Accuracy of dose calculation algorithms for static and rotational IMRT of lung cancer: A phantom study

Determination and comparison of dosimetric parameters of three-dimensional conformal radiotherapy, field in field, and intensity-modulated radiotherapy techniques in radiotherapy of breast conserving patients

Purpose

Three radiation therapy techniques for breast are common, namely three-dimensional conformal radiotherapy (3D-CRT), Field-in-Field (FIF), and Intensıty-Modulated Radıotherapy (IMRT). The purpose of this study was to determine and compare dosimetric parameters of three different treatment planning planning types; 3D-CRT, FIF, and IMRT in target and normal tissues after breast-conserving surgery.

Methods

One hundred patients with left or right breast cancer cooperated in this study. They were divided into three categories (small, medium, and large size) based on breast volume. Three treatment planning techniques were carried out by planner for each patient in Prowess® 5.2 Treatment Planning System. The dosimetric parameters were obtained from dose-volume histograms using the CERR software (MATLAB Company, Washington, USA), which runs as an add-on in MATLAB software.

Results

3D-CRT technique with the highest value of Dmax creates more hot spots than the other techniques in the tumor region (P = 0.013). IMRT and FIF showed the best uniformity compared to 3D-CRT in all groups with respect to the values of the parameters D98 and D2. IMRT provided the best coverage in the tumor compared to other methods (P < 0.001). 3D-CRT technique yielded a high volume receiving ≥107% of the prescription dose (P < 0.001). Among the three methods, the FIF method results in a lower dose to the lung for treatment based on the V5 and V20 parameters (P < 0.001). Homogeneity index for IMRT was better than FIF, as well as, conformity index (CI) for IMRT and FIF was better than 3D-CRT.

Conclusion

IMRT and FIF plans offered excellent target coverage and uniformity, whereas FIF had better protection of healthy tissues. Thus FIF method is an efficient method to improve the quality of treatment for breast cancer patients.

Dosimetric accuracy of three dose calculation algorithms for radiation therapy of in situ non-small cell lung carcinoma

Background

Study determines differences in calculated dose distributions for non-small cell lung carcinoma (NSC LC) patients. NSC LC cases were investigated, being the most common lung cancer treated by radiotherapy in our clinical practice.

Materials and methods

A retrospective study of 15 NSCLC patient dose distributions originally calculated using standard superposition (SS) and recalculated using collapsed cone (CC ) and Monte Carlo (MC) based algorithm expressed as dose to medium in medium (MCD_m) and dose to water in medium (MCD_w,) was performed so that prescribed dose covers at least 99% of the gross target volume (GTV). Statistical analysis was performed for differences of conformity index (CI), heterogeneity index (HI), gradient index (GI), dose delivered to 2% of the volume (D_2%), mean dose (D_mean) and percentage of volumes covered by prescribed dose (V_70Gy). For organs at risk (OARs), D_mean and percentage of volume receiving 20 Gy and 5Gy (V_20Gy, V_5Gy) were analysed.

Results

Statistically significant difference for GTVs was observed between MCD_w and SS algorithm in mean dose only. For planning target volumes (PTVs), statistically significant differences were observed in prescribed dose coverage for CC, MCD_m and MCD_w. The differences in mean CI value for the CC algorithm and mean HI value for MCD_m and MCD_w were statistically significant. There is a statistically significant difference in the number of MUs for MCD_m and MCD_w compared to SS.

Conclusion

All investigated algorithms succeed in managing the restrictive conditions of the clinical goals. This study shows the drawbacks of the CC algorithm compared to other algorithms used.

Development of a Tongue Immobilization Device Using a 3D Printer for Intensity Modulated Radiation Therapy of Nasopharyngeal Cancer Patients

Purpose

This study aimed to reduce radiation doses to the tongue, a patient-specific semi-customized tongue immobilization device (SCTID) was developed using a 3D printer for helical tomotherapy (HT) of nasopharyngeal cancer (NPCa). Dosimetric characteristics and setup stability of the SCTID were compared with those of a standard mouthpiece (SMP).

