Evaluate dose distribution of imrt and vmat technique in radiotherapy for head and neck cancer using truebeam stx linear accelerator – Pham Hong Lam

Journal of military pharmaco-medicine n o 1-2019 180 EVALUATE DOSE DISTRIBUTION OF IMRT AND VMAT TECHNIQUE IN RADIOTHERAPY FOR HEAD AND NECK CANCER USING TRUEBEAM STX LINEAR ACCELERATOR Pham Hong Lam1; Nguyen Thi Van Anh2; Pham Quang Trung2 SUMMARY Objectives: To evaluate and compare dose distribution between IMRT and VMAT plan in radiotherapy for head and neck cancer using TrueBeam STx accelerator. Subjects and methods: CT imaging of head and neck cancer’s patients treated with VMAT on the TrueBeam STx accelerator was used to replan using IMRT technique in TPS Eclipse v13.6. Conformity index, gradient index and homogeneity index were used to compare plan quality and dose distribution at planning target volume, organs at risk. Results: The dose distribution on planning target volume of IMRT technique based on CI100 - Paddick, HIRTOG index (0.82 ± 0.04, 1.085 ± 0.014) gave the same value as the VMAT technique (0.81 ± 0, 44, 1.094 ± 0.014). Maximum doses on organs at risk such as spinal cord, brainstem, and mandible received from IMRT technique were a little lower than the doses from VMAT technique. Conclusion: The IMRT technique is equivalent to the VMAT based on coverage, ability to focus dose on the planning target volume and the ability to spare dose to critical organs. Both the IMRT and VMAT technique on the TrueBeam STx Linac can be selected to treat head and neck cancer patients in 108 Military Central Hospital. * Keywords: Head and neck cancer; IMRT; VMAT; Conformity index; Homogeneity index; Gradient index. INTRODUCTION Radiation therapy is one of the main modalities for cancer treatment. The linear accelerator (Linac) is an indispensable device and it is the most basic component of an external radiotherapy. Especially, radiation therapy plays an important role in head and neck cancer treatment. The biggest difficulty with the treatment of head and neck cancer by radiotherapy is that it has a large number of critical organs near treatment volume. Organs at risk that need to be protected during radiotherapy include: brainstem, spinal cord, salivary glands, esophagus, larynx, mucosa... Whereas the head and neck area has relatively small surface area [1]. In most early 3D-CRT cases, it is inevitable that these organs will be overdosed to reach doses in the tumor. This can have serious consequences for the quality of life of the patient. 1. 103 Military Hospital 2. 108 Military Central Hospital Corresponding author: Pham Hong Lam ([email protected]) Date received: 20/10/2018 Date accepted: 04/12/2018 Journal of military pharmaco-medicine n o 1-2019 181 Nowadays, many generations of modern linear accelerators have been introduced and put into clinical applications, these radiotherapy systems are equipped with many advanced features and many new techniques. Advanced radiotherapy techniques, such as IMRT, VMAT have been used to treat head and neck cancer, in which, planning target volume (PTV) doses can be optimized meanwhile organs at risk (OARs) are protected. In Vietnam, in recent years, many oncology centers have been established and equipped with modern accelerators. In 2017, the TrueBeam STx Linear Accelerator (Linac) has been installed and put in use to treat patients at the 108 Military Central Hospital. The Linac system is the latest generation of radiotherapy accelerators from Varian manufacture. It is integrated a number of technologies that enable the implementation of radiation techniques with optimized dosage. To understand the new techniques, this report will focus on: Comparing and evaluating the quality of the VMAT and IMRT plan to point out the optimal treatment for patients with head and neck cancer. The entire research process was performed on the TrueBeam STx Accelerator (Eclipse v13.6) at Radiotherapy & Radiosurgery Department - Cancer Institute, 108 Military Central Hospital. SUBJECTS AND METHODS 1. Subjects. A total of 30 patients with head and neck cancer, who received radical radiotherapy treatment on TrueBeam STx accelerator at Radiotherapy & Radiosurgery Department, 108 Military Central Hospital, enrolled in the study from January 2018 to September 2018. 