Introduction: Intensity Modulated Arc Therapy (IMAT/VMAT/Rapid Arc) is now considered as a better choice of intensity modulated radiotherapy treatment technique because of its advantages like higher patient throughput, expenses of lower monitor units and simplicity in treatment execution. The treatment delivery regimen may be in the form of Sequential Boost (SEQB) or Simultaneous Integrated Boost (SIB). In the SEQB regimen different plans are used for each phase of treatment, where as in SIB regimen a single plan is used throughout the course of treatment delivering differential doses to different target volumes.
Purpose: To compare the radiobiological and dosimetric parameters of the two regimens of IMAT i.e. SIB versus SEQB in locally advanced head and neck cancer patients.
Materials and Methods: A total of 24 previously untreated locally advanced head and neck cancer patients were included in this study. The patients were prospectively randomized into SIB and SEQB arm. The CT data set in treatment position were transferred to the ECLIPSETM Treatment Planning System (Version 11.0.47). All the target volumes i.e. Gross Target Volume (GTV), High Risk (HR), Intermediate Risk (IR), Low Risk (LR) Planning Target Volume (PTV) and Organ at Risk (OAR) were delineated. The treatment plan of each phase in SEQB regimen were independently optimized. All the patients were treated using 6MV linear accelerator (UNIQUETM Performance, Varian Medical System).
Results: In the SIB arm 11 patients and in the SEQB arm 13 patients were enrolled. The BED (10) value for HR PTV was same in both group, whereas for IR PTV and LR PTV the values were 59.0, 63.6 and 50.0, 56.0 for SIB and SEQB arm respectively. The V (95) values were 100% for all the target volumes in both arms of patents. The average D (100) value for GTV, HR PTV an IR PTV were higher in SEQB arm than in SIB arm (7066cGy vs. 6900cGy, 6720cGy vs. 6497cGy and 6308cGy vs. 5917cGy). The average D (100) value for LR PTV were 5037cGy, 4871cGy for SIB and SEQB arm respectively. The spinal cord maximum doses were within the tolerance in both group of patients. The left and right parotid gland sparing was comparable in both group of patients. Average integral dose was 12.8% higher in SIB group than SEQB group. The average total monitor unit per fraction was 25.6% higher in SEQB arm than in SIB arm.
Discussion: The SIB treatment regimen offers more organizational benefits over SEQB regimen in terms of practicality and lesser chances of treatment uncertainty. But SIB treatment when combined with chemotherapy may increase the toxicity profile as larger volume is irradiated throughout the treatment course. To balance the radiobiological volume effect, BED (10) values for IR PTV and LR PTV intentionally kept lower in SIB arm. In the SEQB, each phases were optimized independently, which cause the higher D (100) values for GTV, HR PTV and IR PTV in the plan sum dose volume histogram. D (100) value of LR PTV is lower in SEQB arm as prescription dose is much lesser than SIB arm (5000cGy vs. 5400cGy). As large volume is irradiated throughout treatment course in SIB arm, a clear increase in integral dose is observed. Promising reduction in MUs in SIB treatment may be considered as a good merit of this regimen.
Conclusion: SIB treatment regimen may be considered as more logical and efficient in the treatment of locally advanced head and neck caner with comparable radiobiological and dosimetric parameters. The clinical comparison may explore more pros and cons of SIB and SEQB.