Dosimetric comparison of active scanning Proton Therapy and Helical Tomotherapy in pediatric and adolescent Hodgkin’s lymphoma treated with tomotherapy.

Introduction The aim of this study is to compare radiotherapy plans using helical tomotherapy (HT) and active scanning proton technique (PT) for a group of pediatric-adolescent Hodgkin Lymphoma (HL) patients treated with HT.

Methods Five HL patients (pts), age 13-24 years (mean 16.5), (4 female IIB, IIIB, IVA an IVB, and 1 male stage IIIA), were enrolled in the AIEOP LH 2004 trial. Tumor sites were in the neck- mediastinum (n=1), mediastinum only (n=2), neck-mediastinum-abdomen (n=2). The pts were treated with 4 or 6 COPP-ABV plus involved fields HT, delivered at dose of 14.4Gy/8fr (2 pts in CR) or 25.2Gy/14fr (3 pts in PR). HT plan generated by the Hi-Art TomoPlan with a field of 2.5 cm, mean pitch of 0,215 and mean modulation factor of 1,75. All pts were re-planned with either proton multifield optimization dose (MFO) or single field optimization dose (SFO) active scanning proton technique. The median number of beams used for PT re-planning were 3 (2-5). The quality of target coverage (D2, D98), homogeneity (HI D2-D98), conformity (CI95) and the exposure of normal tissues for selected organs at risk (OARs) and the efficiency of radiation delivery were analyzed.

Results All HT patients are in CR after a median follow-up of 8 years (range 7,5-11). No chronic toxicity, nor second malignancy occurred. All HT plans as well as the PT plans feature excellent PTV coverage, high conformity and homogeneity (CI mean for PT 0.8, for HT 0.8; HI mean for PT 0.9, for HT 0.8). For all OARs, PT showed dose reductions compared with HT, especially in the lower and intermediate dose region of the DVHs, with the main advantages resulting in: heart, breast, lungs, thyroid gland. The dose to the OAR, expressed as average dose reduction (ADR) with PT compared with HT, is lowered as follow: heart ADR of D_mean by 4.9 Gy reduction in all pts; breast tissue ADR in D_mean of 3,3 Gy; lungs ADR in D_mean of about 2 Gy; thyroid ADR of D_mean of 4.5 Gy; esophagus no-differences. Spinal cord maximun dose, average reduction of 1,4 Gy. Noteworthy that one patient with extensive neck/axillary/supra/infradiaphragmatic disease could not be treated with PT because of the time required for the delivery of the treatment.

Conclusion Both techniques achieve high target coverage, homogeneity and conformal treatment plan; all parameter are slightly superior for PT plan. PT features superior dose-sparing of OARs. Dosimetric advantages may have the potential to translate into a reduction of long-term radiation-induced toxicity.