Radiation therapy or radiotherapy is a cancer treatment that uses high doses of X-rays to kill cancer cells and shrink tumors [1].There have been significant advances in the treatment delivery of radiotherapy over the past few decades. These include increased sophistication of imaging techniques, which has resulted in improved accuracy of target volume definition and delineation, as well as developments in treatment planning systems and linear accelerator delivery capabilities leading to improved dose distributions, heterogeneity and conformity. These developments have been mainly driven by the need to reduce the dose to normal tissue structures and thereby reduce the risk of toxicity, which then allows dose escalation to the tumor volume potentially leading to improved locoregional control [2]. To that end, newer radiation techniques, e.g. volumetric modulated arc therapy (VMAT), have been developed. VMAT techniqueincludes simultaneous gantry rotation and multileaf collimator (MLC) motions during irradiation with fluctuating dose rate leading to the construction and development of highly conformal dose distributions in lesser time than IMRT. The advantages of this technique are improved target volume conformity, particularly in volumes and improved sparing of normal tissues and organs at risk (OARs) resulting in reduced acute and late toxicities [3-5]. The RT technique that allows treatment of many volumes having various dose prescriptions results into delivery of high biological effective doses to the target volume and reduction of the dose to the organ at risk and improvement of the toxicity is usually called simultaneous integrated boost (SIB). Mazzola et al reported first clinical trial of the single-stage VMAT-SIB technique and concluded Long-term results and prospective randomized trials are advocated [6].

Aim and Objectives

A sequential two-phase process, initial and boost irradiation, is the common practice for the radiotherapy management of head and neck cancer. In the present work, the purpose of this study is to investigate the Homogeneity index (HI), Conformity Index (CI) of dose distribution inside the target volume and critical organ (OARs) doses of individual tumor sites to evaluate the quality of VMAT-SIB plans for Head and Neck cancer.

In our institution, thestudy was accepted by hospital Committee on 01.03.2024 (# NCCI/IRB/2024/10). Inclusion criteria were age factor, ≥30 years old. Inclusive criteria were no previous history of radiotherapy and no metastasis diseases. Ten patients of carcinoma of head and neck were continuously selected from the list of patients that have delivered VMAT-SIBradiotherapy treatment between 2021 and 2022 at our department. CT scan images of those patients were used for producing SIB plans using VMAT technique. These plans were accepted only if they encounter the set of planning objectives established in the protocol. Patient characteristics are given in Table 1. Volume Delineation Gross tumor volume (GTV) consists of identifying malignant disease and clinical targetvolume (CTV) was then defined as the gross tumor volume plus all sites of potential subclinical disease. The PTV was the final volume for irradiation, and it included the clinical target volume with a 0.5- to 1.0-cm margin for daily setup variation, customized to limitdose to critical normal tissues and to allow for organ motion. These above defined volumes and simultaneous integrated boost volume were planned to receive 50Gy upto 63.25Gy in 25 fractions. These volumes and other organs at risks (OARs) were delineated by the radiation oncologist.

Dose Prescription

The patient identification, site, fraction, prescribe dose and volume are given in Table:1.

Table 1: Showing patients prescription in detail

Dose constraints were used for organs at risk based on the report by Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) [7].

Table 2: Maximum doses of critical structure

For the purpose of the study the Dmax is defined as D0.03cc (Dose received by 0.03cc volume) as used in the RTOG-0825 protocol. Eyes were used as a surrogate for Retinae. Brainstem Dmax limit is kept at 60Gy and D1-10cc can receive up to 59Gy. Full organ of brainstem can receive up to 54Gy.

Treatment planning

For all patients, VMAT-SIB plans were designed using the Monaco TPS version: 5.11.02, optimization process was provided via Monte Carlo (MC) algorithm. These all plans were delivered using the same Elekta Synergy-S linear accelerator having X-ray beam energy of 6 MV with a variable dose rate up to maximum of 600MU/min. Each case was planned with a single or multiple arcs of various angles rotating clockwise and counterclockwise. The collimator angle for each patient was fixed to 0° and 90° during gantry rotation, based on the patient's anatomy, tumor shape and position. The prescription doses in cGy, target volume in cc and tumors are given in Table 1.

Plan evaluation

The dosimetric evaluation is based on the homogeneity index and conformity index for the PTV, CTV, GTV and OARs. All RT plans were investigate using dose volume histogram (DVH) consideration and isodose curves. The parameters for dosimetric evaluations are given in detail below:

Homogeneity index (HI):

In radiotherapy, homogeneity refers to how uniform the radiation dose is across the target volume (usually the tumor or planning target volume, PTV).

If this index is less than or equal to zero, treatment plan is acceptable. An HI value approaching zero indicates a more homogeneous dose distribution within the target volume [8-9].

Conformity index (CIs):

It is defined as the ratio of the prescription isodose volume (PI) divided by the target volume (TV). Following formula was used:

A CI value closer to 1 indicates a more conformal dose distribution to target volume. In addition, target volume coverage was compared in terms of D₉₈, D_mean, V95%, and V107% (the volume receiving 95% or 107% of the prescribed dose, respectively).

