Radiotherapy techniques have evolved into an extremely complex process with new imaging modalities, delivery systems and patient immobilization devices. These technological advances have made it possible to reduce the dose to normal tissue structures and consequently minimize the risk of toxicity and morbidity, while allowing for dose escalation to the tumor volumes, potentially leading to improved loco-regional control [1] . Head and neck cancer is histologically heterogeneous and organs at risk have less tolerance to radiation. Due to complex shape of target and close proximity of critical structure, the most common method of head and neck cancer radiotherapy treatment is intensity modulated radiotherapy (IMRT). These techniques address the drawbacks of conventional 3DCRT techniques, such as under-dosage of target regions and excessive radiation to surrounding normal structures [2] . An advantage of IMRT technology over 3DCRT for the treatment of nasopharyngeal carcinoma is improved dose distribution within the target area, better dose hotspot control, and reduced radiation exposure to organs at risk (OAR), including the parotids, brain stem, spinal cord and oral cavity [3] - [5] .

Large field IMRT (LF-IMRT), is a standard IMRT plan which slightly differs in delivery method. For large field delivery, due to the limitation of mlc travelling length, the standard sliding window IMRT plan can create sub-fields and deliver dynamically. In the standard IMRT, mlc carriage moves to next position to cover the subfields. Each one of these sub-fields require mode up separately to deliver. LF-IMRT does not require to mode up each sub-field separately and the all subfields in a large field does mode up collectively. While moving the mlc carriage to deliver the subsequent subfields dynamically, the dose rate falls to zero to reach to the planned position and continue to deliver the plan. These slightly reduce the overall treatment time.

LF-IMRT plan, is similar to standard IMRT plan and requires multiple fixed-angle radiation beams with many treatment segments or sub-fields, leading to lengthy treatment delivery times. Extended time on the table reduces comfort of patient and increases intra-fraction motion of the tumor or organs at risk (OARs). Therefore, the availability of new technologies is of great interest.

Volumetric modulated arc therapy, based on the original investigation of Otto [6] has been introduced in clinical practice in several institutes after an intensive validation at the planning level, compared with intensity modulated radio therapy (IMRT) or other approaches. Rapid Arc, the Varian medical systems (palo alto, CA) solution of VMAT, is implemented as the progressive Resolution Optimization algorithm-III in the Eclipse treatment planning system (varian medical system). The optimization process is based on an iterative inverse planning process aiming to simultaneously optimize the instantaneous multi leaf collimator (mlc) positions, dose rate, and gantry rotation speed to achieve the desired dose distribution.

Recent studies have examined dosimetric difference between IMRT and VMAT for various cancers, with the consensus being that VMAT produces a similar or superior plan to IMRT with a more efficient delivery [7] . Therefore, the aim of this study was to compare the dosimetric and technical aspects of VMAT and LF-IMRT for the radiotherapy treatment of head and neck cancer.

Figure 1 shows the axial, sagittal and coronal view of dose distribution and refers to one patient. Figure 2 and Figure 3 show the comparison DVH for PTVs and OARs respectively. Table 2 and Table 3 give an overview of the numerical results from average DVH analysis on PTVs and OARs respectively and report mean values ± standard deviation (SD) and “p” values to assess the relative inter-patient variability. Table 4 gives details of the technical characteristics.

As it is now, it is difficult to understand, the target volume was to receive at least 95% of the prescribed dose, but has received higher than 96% of the target volumes in both techniques. Mean dose of PTV_70Gy was achieved better (70.56 ± 1.10 Gy) with Rapid Arc without increasing dose maximum than (71.66 ± 1.08 Gy) with IMRT. Rapid Arc plans achieved higher dose conformity index (CI_95%) (0.906 ± 0.029, closer to a value of 1) than with IMRT (0.868 ± 0.067) and the p = 0.021 shows significant difference. The Homogeneity index (HI_95%) statistically does not differ in both techniques but, mean homogeneity index (HI_95%) shows higher with Rapid Arc (0.089 ± 0.020) than with IMRT (0.097 ± 0.03). For PTV_54Gy the coverage was adequate in both techniques. However, homogeneity index was achieved better with Rapid Arc (0.097 ± 0.3) than with IMRT (0.25 ± 0.10).

