The comparison of parotid and submandibular gland doses in the conventional RT, three dimensional conformal RT (3DCRT) and image-guided RT (IMRT) treatment planning of nasopharyngeal cancer

Aim: The aim was to compare the target volume and normal tissue doses in the conventional, 3DCRT and IMRT techniques using treatment planning Computer Tomography (CT) images of nasopharyngeal cancer patients who were treated previously.Patient and Method: The CT image data of 10 patients who were given radical radiotherapy at Dokuz Eylul University Department of Radiation Oncology between June 2004 and December 2007 were used. The patients had stage T1-4 N0 M0 disease. GTV, CTV, PTV and OAR were defined in each patient according to ICRU 50-62. Conventional, 3DCRT and IMRT treatment planning were performed in each patient. Two isocentric lateral parallel-opposed fields for the primary tumor and upper neck and a single anterior-posterior (AP) field for the lower neck were used for conventional treatment planning. The number of isocentric, non-coplanar fields with different angles was 5 and 7 in 3DCRT and IMRT, respectively. In conventional and 3DCRT a total dose of 70 Gy in 35 fractions (fr) and a total dose of 50 Gy in 25 fr were given to GTV and CTV, respectively with a daily fraction dose of 2 Gy. In IMRT a total doe of 66 Gy was given in 30 fr to GTV with SIB technique, and a total dose of 54 Gy in 25 fr to CTV. The reference isodoses were selected and the dose volume histograms (DVH) of three different treatment plans for each patient were evaluated. For target volume doses the mean of mean percent dose of GTV and PTV 70 were detected. The mean of mean absolute dose of spinal cord and brain stem, the mean V26 Gy values of parotid glands and the mean D50 values of submandibular glands were determined. Analyses were performed for conventional-3DCRT, conventional-IMRT and 3DCRT-IMRT comparisons. Paired samples t-test was performed using SPSS 11 program for evaluating the differences between the treatment planning types.Results: For target volumes (GTV and PTV 70) higher mean percent doses were obtained in IMRT compared to 3DCRT (p=0.003 and p=0.01). However there wasn?t any significant difference in the other two paired comparisons. The mean absolute dose of spinal cord was lower in 3DCRT and IMRT compared to conventional (p<0.001). No significant benefit was observed with 3DCRT and IMRT over conventional in terms of mean absolute dose to brain stem. In terms of V26 Gy percent volume of parotid glands, there wasn?t any significant difference between conventional and 3DCRT for the right parotid, however 3DCRT lowered the V26 Gy percent volume for the left parotis. IMRT was superior compared to both conventional and 3DCRT (p<0,001, p=0,002 (right)) and (p=0,003, p=0,006 (left) respectively) for both parotid glands. As for the D50 dose to both submandibular glands, the only statistically significant difference was observed between 3DCRT and IMRT in favor of IMRT (p=0,01 and p=0,017 for right and left, respectively).Conclusion: In nasopharyngeal cancer patients without neck involvement, IMRT provided higher percent doses in the target volumes compared to 3DCRT, and lower V26 Gy percent volumes compared to both conventional and 3DCRT. While no significant difference was observed in all the other paired comparisons for submandibular gland D50 dose, IMRT - 3DCRT comparison revealed the superiority of IMRT.