2. Methods. - 30 patients were assigned to receive radiation therapy with VMAT. Simulated imaging data of 30 patients were re-used, plan using IMRT technique on Eclipse v13.6. - In order to facilitate the comparison of the quality of the plans, the prescribed dose, the number of fraction is the same 70 Gy/35 Fx. This study focused on the evaluation of dose distribution on the PTV of 70 Gy. - Setting of beam energy parameters, specific field size for each plan. Table 1: Beam summary report. VMAT IMRT Energy 6 MV 6 MV Dose rate 600 MU/min 600 MU/min Prescription dose 70 Gy 70 Gy Number of fraction 35 35 Number of field 3 arcs 9 fields From the obtained plans, the dose volume histogram is studied to compare and evaluate the damage at critical organs. The dose for each organ is recommended by the Radiation Therapy Oncology Group (RTOG) (table 2). Journal of military pharmaco-medicine n o 1-2019 182 Table 2: Dose tolerance of organs at risk. Organs at risk Volume (cc) Dtotal (Gy) Dmax (Gy) Reference Spinal cord - 45 RTOG 0623 [2] Brainstem 1% 60 54 RTOG 0225 [3] Optic chiasm 1% 60 54 RTOG 0225 Parotid (ipsilateral) Mean 26 - RTOG 0912 [4] Esophagus Mean 35 - RTOG 0920 [5] Mandible - - 70 RTOG 0225 Lens - - 25 RTOG 0615 [6] Cochlea Mean 50 - RTOG 0225 Coverage index (CI), conformity index (CI), gradient index (GI) and homogeneity index (HI) are included to compare the quality of VMAT and IMRT plans. Table 3: Formulas for calculating plan evaluation indicators. Index Formula Ideal value Reference Coverage min P D D 0,9 ≤ A < 1 RTOG (1993) [7] 100 PTV V V A = 1 ICRU - 62 [8] CI100 2 100 100 ( )PTV PTV V V V× A = 1 Paddick [9] 5 95 P D D D − A = 0 Wu Qiuhen [10] HI max P D D A ≤ 1,1 RTOG (1993) GI 50 100 V V Paddick [11] (Dmin: Minimum dose value; Dmax: Maximum dose value; DP: Prescription dose; VPTV: PTV Volume; VPTV100: Volume of PTV receiving 100% prescribed dose; V50, V100: the volume is covered by 50% and 100% isolines;D5%, D95%: Minimum dose delivered to 5 and 95% volume of PTV; A: ideal value) Journal of military pharmaco-medicine n o 1-2019 183 RESULTS 1. Mean doses. Table 4: Mean doses value. DP (Gy) Fx Dmax (%) Dmin (%) Dmean (%) VMAT 70 35 109.4 80.4 103.9 IMRT 70 35 108.5 83.2 103.4 Dmax, Dmean, and Dmin dose values are averaged over all plans. The VMAT and IMRT plans are guaranteed at least 95% of the tumor volume received 100% of the prescribed dose. The maximum dose (Dmax) of the techniques was 109.4% (VMAT); 108.5% (IMRT). 2. Plan evaluation index. Table 5: Plan evaluation index. Coverage CI RTOG 1993 CI - ICRU CI100 - Paddick VMAT 0.8 ± 0.2 1.08 ± 0.04 0.811 ± 0.045 IMRT 0.83 ± 0.14 1.09 ± 0.04 0.817 ± 0.042 GI HI Paddick Quihen Wu RTOG (1993) VMAT 27.0 ± 15.6 0.063 ± 0.009 1.094 ± 0.014 IMRT 30.2 ± 13.6 0.050 ± 0.004 1.085 ± 0.014 With prescribed dose of 70 Gy/35 Fx, the coverage, CI, HI and GI values are shown. On average, coverage, CI and HI of the IMRT plans are closed to ideal values. According to the formula given by Paddick (2000), the CI100 index shows the intersection between the volume receiving 100% of the prescribed dose (V100) and the volume of PTV (VPTV). The VPTV100/VPTV ratio is used to evaluate the volume of tumor receiving 100% the prescribed dose. The CI100 - Paddick values for the two subjects were 0.811 ± 0.045 (VMAT); 0.817 ± 0.042 (IMRT). The specific value of each component ratio is given in figure 1. Journal of military pharmaco-medicine n o 1-2019 184 Figure 1: Value of CI100 Paddick index. 