Materials and Methods

For displacement and robust immobilization of the tongue, the SCTID consists of four parts: upper and lower tooth stoppers, tongue guider, tongue-tip position guide bar, and connectors. With the SCTID and SMP, two sets of planning computed tomography and HT plans were obtained for 10 NPCa patients. Dosimetric and geometric characteristics were compared. Position reproducibility of the tongue with SCTID was evaluated by comparing with planned dose and adaptive accumulated dose of the tongue and base of the tongue based on daily setup mega-voltage computed tomography.

Results

Using the SCTID, the tongue was effectively displaced from the planning target volume compared to the SMP. The median mucosa of the tongue (M-tongue) dose was significantly reduced (20.7 Gy vs. 27.8 Gy). The volumes of the M-tongue receiving a dose of 15 Gy, 30 Gy, and 45 Gy and the volumes of the mucosa of oral cavity and oropharynx (M-OC/OP) receiving a dose of 45 Gy and 60 Gy were significantly lower than using the SMP. No significant differences was observed between the planned dose and the accumulated adaptive dose in any dosimetric characteristics of the tongue and base of tongue.

Conclusion

SCTID can not only reduce the dose to the M-tongue and M-OC/OP dramatically, when compared to SMP, but also provide excellent reproducibility and easy visual verification.

Questionnaire survey on treatment planning techniques for lung stereotactic body radiotherapy in Japan

This study aimed to obtain details regarding treatment planning techniques for lung stereotactic body radiation therapy (SBRT) employed at each institution in Japan by using a questionnaire survey. An Internet questionnaire survey on SBRT procedures performed in 2016 was conducted by the QA/QC committee of the Japan Society of Medical Physics from April to June 2017. The questionnaire assessed two aspects: the environment for SBRT at each institution and the treatment planning techniques with and without respiratory motion management techniques (RMMT). Of the 309 evaluated responses, 218 institutions had performed SBRT. A total of 186 institutions performed SBRT without RMMT and 139 institutions performed SBRT with RMMT. When respiratory motion was ≥10 mm, 69 institutions applied RMMT. The leading RMMT were breath holding (77 institutions), respiratory gating (49 institutions) and real-time tumor tracking (11 institutions). The most frequently used irradiation technique was 3D conformal radiotherapy, which was used in 145 institutions without RMMT and 119 institutions with RMMT. Computed tomography (CT) images acquired under free breathing were mostly used for dose calculation for patients treated without RMMT. The usage ratio of IMRT/VMAT to SBRT is low in Japan, compared to elsewhere in the world (<20% vs ≥70%). Among the available dose calculation algorithms, superposition convolution was the most frequently used regardless of RMMT; however, 2% of institutions have not yet made heterogeneity corrections. In the prescription setting, about half of the institutions applied point prescriptions. The survey results revealed the most frequently used conditions, which may facilitate standardization of treatment techniques in lung SBRT.

Development of a semi-customized tongue displacement device using a 3D printer for head and neck IMRT

Purpose

To reduce radiation doses to the tongue, a patient-specific semi-customized tongue displacement device (SCTDD) was developed using a 3D printer (3DP) for head and neck (H&N) radiation therapy (RT). Dosimetric characteristics of the SCTDD were compared with those of a standard mouthpiece (SMP).

Materials and methods

The SCTDD consists of three parts: a mouthpiece, connector with an immobilization mask, and tongue displacer, which can displace the tongue to the contralateral side of the planning target volume. Semi-customization was enabled by changing the thickness and length of the SCTDD. The instrument was printed using a 3DP with a biocompatible material. With the SCTDD and SMP, two sets of planning computed tomography (CT) and tomotherapy plans were obtained for seven H&N cancer patients. Dosimetric and geometric characteristics were compared.

