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The choice of the optimal therapeutic strategy in pediatric cancers remains an important issue, especially in radiotherapy. Indeed, due to the age of the patients, it is important to drastically reduce the doses received by the healthy organs. In external beam radiation therapy (EBRT), a prospective comparison between protons, electrons and photons is possible. However, the dosimetric result of brachytherapy (BT) is highly dependent on the quality of the implantation. In this study, a retrospective dose comparison was carried on pediatric patients with rhabdomyosarcoma treated with brachytherapy.

For all patients, 7 irradiation techniques were compared, including brachytherapy (Oncentra) and 6 EBRT techniques: 3D conformational EBRT with electrons and photons (Raystation), intensity-modulated radiation therapy (IMRT, Raystation), volumetric modulated arc therapy (VMAT, Raystation), tomotherapy (Precision), and proton therapy (Eclipse). The tumors of the 4 considered patients were located in the trunk (right humerus, supraclavicular region), in the head and neck region or in the vesico-prostatic region. The prescribed dose was equal to 60 Gy, except for the supraclavicular tumor (prescription equal to 55 Gy in brachytherapy and 54 Gy in EBRT). The brachytherapy treatment was delivered using a pulsed dose rate technique (PDR-BT) with 0.42 Gy pulse per hour. Data were compared considering equivalent doses of 2 Gy fractions (EQD2).

All dosimetries were produced except for patient suffering from the tongue tumor for whom the proton therapy plan was not feasible due to the presence of teeth. Figure 1 compares dose maps obtained by VMAT and tomotherapy, protons and brachytherapy for the patient treated for a vesico-prostatic tumor. As expected, VMAT resulted in a large bath of low doses. The 40 Gy isodose decreased from 149.3 cc to 129.9 cc and again to 59.6 cc when switching from VMAT to protons and then to brachytherapy. For the patient treated for the tongue tumor, maximum dose to the mandible was decreased from 60.9 for 3D CRT photons to 47.4 Gy for brachytherapy treatment. The same was observed for cartilage volumes for rhabdomyosarcoma located inside the humerus with maximum dose values less than 38.3 Gy for cartilage in the surrounding bones for BT. The corresponding maximum values were equal to 60.6 and 60.7 Gy for 3D CRT and proton therapy photons, respectively.  

These results once again demonstrated the ability of BT to drastically spare healthy tissue surrounding the tumor. Proton therapy is an encouraging alternative for many of the locations analyzed. This exhaustive study in the techniques implemented provides quantitative arguments as to the clinical options to be considered. The database will be continued in order to increase its completeness.