3. Tolerance dose of organs at risk. Table 6: VMAT IMRT Spinal cord D1% (Gy) 39.3 ± 2.7 35.7 ± 11.6 Brain stem D1% (Gy) 39.7 ± 12.4 35.1 ± 14.9 Optic nerve (left) D1% (Gy) 35.0 ± 23.8 38.2 ± 25.9 Optic nerve (right) D1% (Gy) 29.0 ± 22.2 37.1 ± 25.2 Parotid gland (left) Dmean (Gy) 22.1 ± 4.6 22.2 ± 7.7 Parotid gland (right) Dmean (Gy) 24.4 ± 6.1 24.1 ± 9.3 Esophagus Dmean (Gy) 16.4 ± 11.8 16.7 ± 11.7 Mandible Dmax (Gy) 69.2 ± 5.1 68.8 ± 23.3 Len (left) Dmax (Gy) 2.61 ± 2.96 2.53 ± 2.82 Len (right) Dmax (Gy) 2.41 ± 2.61 2.25 ± 2.38 Cochlea (left) Dmean (Gy) 22.9 ± 19.6 24.7 ± 21.9 Cochlea (right) Dmean (Gy) 24.9 ± 21.7 25.1 ± 22.2 MUs 477 ± 83 1864 ± 623 Comparison of tolerance dose at critical organs between VMAT and IMRT, for spinal cord, brainstem and optic chiasm we consider the value of D1% (dose at 1% of organ volume). The data obtained were compared with the tolerance dose range recommended by RTOG. Journal of military pharmaco-medicine n o 1-2019 185 DISCUSSION The biggest demand for radiotherapy in cancer treatment is how to focus the dose on the target volume and minimize the dose to the surrounding normal tissues. However, for head and neck cancer radiotherapy, the organs at risk are closed to the location of the tumor so that the requirement becomes more difficult to achieve. Using evaluation indicators, we can compare and evaluate the quality of each plan, selecting the best treatment for head and neck cancer patients. * Coverage: With the data obtained, the study demonstrated that both plans VMAT and IMRT achieved a TV coverage greater than 0.8: 0.8 ± 0.2 and 0.83 ± 0.14, respectively. * CI: In term of conformity, the VMAT and IMRT plans both give the same CI index and it is also close to the ideal value: CIICRU (1.08 ± 0.04 and 1.09 ± 0.04), CI100 - Paddick (0.81 ± 0.44 and 0.82 ± 0.04). This may be because the TrueBeam STx uses a high resolution multi leaf collimator (MLC) HD120, which offers flexible dose modulation, with 32 pairs of central leaves of 2.5 mm thickness and 28 pairs of mini-leaves 5 mm thickness. The CI100 index given by Paddick is calculated by the intersection between volumes received prescribed dose and PTV. Based on the VPTV100/V100 ratio, normal tissue areas receiving high doses are also considered. This ratio averaged over 30 patients (0.869) (VMAT) and 0.862 (IMRT). * HI: In a study by Q. Shamsi et al [12], the analysis and evaluation of the IMRT plan for treating head and neck cancer on a varian clinac DHX, the study provided the HIRTOG value (1.15 ± 0.05). Meanwhile, the plans on the TrueBeam STx in this study provide near-ideal results: VMAT (1.094 ± 0.014), IMRT (1.085 ± 0.014). We also compared the homogeneity in dose distribution in the treatment volume by the HI index given by Quihen Wu (2003). Specifically, the HI with IMRT plans (0.050 ± 0.004) was closer to the ideal value than the VMAT (0.063 ± 0.009). This suggests that, with the TrueBeam STx, the IMRT technique could provide better uniformity in dose distribution at PTV. * GI: In terms of the possibility of reducing the dose when going out of the tumor volume, our study also showed that the dose-reduction value - GIPaddick (2006) with VMAT (27.0 ± 15.6) better than the value with IMRT plan (30.2 ± 13.6). These results showed that in radiotherapy for head and neck cancer, VMAT can reduce the dose from 100% to 50% better than IMRT. * Doses in organs at risk: According to statistics, with the plans on the TrueBeam STx, the normal tissues receive quite small dose, the mean dose Journal of military pharmaco-medicine n o 1-2019 186 in the two salivary glands and mandible was below the tolerance dose range. In a previous study by Braam et al [13], referring to the comparison of the quality of head and neck cancer treatment between IMRT and conventional radiotherapy, the authors point out that the Dmean dose at each salivary gland which are higher than 26 Gy can cause xerostomia for patients after radiation therapy. * MUs and delivery time: Number of MUs in VMAT plan (477 ± 83 MU) was 2.9 to 3.6 times fewer than IMRT plan (1864 ± 623 MU). Small MUs help to reduce delivery time in VMAT plan, minimizing fatigue for patients and increasing treatment outcome. CONCLUSION - In term of dose distribution on tumor, IMRT technique had CI100-Paddick (0.82 ± 0.04) and HIRTOG (1.085 ± 0.014) were similar to those of the VMAT technique (0.81 ± 0.44 and 1.094 ± 0.014). - The IMRT technique also offers better protection based on the ability to protect the organs at risk. 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