Results

Using the SCTDD, the tongue was effectively displaced from the planning target volume without significant tongue volume change compared to the SMP. The median tongue dose was significantly reduced (29.6 Gy vs. 34.3 Gy). The volumes of the tongue receiving a dose of 15 Gy, 30 Gy, 35 Gy, 45 Gy, and 60 Gy were significantly lower than using the SMP.

Conclusion

The SCTDD significantly decreased the radiation dose to the tongue compared to the SMP, which may potentially reduce RT-related tongue toxicity.

A Feasibility Study of IMRT of Lung Cancer Using Gafchromic EBT3 Film

BACKGROUND

Intensity modulated radiation therapy (IMRT) is an advanced method for delivery of three dimensional therapies, which provides optimal dose distribution with giving multiple nonuniform fluency to the patient. The complex dose distribution of IMRT should be checked to ensure that the accurate dose is delivered. Today, film dosimetry is a powerful tool for radiotherapy treatment Quality Assurance (QA) and a good method to verify dose distribution in phantoms.

OBJECTIVE

This study aimed to evaluate the accuracy of IMRT treatment planning system, Prowess Panther® software, with Gafchromic EBT3 films in a inhomogeneity phantom.

MATERIAL AND METHODS

The IMRT plan was generated by Prowess Panther® treatment planning system (TPS) version 5.2 on a inhomogeneity phantom, then it was irradiated by ONCOR linear accelerator (Linac) with 6 (MV) photon beam energy. The Gafchromic EBT3 film located between the phantom has measured the dose distribution. To compare between TPS calculated doses and film measured doses, Gamma criteria 3%/3 mm, 4%/4 mm, 5%/5 mm, 6%/6 mm and 7%/7 mm Dose Difference (DD) and Distance to Agreement (DTA), respectively were used.

RESULTS

Gammas passing rates for PTV are obtained 67.5% for 3%/3mm, 78.8% for 4%/4mm, 86.3% for 5%/5mm, 91.2% for 6%/6mm and 94.3% for 7%/7mm and for organs at risk is 72.4% for 3%/3mm, 82.8% for 4%/4mm, 89.8% for 5%/5mm, 93.3% for 6%/6mm and 95.4% for 7%/7mm (respectively DD/DTA). By increasing the range of criteria the capability increased.

CONCLUSION

The results show that the use of EBT3 film in a inhomogeneity phantoms allows us to evaluate the dose differences between the EBT3 measured dose distribution and TPS calculated dose distribution .Hence, a result Prowess Panther® TPS can be used for IMRT technique treatment.

Correlation between gamma passing rate and complexity of IMRT plan due to MLC position errors

PURPOSE

This study evaluates the correlation between the susceptibility of the γ passing rate of IMRT plans to the multi-leaf collimator (MLC) position errors and a quantitative plan complexity metric.

METHODS

Twenty patients were selected for this study. For each patient, two IMRT plans were generated using sliding window and step-&-shoot techniques, respectively. Modulation complexity score (MCS) was calculated for all IMRT plans, and symmetric MLC leaf bank errors, ranging from 0.3 mm to 1 mm, were introduced. Original and modified plans were delivered using Varian's Clinac iX. The obtained dose distribution using ArcCHECK was then compared with the TPS calculated dose distribution of the original plans. 3D gamma analysis was performed for each verification with passing criteria of 2%/2 mm. The γ passing rate decreasing gradient were calculated to evaluate relationship between variation of γ passing rate due to MLC errors and complexity.

RESULTS

A linear regression analysis was applied between γ gradient and complexity, and the results showed a linear correlation (R² = 0.81 and 0.82 for open and closed MLC error types, respectively) indicating the more complex plans are more susceptible to MLC leaf bank errors. Meanwhile, correlation of re-normalized γ passing rate and complexity for all errors scenarios also presented a strong correlation (r > 0.75).

