Accelerated Modulated Fractionation (SIB-IMRT) for Head and Neck District

Overview

Nowadays the association between radiotherapy and the anti- Epidermal Growth Factor Receptor (anti-EGFR) monoclonal antibody Cetuximab represents a valid option in the treatment of head and neck locally advanced squamous neoplasm and, particularly, of oropharynx carcinoma. Up to date we have only indirect comparison with the standard curative treatment (i.e. concurrent radiochemotherapy) and the preliminary data show equivalent efficacy of both regimens. For this reason, concurrent Cetuximab and radiotherapy is administered in patients not eligible to chemoradiotherapy. The introduction of Cetuximab has been associated to new kind of toxicities, especially cutaneous, that have increasingly reported. The aim of our study is to improve the toxicity/benefit ratio in patients receiving concurrent radiotherapy and cetuximab for locally advanced head and neck neoplasm. Hence, this improvement could be achieved by modulating radiation therapy dose per fraction following Cetuximab pharmacokinetics.

Radiobiological rationale Overexpression of EGFR has been found to be involved into two different mechanisms of response to ionizing radiations: the former is related to the increased cell proliferation rate, the latter leading to lower radiosensitivity. 1. Increasing of cell proliferation rate From an analysis of the already reported evidences, we quantified the effect of a higher cell proliferation rate due to overexpression of EGFR. These findings were reported in patients affected with squamous carcinoma of head and neck district and evaluated for EGFR status. In those patients, local recurrence was compared in patients with different length of treatment but at the same fractionation. Consequently, the real doubling time in the subgroups with high and low EGFR expression was extrapolated . From these extrapolated values, equivalent doses of each day of non-delivered treatment have been calculated for patients who received accelerated schedule. Furthermore, doses were reported in relation to the different primary sites (oropharynx, larynx, oral cavity, hypopharynx). Modification of doubling time (TD) of neoplastic cell population in patients with Low and High EGFR expression. 2. Reduction of radiosensitivity Unfortunately, how radiosensitizing drugs can modify radiosensitivity cannot be evaluated retrospectively. Actually, the efficacy is evaluated on the basis of an overall effect of the treatment but radiosensitivity changes day by day in radiation and pharmacological concurrent treatment. However, these changing in radiosensitivity can be analysed in pre-clinical setting. In many reports, a progressively higher radiosensitivity has been shown in cellular survival curves by increasing EGFR inhibitor concentration. Nevertheless, the concentrations of EGFR inhibitor do not correspond to in vivo concentration during radiotherapy delivery. Fortunately, this data is obtainable from Cetuximab pharmacokinetic curve. Therefore, cell survival curves corresponding to drug concentration were obtained by performing a graphic interpolation. From these curves, daily radiosensitivity parameters were found. Subsequently, daily doses with respect to daily radiosensitivity were identified by radiobiological calculation model. This fractionation, designed on Cetuximab pharmacokinetics, have been calculated to be equivalent to curative treatment (70Gy given with conventional schedule). At the same time, lower cutaneous toxicity is expected with this "modulated" schedule due to the possibly increased cellular repair. Pharmacokinetics data of Cetuximab we found on the population: Vmax 4.38mg/h (15.4%), Km 74g/ml, central compartment volume Vl 2.83 l (18.6%), peripheral compartment volume 2.43 l (56.4%) and intercompartmental clearance 0.103 l/h (97.2%). Those parameters remain unmodified also during prolonged therapies. Administrated doses have been found to be adequate to cell wall receptors saturation. A definition of overexpression of EGFR is still lacking. Different cut-offs have been proposed to distinguish patients with "high expression" from patients with "low expression" of EGFR. In a recent study the adoption of accelerated fractionation showed an advantage for those patients with an expression of EGFR\>50%. Thus, we adopted this cut-off in our study.