Heterogeneity index:

A heterogeneity index in radiotherapy refers to a localized spot either within the target volume or outside it, in surrounding tissue that receives a radiation dose significantly higher than the prescribed dose.

If this index was less than or equal to 107%, treatment plan was acceptable. [8-9].

RESULT AND DISCUSSION

The conformity index of all patients is shown in figure 1.

Figure 1 Show Bar Histogram of CI values of 10 patients

Figure 2 Show Bar Histogram of Her I values of 10 patients

Figure 3 Show Bar Histogram of HI values of 10 patients

Plan Quality

Dosimetric analysis of the 10 VMAT-SIB plans demonstrated favorable target coverage with acceptable conformity and homogeneity across all patients. The Conformity Index (CI), shown in Figure 1, ranged from 0.54 to 0.81 with a mean value of 0.67, indicating that the prescription isodose volume closely approximated the target volume in most cases, particularly in Patients A and D (CI = 0.81 and 0.80, respectively), which showed the best conformity. Patients G and I recorded the lowest CI values (0.54), suggesting comparatively less conformal coverage, likely attributable to irregular or concave target geometry requiring greater compromise between target coverage and OAR sparing.

The Homogeneity Index (HI), presented in Figure 2, ranged from 0.106 to 0.207 with a mean of 0.130. Patients D, E, and G demonstrated the most homogeneous dose distributions (HI ≈ 0.106–0.110), while Patient A showed the greatest dose heterogeneity (HI = 0.207). Notably, Patient A's plan also achieved the highest CI, illustrating the well-recognized trade-off in VMAT-SIB optimization: pursuing tighter conformity to an irregular target volume can introduce localized dose gradients (hot and cold spots) that modestly increase heterogeneity within the target.

The Heterogeneity Index (HetI), shown in Figure 3, ranged from 102% to 106% (mean ≈ 104.6%), remaining below the 107% acceptability threshold defined in our planning protocol for all 10 patients. This confirms that despite inter-patient variability in dose distribution, no plan exceeded the pre-specified limit for clinically significant hot spots, supporting the overall dosimetric acceptability of the VMAT-SIB technique in this cohort.

The mean CI of 0.67 and mean HI of 0.130 obtained in our study are consistent with previously reported VMAT-SIB series for head and neck cancer, where CI values in the range of 0.6–0.9 and HI values below 0.15–0.20 are generally considered indicative of clinically acceptable plan quality [8-9]. The relatively wide range in CI (0.54–0.81) across our small cohort reflects the anatomical heterogeneity inherent to head and neck targets — patients with more irregular or horseshoe-shaped PTVs (e.g., those wrapping around the spinal cord or brainstem) inherently achieve lower conformity, as the optimizer prioritizes OAR sparing over strict conformity to an geometrically complex target. The inverse relationship observed between CI and HI in individual patients (e.g., Patient A) is a recognized phenomenon in VMAT planning: maximizing conformity around complex target shapes can require steeper dose gradients, which in turn slightly increases within-target dose heterogeneity. Despite this, all HetI values remained within the 107% acceptability threshold, indicating that the SIB technique successfully avoided clinically significant hot spots even in the least conformal plans. This finding supports the safety and reproducibility of VMAT-SIB for simultaneous delivery of differential doses to gross disease and subclinical/nodal volumes without unacceptable dose escalation to normal tissue. Taken together, these results demonstrate that VMAT-SIB planning at our institution consistently met internationally accepted benchmarks for both conformity and homogeneity, while allowing simultaneous dose escalation to gross tumor volume and adequate sparing of adjacent organs at risk — consistent with the technique's established advantages over sequential-phase or IMRT-based approaches [3-6]. Organ at Risk Dosimetric evaluation of the organs at risk (OARs) confirmed that all critical structures were maintained within their respective dose tolerance limits as defined by QUANTEC guidelines and the institutional planning protocol [7]. Across all 10 patients, maximum doses (Dmax, defined as D0.03cc) to the brainstem, spinal cord, parotid glands, and optic structures remained below their prescribed constraints, with no plan exceeding the tolerance thresholds outlined in Table 2. The brainstem Dmax was consistently kept under the 60 Gy limit, with D1–10cc below 59 Gy and the full-organ dose below 54 Gy in all cases. These results demonstrate that the VMAT-SIB technique achieved adequate sparing of adjacent normal tissues while simultaneously delivering differential boost doses to the target volumes, without compromising organ-specific safety limits. This consistent OAR sparing, achieved alongside acceptable conformity and homogeneity indices, further supports the clinical feasibility of VMAT-SIB for head and neck cancer treatment, minimizing the risk of treatment-related toxicity while maintaining high-quality target coverage.

VMAT-SIB treatment planning technique showed good conformal and homogeneous target coverage with sparing of organs at risk and reduced treatment delivery time. With these features, safety of simultaneous integrated boost may allow its routine clinical use, though it is still under investigation in many areas.

Abbreviations Used:

CI: Conformity Index, HI: Homogeneity Index, GI: Gradient Index, SIB: Simultaneous Integrated Boost, VMAT: Volumetric Modulated Arc Therapy, MLCs: Multi-leaf Collimators, MU: Monitor Units, TV: Target Volume, Dmax: Maximum Dose, PD: Prescribe Dose, Linac: Linear accelerator.