Intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) became a standard treatment approach in head and neck (H&N) cancer radiotherapy due to the ability of dose escalation to the tumor with reducing/limiting the doses to critical structures. In H&N cancers IMRT and VMAT published clinical results shows better outcomes in tumour control and quality of life compare to the conventional radiotherapy. The results do confirm that IMRT does decrease xerostomia compared with conventional radiotherapy [1]. Low dose volume regions are a major worry in modulated radiotherapy because of its risk of secondary malignancies incidence, especially in the adult and pediatric patients. The prevalence of second malignancies after radiotherapy for pediatric and young adult populations is well established as one of the significant long-term sequelae of radiation treatment [2]. In sliding window IMRT technique the multileaf collimators (MLC) deliver doses by moving from bank ‘B’ towards ‘A’ bank with variable speed and dose rate with fixed jaws. Jaw tracking techniques offers to reduce the MLC transmission during the IMRT and VMAT, during the jaw tracking treatment the secondary jaws (Y1, Y2, X1 & X2) will dynamically move as close to the MLC segments to further minimize the MLC transmission. Typically values for the MLC transmission for the Millennium 120 leaf MLC (Varian Medical Systems, Palo Alto, CA, USA) were between 1.6% to 2.5% for beam energy 6 MV to 18 MV X-ray photon beams, with the use of jaw tracking by combining the MLC and secondary jaws we can further reduce the leakage radiation to negligible amount. According to the study by LoSasso et al. [3], the MLC transmission increases with increasing jaw field size and beam energy. Cadman et al. [4] found that the transmission through the jaw and the MLC together is smaller than 0.1%.

TrueBeam medical linear accelerator (Varian Medical Systems) is equipped with jaw tracking technique to reduce the MLC transmission by dynamic tracking of the secondary jaws during the IMRT and VMAT delivery was used in this study. The jaws can move at the maximum speed of 2 cm/s, during the IMRT or VMAT dose delivery as close as possible to the MLC aperture, and further minimizes leakage and transmission through the MLC leaves [5].

In this study we intent to estimate the dosimetric influence of the jaw tracking technique in H&N cancers patients with sliding window IMRT and VMAT techniques to compare with standard static jaws sliding window IMRT and VMAT plans by evaluating the doses to the critical structures, low dose volumes by keeping the priorities and constraints same with comparable plan quality indexes.

We retrospectively selected ten previously treated H&N cancer patients stage (T1/T2, N1, M0) in this study. All the patients were planned for sliding window IMRT and VMAT with simultaneous integrated boost (SIB) technique to deliver a differential dose per fraction to the high, intermediate and low risk volume using a single plan. We intend to deliver 70 Gy to high risk volume, 64 Gy to intermediate risk volume and 56 Gy for the low risk volume in 35 fractions. All the IMRT/VMAT plans were planned with flattened 6 MV photons using Millennium 120 leaf MLC with and without jaw tracking by keeping the same priorities for the target volumes and critical structures for a particular patient. Eclipse treatment planning system, ver. 11.0 (Varian Medical Systems), was used in this study. To standardize all the IMRT and VMAT plans with and without jaw tracking technique were normalized at the target mean of the high risk volume (planning target volume [PTV], 70 Gy).

In the present study, IMRT and VMAT plans were compared with IMRT and VMAT with and without JT technique. Plan quality was assessed by comparing HI and CI. The target coverage for HR-PTV, IR-PTV, and LR-PTV were almost equally good for all IMRT and VMAT with and without JT plans. The quality of the plan remains almost same for the all the PTV’s.

Jaw tracking methods have also been used in treatment modalities aiming to achieve better target coverage and critical structure sparing. Feng et al. [5] found both JT-IMRT and SJ-IMRT plans can achieve comparable target dose coverage. Our results are shows both JT and SJ IMRT/VMAT method has a comparable target conformity and homogeneity with appreciable OAR sparing. JT displays superior OARs sparing than SJ plans and they are of clinical importance, especially for the patients with large and complex targets but close to some highly radio-sensitive organs to spare. The mean low doses reduction for OARs ranged from 0.5% to 7.64% in our study. The JT-IMRT plans deposited significantly mean lower dose to various OARs, the mean doses reductions for these OARs and the value reductions depend on the volume and the location of the OARs. In JT-IMRT periphery OAR has an lesser low dose and mean dose reduction compared to the OAR in the center or close to the PTV’s, the maximum of 7.64% reduction in the periphery structure of parotid compare to 1.4% reduction in the thyroid.

Snyder et al. [7] evaluated dosimetric benefits of JT for spine stereotactic radiosurgery using IMRT and VMAT. It showed that, Jaw tracking decreased spinal cord for both IMRT and VMAT plans, but a larger decrease 10% was seen with the IMRT plans (p = 0 .004 vs. p = 0.040).

