The necessity of replanning during the intensity-modulated radiotherapy (IMRT) for head and neck cancer, to ensure adequate coverage of target volume

The necessity of replanning during the intensity-modulated radiotherapy (IMRT) for head and neck cancer, to ensure adequate coverage of target volume Naveen B.1*, S. Narayanan G.2, Narayanan S.3 DOI: https://doi.org/10.17511/ijmrr.2020.i02.10 1* Naveen B., Assistant Professor, Department of Radiation Oncology, VTSM Peripheral Cancer Centre, Kalaburagi, Karnataka, India. 2 Geeta S. Narayanan, Professor and HOD, Department of Radiation Oncology, Vydehi Institute of Medical Sciences and Research Centre, Bengaluru, Karnataka, India. 3 Sowmya Narayanan, HOD, Department of Radiation Physics, Vydehi Institute of Medical Sciences and Research Centre, Bengaluru, Karnataka, India.


Introduction
Radiotherapy (RT) plays an important role in the management of head and neck cancer. Intensitymodulated radiotherapy (IMRT) can conform the dose to target with complex shape and spare organs at risk (OAR), when compared to three-dimensional conformal radiotherapy (3DCRT). Location, shape, and size of disease and anatomy change over 6-7 weeks radiotherapy.
Much research has been reported on the sparing of the parotid gland with IMRT. This organ irresponsible for 60% to 65% of the saliva produced and xerostomia is a major acute and late side effect that can have a significant negative impact on a patient's quality of life. Six months post-treatment, the stimulated salivary flow is reduced exponentially for each parotid gland at a rate of approximately 4% per Gy of mean parotid dose [1].
Eisbruch et al. [2] reported that the sparing of major salivary glands by IMRT increased late salivary flow rates, and improved xerostomia. They also noted that the sparing of minor salivary glands in the oral cavity was a significant independent predictor of xerostomia.
An analysis of dose, volume, and function relationships in the parotid glands after IMRT suggested that a mean parotid dose of 26 Gy was necessary for the substantial sparing of the gland. Barker et al [3] conducted a pilot study to quantify the magnitude of the anatomical changes using an integrated CT linear accelerator system. They concluded that GTVs decreased throughout the course of RT, at a median rate of 1.8% per treatment day. On the last day of treatment, this corresponded to a median total relative loss of 69.5% of the initial GTV.
In addition, the center of mass of the GTV changed position with time, indicating that tumor loss was frequently asymmetric. At treatment completion, the median center of mass displacement was 3.3 mm. Parotid glands also decreased in volume by 0.6% per treatment day and shifted medially (median shift of 3.1 mm) with time. This medial displacement of the parotid glands correlated with the weight loss that occurred during treatment. Hansen

Results
The most common age for affected individuals in the present study is between 51-60 years of age comprising 43.3% of patients ( Figure 9). The majority of the patients were male constituting 70% of the population   Most patients in the present study had advanced Disease.   Most of the patients had Stage IVA disease.

PTV70Gy
The comparison was done by using D95 (Minimum Dose encompassing 95% target volume) and V95 % (Volume receiving 95% of the dose or more).  The present study had planned to repeat the CT scan using Cone-beam CT (CBCT) images during the fourth week. But it was found that the target delineation was difficult using CBCT images due to poor soft-tissue resolution. Hence, CT images were repeated using CT simulator.
The present study did not quantify anatomical changes that occurred over the course of four weeks of treatment but assessed the dosimetric impact of anatomical changes. CT images were fused repeated with the pre-planning images and evaluated the initial IMRT plan and thereby need for replanning was evaluated.
In An increased delivered dose was also observed in the volumes that received high doses, for example, V 100%, V95%, V90%.
As discussed above, several studies have reported that due to modifications of the patient anatomy during treatment, the dose actually delivered was higher than the planned dose as shown in Table 7. Stem and spinal cord. Also the mean dose to the parotid glands increased in repeat CT images compared to pre-treatment images.
Hence repeat acquisitions of CT images during the course of IMRT for patients with H and N cancer may become essential to identify volumetric changes with potential dosimetric consequences.
In particular, it appears that parotid glands are at significant risk to get a higher dose than planned because of a medial shift towards the high isodose volumes.

Author's contribution
Dr. Naveen B.: Study design and concept,