Application of stereotactic radiotherapy in the management of nasopharyngeal carcinoma: Dosimetric comparison

Abstract

Radiation treatment of nasopharyngeal carcinoma (NPC) requires delivery of 70 Gy to the primary tumor GTV. Because of the limitations of conventional conformal radiation in achieving this objective while sparing critical organs at risk (OARs) we have introduced stereotactic radiation therapy (SRT) into the final phase of NPC treatment as a boost of 10 to 24 Gy to the GTV, depending on the nodal involvement. The purpose of this study was to compare different SRT planning techniques, including intensity modulated stereotactic radiation therapy (IMSRT), with non-stereotactic techniques for the boost phase. The comparison was made to explore advantages and disadvantages of each technique in terms of target volume dose coverage and sparing of organs at risk. Radionics treatment planning and delivery system, including circular collimators, a Mini Multileaf Collimator (MMLC) and 6 MV photon beams on a dedicated Clinac 2100C are used at our institution for stereotactic radiotherapy of intracranial lesions. Stereotactic treatment planning techniques include non-coplanar arcs using circular collimators, 3D conformal and step and shoot intensity modulated stereotactic radiotherapy. Non-stereotactic techniques include Cadplan 3D conformal, Cadplan/Helios IMRT and Pinnacle IMRT. The treatment plans comparison was made for small (31cc), medium (95cc) and large (215cc) size target volumes. Identical CT data and contours were used on all planning systems and a 20 Gy boost dose was selected for all cases in this study. Treatment plans were evaluated based on 2D and 3D dose distributions, dose volume histograms (DVH), maximum (Dmax), minimum (Dmin) and median (Dmedian) doses as well as dose of 95% volume (D95), conformity index (CI), and dose heterogeneity (DH). Parameters used for OARs dose comparison were DVHs, Dmax and Dmedian as well as dose of 10% to 50% volumes (D10, D20 and D50), when the dose values were normalized so that 95% of the target volume received at least 95% of the prescribed dose. It was found that the IMRT techniques produced superior results compared with non-IMRT techniques for medium and large size tumors. For small size tumors, however, stereotactic non-coplanar arcs technique using circular collimators showed comparable results with less dose heterogeneity. There were some limitations for IMRT of small tumors, especially by Helios and Pinnacle. IMSRT resulted in a better tumor dose coverage than other IMRT techniques for small and medium size tumors by producing sharper dose fall off and up to 70% less dose heterogeneity. This is mainly due to smaller MMLC leaf thickness of 4mm compared to 5mm to 10mm for Helios and Pinnacle. For the large size tumor, however, the above results were comparable for all IMRT techniques. Pinnacle and Helios IMRT produced comparable results for the tumor dose coverage to that of stereotactic 3D conformal with greater dose heterogeneity but less dose to OARs. Also, Cadplan conformal produced inferior results, compared to all other techniques, for the target volume dose coverage and sparing OARs. OARs doses were compared for the brain stem and optic chiasm for the same tumor dose coverage that is for a normalized dose. The normalization was performed so that 95% of the target volume would receive 19 Gy, that is 95% of the prescribed dose. It was shown that IMSRT would result in up to 40% less dose to OARs for the small and medium size tumors, compared to other IMRT techniques for the same tumor dose coverage. Pinnacle IMRT results were slightly better results than those of Helios. Bases on the results of this study, it is concluded that IMSRT boost to the primary lesion following or overlapping with standard radiation therapy would result in a better dose coverage of the target volume and a reduced dose to OARs. This, together with stereotactic superior localization and immobilization, will allow greater confidence in target coverage, OARs avoidance and therefore opportunities for dose escalation