Joy et al. [8] evaluated the dosimetric effects of JT in stepand- shoot IMRT, showing an overall reduction of normal tissue doses. Most patients had reductions of V₅, V₁₀, and V₂₀ by less than 2% in the normal tissues. The maximum reduction in V₅ was 16.7%. In our study we found for the volume of V₅, V₁₀, and V₂₀ an average reduction of 4.69%, 2.44%, and 2.12% for JT-IMRT compare to SJ-IMRT, whereas a reduction of 2.75%, 2.06%, and 1.06% were found between JT-VMAT and SJ-VMAT.

Kim et al. [9] assessed the potential advantages of JT technique by using control point sequences of VMAT planning, showing that, for H&N cases, the mean dose reductions for all the OARs ranged from 4.3% to 11.9%. In prostate patients, the organs far from the target showed larger sparing (3.7% to 8.1% on average) in JT than the organs adjacent to the target (1.1% to 1.5%). The dose reductions were more significant in the dose regions of D₈₀, D₉₀ and D₉₅ for all the patients in JT plans.

Schmidhalter et al. [10] evaluated the leaf transmission reduction using moving jaws in dynamic MLC IMRT, and demonstrated that the undesired doses to the body volume minus the PTV decreased by up to 1.8% and 1.5% for prostate and H&N patients. Simultaneously, the monitor unit (MU) increased by up to 3.1% and 2.8%, respectively. In our case the low dose volume V₅ reduced to 4.69% and 2.75% for the JT-IMRT and JT-VMAT compared to the SJ-IMRT and SJ-VMAT. The total MU (x¯ ± σx¯) for all the patients for JT-IMRT, SJ-IMRT, JT-VMAT, and SJ-VMAT were 1,385 ± 266, 1,329 ± 226, 484 ± 58, and 486 ± 58. The MU increased in JT-IMRT by 4.21% compared to SJ-IMRT, but there is no difference in the total MU for the JT-VMAT and SJ-VMAT plans. The design of the Varian MLC requires fields >14 cm in width to be split into 2 or more carriage movements [11]. With the split-field technique, both the number of MUs and total treatment time are significantly increased [11]. Since we used smart leaf motion calculator (LMC) ver. 10.0, there is no split of larger field sizes. If the same study simulated with the older LMC instead of the smart LMC, both the MU and treatment time is going to increases significantly and end up in increased lower dose volumes and OAR doses in the SJ-IMRT technique.

Head and neck site itself large and complex target surrounded by many critical tissues and sensitive structures such as spinal cord, brainstem, thyroid, inferior constrictor, and parotid glands. This study demonstrates that the mean values of the maximum and mean doses to the spinal cord, brainstem, and parotid are significantly reduced in the IMRT and VMAT with JT plan in comparison of without JT plan as shown in Tables 4 and 5. Perhaps these OARs do not overlap with the target in the selected cases, yet the thyroid and inferior constrictor are partially or entirely covered by PTVs so dose reduction in these OARs is slightly lower. The dose reduction to the OARs (parotid glands, spinal cord, etc.), and normal tissue may reduce risk of radiation induced injury and complication such as xerostomia [12,13]. JT technique is also more meaningful to reduce the low dose volume for the radiotherapy patients with local recurrent or second primary malignant lesion within or adjacent to a previously irradiated area.

The risk of radiation induced secondary malignancies is another concern, which is strongly associated with low dose exposure of normal tissues during IMRT/VMAT. The JT technique significantly reduces the low dose regions. In the H&N cases, the JT (IMRT/VMAT) technique significantly reduces the V₅, V₁₀, V₂₀, V₃₀, and mean dose to the normal tissue body, compared to the SJ (IMRT/VMAT). The maximum reduction was observed in V₅ and minimum at V₃₀ as shown in Tables 4 and 5.

In our study we also found that the JT-IMRT plan gives significantly lower mean doses to all OARs, and also significantly reduction in low dose receiving volume V₅, V₁₀, V₂₀, and V₃₀ which are shown in Table 4. The mean doses of OARs have been widely used to predict the probability of radiation toxicity [12] like xerostomia. Therefore, JT plans may decrease the probability of developing acute and late side effects or secondary neoplasm.

In overall analysis of our study shows that, JT-IMRT plans gives superior dose reduction in all OARs and in the low dose volume then that of JT-VMAT plans, may be this is due to high monitor unit used in IMRT in comparison to VMAT plans.

In conclusion, this study demonstrates that all the JTIMRT and SJ-IMRT plans can achieve comparable target dose coverage and are clinically acceptable. JT-IMRT displays superior OARs sparing compare to (static) IMRT plans. These results are of clinical importance, especially for the patients with large and complex targets, but close to some highly radio-sensitive organs to spare, and for patients with local recurrent or secondary primary malignant lesion within a previously irradiated area. Also the JT-IMRT plan significantly reduces the low dose volume compare to the without JT-IMRT plan. JT plans may decrease the probability of developing late side effects or secondary neoplasm.

In comparison of overall plan, JT-VMAT shows insignificant results compare to SJ-VMAT plans in OARs and low dose volumes sparing. The JT-VMAT plans were not clinical significant might be due to less amount of MU compare to IMRT and the delivery method of JT-VMAT.