ESTRO 38 Abstract book

S1145 ESTRO 38

mARC prostate treatment plans have generated clinically acceptable dose distributions and have shown similar or better protection of the evaluated OARs. Regarding the rectum, the greatest advantage of the mARC technique has been obtained in the PR+SV+PLN group, in which the mARC plans have yielded a statistically significant mean reduction in the mean dose of 3 Gy. Considering the bladder, mARC plans have shown a systematic sparing over the entire dose range in the PR+SV group with a statistically significant mean reduction in the mean dose of 6.5 Gy. With respect to the femoral heads, mARC dose distributions have yielded statistically significant reductions in the mean and maximum doses of every group, showing highly significant mean reductions of 5.3 Gy in the mean dose and 8.4 Gy in the maximum dose in the PR+SV group. EP-2076 Is there any advantage in using helium ions over protons for minibeam radiation therapy? T. Schneider 1 , A. Patriarca 2 , Y. Prezado 1 1 Imagerie et Modélisation en Neurobiologie et Cancérologie IMNC, CNRS - Université Paris 11 - Université Paris 7, Orsay, France ; 2 Institut Curie - PSL Research University, Centre de Protonthérapie d’Orsay - Radiation Oncology Department, Paris, France Purpose or Objective Hadron minibeam radiation therapy (hMBRT) is a novel concept combining the normal tissue sparing of submillimetric, spatially fractionated beams [1,2] with the improved dose deposition of ions. The increase of normal tissue tolerances may allow a safe dose escalation in the tumour. Along this line, proton minibeam radiation therapy [3] has already shown a widening of the therapeutic window for radioresistant tumours [4,5]. Next to protons, a possible choice for minibeam radiation therapy are helium ions. Their reduced multiple coulomb scattering compared to protons could lead to more favourable dose distributions, without the possible concerns related to nuclear fragmentation of heavier ions [6,7]. Material and Methods The dose distributions of protons and helium minibeams with the same range (7.7 cm) both in a water phantom and in the computer tomography images of an anonymized human patient (brain tumour) have been assessed by means of Monte Carlo simulations (GATE v.8.0 [7]). Two different full widths at half maximum (FWHM) at the entrance were considered: 1 and 3 mm. Different minibeam spacings and divergences were also evaluated. The peak-to-valley dose ratio (PVDR), entrance-to-Bragg peak dose ratio (EBDR) and lineal energy transfer (LET) were used as figures of merit. Results It was found that the use of helium ions can significantly improve the EBDR yielding a two-fold increase for 3-mm beams and a more than three-fold increase for 1-mm beams, which favours tissue sparing. Due to their reduced lateral scattering, helium ions yield slightly larger PVDRs than proton minibeams, which ultimately might make it challenging to obtain a homogeneous lateral dose distribution at Bragg peak depth with only one array. However, in recent studies in pMBRT, tumour sterilization was achieved even in configurations leading to highly heterogeneous dose distributions [5]. Lastly, no significant difference was observed in terms of LET maps between protons and helium. Conclusion Helium ions may be an optimum alternative for minibeam radiation therapy since they offer improved dose distributions with respect to protons without the possible drawbacks linked to nuclear fragmentations of heavier ions.

results corresponding to 60 seven-field step and shoot IMRT plans that had been previously delivered in our centre. Both samples were formed by 3 groups of 20 patients each, regarding the target volume extent: prostate (PR), prostate and seminal vesicles (PR+SV) and prostate, seminal vesicles and pelvic lymph nodes (PR+SV+PLN). All treatments had a prescription dose to the prostate PTV of 70 Gy in 28 fractions (2.5 Gy/fraction). In case of seminal vesicle involvement, the corresponding PTV had a prescription dose of 56 Gy (2 Gy/fraction). If the pelvic lymph nodes were also considered for treatment, the prescribed dose to their PTV was 50.4 Gy (1.8 Gy/fraction). Plans were optimised to meet the dosimetric requirement of at least the 95% of each PTV receiving the 95% of its prescription dose, with the OARs being irradiated with the lowest dose as possible. Several dosimetric quantities and the mean DVHs have been evaluated for the rectum, bladder and femoral heads. The unpaired t-test or the Mann-Whitney test have been conducted on the evaluated parameters to assess statistically significant differences among both techniques, with a 5% significance level. Results Every plan was considered valid for treatment by the radiation oncologists. The table lists the results of the analyses (mean values ± standard deviations; n.s.: not significant). The mean DVHs are displayed in the figure.

Conclusion