Radiotherapy for head and neck tumours in 2012 and beyond: conformal, tailored, and adaptive?

Abstract

Intensity-modulated radiation therapy (IMRT) is a conformal irradiation technique that enables steep dose gradients. In head and neck tumours this approach spares parotid-gland function without compromise to treatment efficacy. Anatomical and molecular imaging modalities may be used to tailor treatment by enabling proper selection and delineation of target volumes and organs at risk, which in turn lead to dose prescriptions that take into account the underlying tumour biology (eg, human papillomavirus status). Therefore, adaptations can be made throughout the course of radiotherapy, as required. Planned dose increases to parts of the target volumes may also be used to match the radiosensitivity of tumours (so-called dose-painting), assessed by molecular imaging. For swift implementation of tailored and adaptive IMRT, tools and procedures, such as accurate image acquisition and reconstruction, automatic segmentation of target volumes and organs at risk, non-rigid image and dose registration, and dose summation methods, need to be developed and properly validated.

Figure 1:. Comparison between the planned isodose distribution and the dose distribution actually delivered on the 25th fraction

(A) Kilovoltage CT with contrast enhancement and (B) megavoltage CT without contrast enhancement for a patient with a T2-N3 oropharyngeal squamous-cell carcinoma. The patient was treated with chemoradiotherapy. Planned radiotherapy dose was 50·0 Gy administered as 25 fractions of 2·0 Gy, and planning target volumes are delineated in red. The left parotid gland is delineated in blue. Arrows indicate the overdose of the left parotid gland compared with the planned dose.

Figure 2:. Typical diagrammatic representation of an adaptive treatment strategy

The main difference between adaptive and classic treatment strategies is that images acquired during treatment may be used for set-up and dose re-calculation. The diagram relies on two assumptions. First, the quality of in-room imaging is sufficient to compute a provisional dose just before set-up and delivery. Second, the dose effectively received by the patient can be measured after delivery. If not, a natural surrogate would be the provisional dose. Set-up may be guided by the images (image-guided adaptive radiotherapy) and the computed provisional dose (dose-guided adaptive radiotherapy).

Figure 3:. Typical schematic treatment plans for a dose-painting phase 3 trial protocol

Numbers represent doses in Gy. (A) In the standard arm a median dose of 70 Gy is homogeneously delivered to the GTV. (B) In the experimental arm boost doses are delivered to the GTV that range from 70 to 86 Gy and are based on the PET-signal intensity in each voxel. In both arms a median dose of 70 Gy is administered to the CTV and PTV. PTV=planning target volume. CTV=clinical target volume. GTV=gross target volume.