Spinal Cord dose was below the tolerance limit for both techniques. However, D_1% of Spinal cord with Rapid Arc was 37.7 ± 4.67 Gy and with IMRT was 40.68 ± 7.14 Gy. Both left parotid and right parotid received with

Dx% = dose received by the X% of the volume; Vx% = Volume receiving at least X% of the prescribed dose; CI = Conformity Index; HI = Homogeneity Index.

Dx% = dose received by the X% of the volume; Vx% = Volume receiving at least X% of the prescribed dose.

GAI = Gamma Agreement Index.

VMAT are 21.26 ± 8.5 Gy and 22.37 ± 7.44 Gy which are lesser than with IMRT 22.78 ± 11.2 Gy and 24.1 ± 7.96 Gy respectively. Average mean dose Larynx was (37.49 ± 2.7 Gy) with Rapid Arc than with IMRT (41.7 ± 4.2 Gy). Likewise, the average mean dose of uninvolved oral cavity also was lesser with rapid Arc. Concerning Healthy Tissue, Lower dose irradiation volume V_5Gy was little higher with Rapid Arc (37.90% ± 9.33%) than with IMRT (37% ± 9.67%). However, higher dose irradiation volume V_30Gy was lesser with Rapid Arc (10.07% ± 3.93%) than with IMRT (13.1% ± 5.2%). The Integral dose was with Rapid Arc (1.1475 × 10⁵ ± 0.705 × 10⁵ cm³∙Gy) and with IMRT (1.489 × 10⁵ ± 0.778 × 10⁵ cm³∙Gy).

The average monitor unit(MU) needed to deliver the dose of 200 cGy/fraction was 523 ± 116 MU for Rapid Arc and 2127 ± 570 for the 9 field IMRT plan (p < 0.00001). The total Beam On time (Excluding patient setup and Image verification) was 2.14 ± 0.43 min for Rapid Arc and 9.16 ± 2.5 min for IMRT (p < 0.00001). Pre- treatment quality assurance of all plans resulted in average gamma agreement index (GAI). The GAI scored with 3 mm and 3% constraints was 97.84% ± 1.1% with Rapid Arc plans and 95.23% ± 1.7% with IMRT plans. The acceptable GAI threshold was set to 95% as a reference in the hospital.

Different planning studies comparing the volumetric modulated arc therapy with conventional IMRT in different sites have been published [12] - [15] . The study published by Richard et al. [16] . Shows VMAT had shorter Beam-On time and more homogeneous dose distribution compared with 7 field, step and shoot IMRT for prostate cancer. In another study published by Mahantshetty, swamidas Jamema et al. [17] shows that Rapid Arc allows the generation of adequate dose distribution with high target homogeneity, sufficient sparing of organs at risk and minimization of patient moment compared to IMRT. The mean dose of PTV_70Gy received adequately with both techniques, however, Rapid Arc mean dose was differ significantly (p = 0.011) than IMRT. Conformity Index (CI_95%) (p = 0.021), dose of D_80% (p = 0.002) and D_50% (p = 0.003) were significantly higher with Rapid Arc than IMRT, while the V_107% (p = 0.288) and homogeneity Index (HI_95%) (p = 0.28) did not differ significantly with IMRT. Therefore, the dose fall-off with Rapid Arc is higher than IMRT. The advantage of steep dose fall-off with Rapid Arc technique achieved higher Homogeneity for proximity target with low dose prescription PTV_54Gy significantly (p = 0.0001) than IMRT.

When using IMRT for irradiation of oropharyangeal cancer salivary functions was less impaired, but the majority of the patients still suffered from some degree of xerostomia [18] - [20] . Braam et al. showed--that the normal tissue complication probability (NTCP) at several time points after radiation therapy was less than 20% only if the mean dose to the parotid glands was lower than 25 Gy [21] . The average mean dose of both left parotid and right parotid received with VMAT were lesser than with IMRT. However, significantly not differ. V_30Gy of parotid was also lesser with Rapid Arc than IMRT, It ensures that irradiation of higher dose volume is reduced.

