Abstract Book

S68

ESTRO 37

corresponds to an air gap between the RS and the patient surface of 30 and 5 cm, respectively. PTV dose statistics were recorded. The same planning configurations were adopted for the experimental benchmark. A dedicated water equivalent RW3 phantom was used. The dose distribution on the coronal plane was measured with a 2D array of ionization chambers and compared to PB and MC calculated ones by means of a gamma analysis (agreement criteria 3%, 3mm). Results The results of the TPS algorithm comparison are summarized in Table 1. A good PTV coverage is obtained in the PB plan, while a lower homogeneity index (HI, i.e. (D2-D98)/D50) results from the same plan recomputed by the MC algorithm, especially in presence of a large air gap. MC also results into lower average PTV doses. The same trend is observed in the phantom study, as shown by the gamma analysis in Figure 1. While a passing rate close to 90% or larger is obtained for the PB plans with extended snout position, the value drops to about 63% for retracted snout. Remarkably, the passing rate is always above 95% for the MC plans. Conclusion When using proton PBS for BC treatment, the PB algorithm does not allow obtaining sufficient accuracy, especially with large air gaps. On the contrary, MC algorithm resulted into much higher accuracy in all beam configurations tested and has to be recommended. Centres where a MC algorithm is not yet available should consider a careful use of PB, possibly combined with a movable snout system or in any case with strategies aimed at minimizing air gaps.

Conclusion The concerns about possible increased secondary cancers risks for modern APBI techniques are not justified. The risks for all APBI techniques are lower than for WBI. Since the lungs showed the highest risk of secondary cancer per Gray mean dose, we recommend focusing primarily on this parameter to reduce the total LAR of secondary cancer. PV-0137 Dosimetric uncertainties in pencil beam proton therapy for breast cancer F. Tommasino 1 , S. Lorentini 2 , M. Schwarz 2 , F. Fellin 2 , P. Farace 2 1 University of Trento, Physics, Trento, Italy 2 Azienda Provinciale per I Servizi Sanitari APSS, Protontherapy Department, Trento, Italy Purpose or Objective Proton therapy (PT) for the treatment of breast cancer (BC) is acquiring increasing interest. The main motivation for the use of protons lies in the potential reduction of radiation-induced side effects such as cardiac toxicity (especially for left-side BC patients) and pulmonary toxicity. While several in silico studies demonstrated the gain in plan quality offered by pencil beam scanning (PBS), dosimetric uncertainties have been poorly investigated so far. Purpose of the present study is twofold: a) to compare dose distributions obtained with a commercial analytical pencil beam (PB) and Monte Carlo (MC) TPS in a BC patient and b) to benchmark such dose distributions with dedicated measurements. Material and Methods A representative BC patient was planned with intensity modulated proton therapy (IMPT) calculated with PB and MC dose calculation algorithms. Plans were optimized with PB and then MC was used to recalculate dose distribution, with 1% statistical uncertainty. Prescription dose was set to 50 Gy. Due to the shallow extension of the target, plans required the use of a range shifter (RS) device. Movable snout and beam splitting techniques (i.e. the field is divided into two sub-fields, one with and the other without RS) were considered in order to reduce the lateral penumbra and minimize the use of a RS. Plans were calculated for the most retracted (SNOUT 42) and most extended (SNOUT 17) snout positions. This

PV-0138 A multi-modal MRI and CT imaging method for the accurate calculation of proton stopping power ratios A. Sudhyadhom 1 , J. Scholey 1 , T. Solberg 1 1 University of California UCSF, Radiation Oncology, San Francisco CA, USA Purpose or Objective The purpose is to report on a novel method to accurately calculate proton SPR using a combination of MRI and CT imaging. Material and Methods Proton SPR was calculated by the Bethe-Bloch equation. In this equation, the component of greatest uncertainty is the mean ionization potential (also known as the mean excitation energy), as electron density can be determined by CT. A method was devised to determine