Session Item

Proton therapy (PT) for mediastinal tumors has the potential to reduce doses to normal tissue and organs at risk (OARs) compared to photon radiotherapy. However, there is a coherent complexity with PT and treatment in the thorax region can impose a major challenge due to respiratory motion. To minimize the uncertainties associated with PT, robust optimization (RO) and motion-mitigation techniques, e.g. Deep Inspiration Breath-Hold (DIBH), can be applied. The aim of this study was to evaluate pencil beam scanning (PBS) PT in DIBH for mediastinal lymphoma patients, with a focus on treatment characteristics and plan robustness to the clinical target volume (CTV) and OARs throughout treatment.

Sixteen patients diagnosed with mediastinal lymphomas who underwent PBS proton therapy in DIBH were included in this study. Treatment plans with two to three fields (gantry angle 335°-25°) were robustly optimized on the CTV (7mm/4.5%). Single field uniform dose optimization was used for all patients, except for one for which multifield optimization was used. Target coverage and OAR doses were robustly evaluated (7mm/4.5%). Weekly verification CTs (vCT) were acquired during the treatment course and structures (CTV, heart, lungs) were transferred from the planning CT to the vCTs with deformable image registration. The original treatment plan was recalculated on the vCTs as if the full treatment would be delivered on those particular CTs. Thereafter OAR doses and target coverage (CTV D98%) were computed and compared with the nominal plan values. In addition, variation in lung volume was computed.

The median beam-on time was 2:33 min:sec (range 1:22-6:55) while median session time was 25 min (range 17-37) and the median number of breath-holds required was estimated to be 11 (range 5-24). The implemented procedure demonstrated high robust target coverage throughout treatment with CTV D98% deviations within 2% for most patients and above the requirement of CTV D98% ≥ 95% at 46 out of 49 vCTs (fig 1). However, two patients did not achieve a robust dose to CTV due to poor DIBH reproducibility, with CTV D98% as low as 86 and 93% respectively. In one case re-planning was considered necessary. Dose deviation to OARs were low for most patients. The mean dose increase to the heart was below 2 Gy(RBE) for 45/49 vCTs and 3-5 Gy(RBE) for the remaining four vCTs. The corresponding increase for the lungs was below 1 Gy(RBE) for all vCTs. In addition, the total lung volume variation was below 10% for 39/49 vCTs (fig 2).

This evaluation of weakly vCTs showed that PBS PT in DIBH is generally a feasible and robust technique for treatment of mediastinal lymphomas. However, it is important to closely evaluate the DIBH reproducibility during the treatment course to avoid underdosing CTV and to achieve sufficient dose sparring of the OARs.