Title:
Adaptive functional image-guided IMRT in pharyngo-laryngeal squamous cell carcinoma: is the gain in dose distribution worth the effort?

Abstract:
The planning process in radiotherapy (RT) typically involves the acquisition of a unique set of CT images - and eventually of functional images - which is used for delineation of target volumes (TV) and organs at risk (OAR) and for dose calculation. Restricting the delineation and dose calculation solely on pre-treatment images is an oversimplification as it is only a snapshot of the patient's anatomy. The objectives of the present study were (1) to assess the consequences of anatomic modification in dose distribution for both TVs and OARs; (2) to assess the potential benefit of adaptive strategies using Helical Tomotherapy (HT); and (3) to compare CT-based and FDG-PET-based adaptive planning strategies.
Ten patients with H&N SCC were imaged before and during concomitant chemo-RT using CT and FDG-PET acquisition after a mean dose of 14.2, 24.5, 35.0 and 44.9 Gy. Simultaneous integrated boost IMRT planning was performed using HT. We compared (1) the planned dose distribution, (2) the delivered dose distributions that took into account impact of anatomical modifications on dose distribution, (3) the adaptive dose distributions after replanning to take into account the anatomic modifications and the anatomic or functional GTV shrinkage.
There was an increase between the planned and the delivered high dose volumes, which correlated with the slope of the GTV shrinkage. The adaptive high dose volumes were significantly smaller than the delivered ones. The difference between the adaptive and the delivered high dose volume also correlated with the slope of the GTV shrinkage. For both parotid glands combined, the delivered D(mean) showed a statistical trend for an increase of 4.4% compared to the planned D(mean). For the ipsilateral parotid glands, there was a correlation between the D(mean) gain and the slope of the GTV shrinkage when an adaptive planning was used. For the oral cavity, the adaptive D(mean) was 10% smaller than the delivered ones. For the PRV around the spinal cord, there was an increase of about 4.5% between the delivered and the planned D(2%). The adaptive planning translated into a decrease in D(2%) of 7.2%. The differences between the delivered and planned D(2%) and between the adaptive and the delivered D(2%) were correlated with the slope of the GTV shrinkage. For the CTV(proph) and PTV(proph) coverage, adaptive strategy induced a better dose conformation. No significant difference was observed in the various figures of merit between PET-based plan and CT-based isodose distributions.
The dose distribution that is actually delivered to patients significantly differs from what was planned because of anatomic modifications. Adaptive multi-modality IMRT is feasible in H&N tumors and could compensate and improve dose distribution. Some useful surrogate criteria or "flags" are, however, needed to identify patients who might benefit from an adaptive strategy. The optimal adaptive strategy still needs to be defined and prospective studies will have to be conducted to address the safety and the clinical impact of such approaches on patient outcome.