Kirk Patrice et al. [22] . reported that the incidence rate for radiation myelitis is 0.2, 6% and 50% the total dose for 50 Gy, 60 Gy, and 69 Gy respectively, when administrated at the conventional fraction of 2 Gy/day. In addition, according to the Radiation Oncology Group of European Organisation for Research and Treatment of Cancer, the maximal radiation dose that should be applied to the Spinal Cord is 45 Gy, and it should not exceed 40 Gy if oxaliplatin Chemotherapy is administered as well [23] . In this study the Spinal Cord dose did not reach the tolerance level in both techniques. Hovever, Sparing of spinal cord was significantly higher with Rapid Arc (p = 0.047) than IMRT. Betzel et al. [24] reported that RapidArc deliveries were found to be more tolerant to variations in dose rate, gantry position, and MLC leaf position than fixed-field IMRT with dynamic SW delivery. Dose rate variations during RapidArc must be large to affect dose distributions significantly; dynamic IMRT is inherently not susceptible to such variations. Clinically acceptable treatments may be delivered accurately using Rapid Arc despite large fluctuations in dose rate and gantry position [24] . The dose to targets was delivered with more degrees of the freedom in Rapid Arc technique allowed to spare significantly the larynx (p = 0.002) and uninvolved oral cavity (p = 0.004) which are located in between the targets.

Concerning Healthy Tissue, the irradiation of higher volume by lower dose (p = 0.828) was not differ significantly. However, higher dose irradiation volume V_30Gy was lesser with Rapid Arc significantly (p = 0.037). Computed integral dose of prescribed dose irradiation at non-tumor volume with Rapid Arc was lesser than with IMRT, there was no significant difference (p = 0.828). Jia.et al. found that the scatter dose predominates in the peripheral dose in VMAT, while the leakage dose predominates in IMRT, Moreover, leakage dose contribution to the peripheral dose becomes quite large and the peripheral dose at distant point is higher [25] . When evaluating radiation dose to healthy tissues, especially at sites remote from the treatment region, radiation leakage may represent an important factor, increasing integral dose to normal structures [26] . Specifically, two main sources of leakage from LINAC can be considered to increase the patient’s Non-Tumour Integral Dose (NTID). (i) transmission of radiation through the collimator leaves and leakage through the primary collimation system. It is generally believed that leakage through the leaves of a conventional collimator might carry a significant contribution to peripheral integral dose (leakage is about 2.5% for 6 MV photon beams). (ii) leakage radiation is the dominant component and is directly related to the number of MU delivered [26] . Such a leakage radiation doses can increase the risk of secondary malignancy. In our study, the required monitor unit to deliver the prescribed dose with similar dose distribution of targets and OARs with IMRT was three times more than Rapid Arc (p < 0.00001). The impact of irradiation of healthy tissues at low doses with the VMAT technique is remains unresolved. Longer treatment delivery times increase the chance of patient displacement during IMRT [27] - [29] . The treatment time for delivering the Rapid Arc plans was significantly lesser (p < 0.00001) than IMRT. This difference may have significantly impact on IMRT outcome, especially for tumors with a low α/β ratio and a short repair half time [30] .

It is important to understand that whether treating with Rapid Arc or IMRT, pre treatment quality assurance is vital to safe and effective patient care. The reliability and efficacy of the Rapid Arc modality as anticipated in dosimetric investigation [31] - [33] . The results of our study was, the GAI scored with 3mm and 3% constraints was more than acceptable threshold of 95% with both techniques. Our QA passing results agree with Verbakel et al. Published report [34] . High resolution of the detector (EPID) emphasizes the difficulties of the treatment planning system in properly managing the interleaf transmission and tongue and groove effect to notify the failing points in the delivery.

The Rapid Arc (VMAT) delivered the dose rapidly with lesser Monitor Unit without compromising in target dose coverage, conformity and higher sparing of OARs than IMRT. Thus, the risk of intra-fractional patient movement is reduced. The collimator transmission as scattered radiation from the Linac, Which is proportional to the number of MU and can increase potential risk of secondary cancer is now largely reduced by the Rapid Arc. These advantages can allow increase in dose/fraction and deliver in very short time a higher dose level at the same time keep maintain lesser OARs dose.

Jayapalan Krishnan,Suresh Rao,Sanath Hegde,  Shambhavi,Jayarama Shetty, (2015) A Dosimetric Comparison of Double Arc Volumetric Modulated Arc Therapy with Large Field Intensity Modulated Radiation Therapy for Head and Neck Cancer. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,04,353-363. doi: 10.4236/ijmpcero.2015.44042