 6 MV photon beam used in the treatment planning system, and the equation for thickness computation was fitted out. The plan file RTPLAN file of each patient was exported from the planning system and transformed to a compensator thickness matrix; therefore, it was input into a numerical control machine for the manufacturing and cutting of the compensators. The CB-IMRT plans obtained were verified on a homogeneous phantom with commercial software. Planar doses were measured by films, and the computed ones were compared using gamma evaluation with 3-mm distance to agreement and 3% dose difference criteria adopting a pass rate of Pγ >90%. The monitor units (MUs) of the multileaf collimator IMRT plan (MLC-IMRT) and the CB-IMRT plans were compared. Depth of cut was computed through the equation fitted from real measurements. The planned RTPLAN files were used to transform the cutting files needed by the numerical control machine. RESULTS: Plan validations show that the minimum and maximum of gamma pass rate among the 10 patients are 90.2% and 98.2%, respectively, which both satisfy the requirements of clinical planning. The MUs of CB-IMRT are significantly smaller compared with MLC-IMRT. CONCLUSION: CB-IMRT satisfies the requirements of clinical therapy and can be used in a radiotherapy routine.

Comparison of gamma- and DVH-based in vivo dosimetric plan evaluation for pelvic VMAT treatments.

BACKGROUND AND PURPOSE: To compare DVH-based quality assurance to a multi-parametric γ-based methodology for in vivo EPID dosimetry for VMAT to the pelvis. MATERIALS AND METHODS: For 47 rectum, 37 prostate, and 44 bladder VMAT treatments we reconstructed the 3D dose distributions of 387 fractions from in vivo EPID dosimetry. The difference between planned and measured dose was evaluated using γ analysis (3%/3mm) in the 50% isodose volume (IDV) and DVH differences (ΔD2, ΔD50 and ΔD98) of targets and organs at risk. The γ-indicators mean γ, γ pass rate and γ1% were compared to DVH-differences and their correlations were studied. DVH-based alerts on PTV and IDV were compared to γ-based alerts. RESULTS: Average PTV D50 and D98 dose differences were 0.0±2.2% (1SD) and -1.4±2.9% (1SD). Alert criteria of |ΔD50|<3.5-4.5% corresponded to an alert rate of about 10%. Strong correlations between mean γ and γ pass rate and difference in PTV ΔD50 were observed for all sites. DVH- and γ-based alerts agreed on >80% of the fractions for the majority of compared alert thresholds and methods. This agreement is >90% for the larger deviations. CONCLUSIONS: Strong correlations between some γ- and DVH indicators were found. Our comparison of multi-parametric alert strategies showed clinical equivalence for γ- and DVH-based methods.

The feasibility of mapping dose distribution of 4DCT images with deformable image registration in lung.

Calculating an accurate cumulative dose through individual phases for four-dimensional computed tomography (4DCT) images from the lung is time-consuming. Although the dose distribution of different phases is similar, copying the dose distribution of one phase directly to another phase would yield a dosimetric error of approximately 4% without further optimization. To reduce the dosimetric error, three-dimensional B-spline elastic deformable image registration (DIR) was used to quickly obtain a relatively accurate cumulative dose of 4DCT images acquired from ten lung cancer patients. The dose distribution of the end-expiration phase was mapped to the end-inspiration phase using DIR. The mapped dose in the end-inspiration phase was then compared with the directly copied dose by analysis (3cm/3%) and the t-test. The results showed that optimization using DIR was significantly better in the average pass rate (by 0.6-4.7%). Our results indicate it is feasible to map the dose distribution of 4DCT images in lung with DIR, and that the motion amplitude of individual respiratory and different DIR algorithms affect the differences between the mapped and actual dose.

In vivo portal dosimetry for head-and-neck VMAT and lung IMRT: linking γ-analysis with differences in dose-volume histograms of the PTV.

PURPOSE: To relate the results of γ-analysis and dose-volume histogram (DVH) analysis of the PTV for detecting dose deviations with in vivo dosimetry for two treatment sites. METHODS AND MATERIALS: In vivo 3D dose distributions were reconstructed for 722 fractions of 200 head-and-neck (H&N) VMAT treatments and 183 fractions of 61 lung IMRT plans. The reconstructed and planned dose distributions in the PTV were compared using (a) the γ-distribution and (b) the differences in D2, D50 and D98 between the two dose distributions. Using pre-defined tolerance levels, all fractions were classified as deviating or not deviating by both methods. The mutual agreement, the sensitivity and the specificity of the two methods were compared. RESULTS: For lung IMRT, the classification of the fractions was nearly identical for γ- and DVH-analyses of the PTV (94% agreement) and the sensitivity and specificity were comparable for both methods. Less agreement (80%) was found for H&N VMAT, while γ-analysis was both less sensitive and less specific. CONCLUSIONS: DVH- and γ-analyses perform nearly equal in finding dose deviations in the PTV for lung IMRT treatments; for H&N VMAT treatments, DVH-analysis is preferable. As a result of this study, a smooth transition to using DVH-analysis clinically for detecting in vivo dose deviations in the PTV is within reach.