Session Item

Seminoma patients with indication for radiotherapy have traditionally been treated with photons – opposing IMRT fields or VMAT. Due to the extent of the target there is a substantial dose bath to surrounding organs at risk (OAR). The patients are young with excellent prognosis so reducing the risk of secondary cancer is of utmost importance.

A comparative dose planning study was done to determine if robust optimized intensity modulated proton therapy (IMPT) could reduce dose to OAR with the aim of clinical implementation of IMPT.

Six patients with seminoma, treated with photons ("dog-leg" 3 field IMRT, n=4 or 2 full arc VMAT, n=2) were used for comparative dose planning. CTV-E (Elective) extended from Th11/12 to top of acetabulum on the ipsilateral side. Prescribed dose ranged from 20–25 Gy RBE to CTV-E. Pathological lymph nodes were subsequently boosted to 10 Gy RBE (Table 1). The clinical photon treatment plan was used as comparator in the photon/proton comparison.

Proton plans were made using 5 field-robust (5 mm setup error; 3.5% range uncertainty) IMPT planning (Multi field optimization, Eclipse v13.7) for the CTV-E (3 posterior supplemented by 2 anterior fields at the level of iliaca arteries). Three posterior robust IMPT fields were used for the paraaortic boost. Boost in the pelvis area had an individual field arrangement. Plan where recalculated to check for changes in bowel gas.

Dose to OARs (body outline, bowel bag, bladder, spinal cord, duodenum, kidneys, pancreas, stomach) where compared for photon vs. proton plans. The risk of secondary cancer was calculated by the model described in Schneider et al., 2011 (Theor Biol Med Model 2011 Jul 26;8:27).

Wilcoxon's signed rank test was used for comparison. A p-value<0.05 was considered statistically significant.

CTV volumes differed with a median (range) of 650 cm³ (418–757 cm³) for CTV-E and 56 cm³ (25–79 cm³) for the boost.

Mean doses to all OARs were significantly lower with protons compared to photons except for spinal cord, due to the posterior beams and for pancreas which for one patient had a higher mean dose with photons, (Figure 1). For left and right kidney, a reduction from 4.9 Gy to 3.3 Gy and 4.7 Gy to 2.6 Gy (p=0.03; p=0.03) was seen, respectively. Mean dose to body outline was reduced from 3.9 Gy to 1.9 Gy (p=0.03).

The excess absolute risk (EAR) of secondary cancer was reduced from 46 to 23 per 10.000 persons per year (PY) for photon vs proton plans (p=0.03) but with considerable variation between opposing IMRT fields and VMAT. VMAT resulted in a higher EAR due to the low dose bath from the VMAT technique.

For all patients, the mean dose to delineated OAR were reduced with protons compared to photons except for the spinal cord and pancreas. Likewise, the radiation dose to all OARs and thereby the risk of secondary cancer was reduced; this was especially pronounced comparing proton planning to VMAT planning.

Robust IMPT is a favorable treatment option for seminoma patients.