Adaptive Planning of Intensity Modulated Radiation Therapy for Head-and-Neck Cancers for Improved Accuracy of Delivered Dose in Patients With Clinically Significant Anatomical Change Due to Therapy

Abstract

Purpose/Objective(s): Adaptive planning of intensity-modulated radiation therapy (AP-IMRT) for head and neck cancer (HNCA) will improve conformality of delivered target dose after significant geometric changes in patient’s anatomy related to therapy response. We examined the effects of mid-course IMRT replanning using FDG PET-CT imaging to determine the relative change in delivered dose to target tissues and normal structures compared with the initial IMRT plan. Materials/Methods: Three HNCA patients with locally advanced, stage IVA/IVB disease with large nodes (4, 5.5, 7 cm) underwent one IMRT replan based upon mid-treatment PET-CT at 36 Gy of planned 72 Gy course. All patients had a partial response to definitive chemoradiation. Primary sites were oropharynx and supraglottic larynx, while one subject had an unknown primary. Adapted clinical target volume (AP-CTV), planning target volume (AP-PTV), and normal structures were recontoured on the new PET-CT for optimized IMRT planning. The change in calculated delivered dose of the new optimized IMRT plan was compared to the initial IMRT plan dose recalculated for the new CT data set and adapted contours. Using rigid image registration, we superimposed the new CT scan into the initial CT scan in order to calculate the dose delivered by the initial IMRT plan to the new CT scan. We evaluated the change in dose delivered to the new regions of interest, which were re-drawn by the treating physician, including AP-CTV, AP-PTV, salivary glands and spinal cord. Results: Mid-treatment PET-CT imaging displayed significant changes in patient anatomy that resulted in differences in both target tissue and normal tissue volumes. In one case, the left parotid volume and APCTV_58 decreased by 27% and 21%, respectively. DVH analysis demonstrated a small but meaningful underdosing to AP-CTV_58 and AP-PTV_58 due to geometric changes in targets. V99 for AP-CTV_58 was <54 Gy and V99 for AP-PTV_58 was <46 Gy with the initial prescribed dose of 58 Gy. The maximum spinal cord dose increased from 27.7 Gy to 35.2 Gy, an increase of 27.1%. The mean right parotid dose increased by 15.8% while the mean left parotid dose remained unchanged. Conclusions: The anatomical changes seen in HNCA patients undergoing definitive IMRT pose a significant challenge in providing accurate dosing during the course of treatment. AP-IMRT provides greater conformality in dose delivery and improved CTV/PTV dose coverage especially when significant clinical response is noted on physical examination or daily pretreatment imaging. AP-IMRT can also maximize sparing of radiosensitive normal structures. Adaptive planning is fast becoming an important aspect of IMRT and will require further research to increase effectiveness for individualized therapy goals. Author Disclosure: M. Bishop: None. A. Al-Basheer: None. J. Greskovich: None.