CONCLUSION

The statistics results revealed variation relationship of dosimetry robust of plans with various complexities to MLC errors. Our results also suggested that the observed susceptibility is independent of the delivery techniques.

Normal lung sparing Tomotherapy technique in stage III lung cancer

PURPOSE

Radiation pneumonitis (RP) has been a challenging obstacle in treating stage III lung cancer patients. Beam angle optimization (BAO) technique for Tomotherapy was developed to reduce the normal lung dose for stage III non-small cell lung cancer (NSCLC). Comparative analyses on plan quality by 3 different Intensity-modulated radiation therapy (IMRT) methods with BAO were done.

MATERIALS AND METHODS

Ten consecutive stage IIIB NSCLC patients receiving linac-based static IMRT (L-IMRT) with total 66 Gy in 33 fractions to the PTV were selected. Two additional Tomotherapy-based IMRT plans (helical beam (TH-IMRT) and static beam (TD-IMRT)) were generated on each patient. To reduce the normal lung dose, Beam angles were optimized by using complete and directional block functions in Tomotherapy based on knowledge based statistical analysis. Plan quality was compared with target coverage, normal organ sparing capability, and normal tissue complication probability (NTCP). Actual beam delivery times and risk of RP related with planning target volume (PTV) were also evaluated.

RESULTS

The best PTV coverage measured by conformity index and homogeneity index was achievable by TH-IMRT (0.82 and 1.06), followed by TD-IMRT (0.81 and 1.07) and L-IMRT (0.75 and 1.08). Mean lung dose was the lowest in TH-IMRT plan followed by TD-IMRT and L-IMRT, all of which were ≤20 Gy. TH-IMRT plan could significantly lower the lung volumes receiving low to medium dose levels: V5~30 when compared to L-IMRT plan; and V5~20 when compared to TD-IMRT plan, respectively. TD-IMRT plan was significantly better than L-IMRT with respects to V20 and V30 and there was no significant difference with respect to V40 among three plans. The NTCP of the lung was the lowest in TH-IMRT plan, followed by TD-IMRT and L-IMRT (6.42% vs. 6.53% vs. 8.11%). Beam delivery time was the shortest in TD-IMRT plan followed by L-IMRT. As PTV length increased, NTCP and Mean lung dose proportionally increased significantly in all three plans.

CONCLUSION

Advantageous profiles by TH-IMRT could be achieved by BAO by complete and directional block functions. Current observation could help radiation oncologists to make wise selection of IMRT method for stage IIIB NSCLC.

MRI-based radiotherapy planning method using rigid image registration technique combined with outer body correction scheme: a feasibility study

An alternative pseudo CT generation method for magnetic resonance image (MRI)-based radiotherapy planning was investigated in the work. A pseudo CT was initially generated using the rigid image registration between the planning MRI and previously acquired diagnostic CT scan. The pseudo CT generated was then refined to have the same morphology with that of the referenced planning image scan by applying the outer body correction scheme. This method was applied to some sample of brain image data and the feasibility of the method was assessed by comparing dosimetry results with those from the current gold standard CT-based calculations. Validation showed that nearly the entire pixel doses calculated from pseudo CT were agreed well with those from actual planning CT within 2% in dosimetric and 1mm in geometric uncertainty ranges. The results demonstrated that the method suggested in the study was sufficiently accurate, and thus could be applicable to MRI-based brain radiotherapy planning.

Quantification of residual dose estimation error on log file-based patient dose calculation

PURPOSE

The log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error.

METHODS AND MATERIALS

Modified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: ±2.0, ±1.0, and ±0.5mm in opposite directions and systematic leaf shifts: ±1.0mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks.

RESULTS

For MLC leaves calibrated within ±0.5mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32±0.27% and 0.82±0.17Gy for PTV and spinal cord, respectively, and in prostate plans 1.22±0.36%, 0.95±0.14Gy, and 0.45±0.08Gy for PTV, rectum, and bladder, respectively.

CONCLUSIONS

In this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy.