The effect of the choice of the prescription point on dosimetric parameters in three-dimensional conformal radiotherapy of the head and neck cancer
Introduction: This study aimed to investigate the effect of different dose prescription points in three-dimensional treatment planning of the head and neck cancer on dosimetric parameters of the target dose distribution.
Materials and Methods: Thirty patients with nasopharynx (18) and oropharynx (12) cancers were selected for this study. Treatment planning was done using single-isocenter half-beam technique for each patient. Then, two sets of dual reference points for dose prescription were considered in two adjacent planning target volumes (PTVs) including anterior lower neck (ALN) and lateral sites. The homogeneity and conformity indexes (HI and CI, respectively) of the dose distribution were calculated for each patient. Then, the analysis of the dosimetric parameters including D95%, Dmax, Dmean, and the maximum dose in the field’s junctions for treatment plans with various prescription points were carried out.
Results: The HI and CI for dose coverage of the PTVs was obtained as (0.04 and 0.15) and (0.02 and 0.13) in dual set 1 and 2, respectively. There was a negligible difference between the dose in prescription points and mean doses. However, the paired t test analysis showed that there was a significant difference (P < 0.05) between the Dmax at treatment plans with various prescription points. Moreover, a small difference, of borderline significance, was observed for the maximum dose at the field’s junctions between two sets of dual prescription points (P = 0.08).
Conclusion: Dose distributions in the two sets of dual dose prescription points showed a good estimation of the mean doses to the PTVs; and then can be used as a representative of tumor dose. However, the Dmax of the target dose distribution for set 2 dose prescription points was lower than set 1 points. The proper comparison of treatment plans can be achieved by using a common reference point which satisfy the requirements of the ICRU reference point.
Keywords: Radiotherapy treatment planning, Single-isocenter half-beam technique, ICRU reference point, Mean dose, Asymmetric jaws.
Allaveisi F. The effect of the choice of the prescription point on dosimetric parameters in three-dimensional conformal radiotherapy of the head and neck cancer. J Radiobiol 2017;4(1):e03.
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