Abstract Book

S1046

ESTRO 37

Conclusion The methodology developed for combining the probabilities of microscopic extension and geometric errors leads to a statistically sound definition of the planning target volume. This concept may lead to more accurate estimations of confidence intervals and/or a significant mitigation of treated volumes. EP-1924 Comparison of dosimetric parameters of treatment planning techniques for anal cancer E. Titovich 1 , M. Piatkevich 1 , M. Mayorova 1 1 National Cancer Center of Belarus, Radiotherapy engineering and medical physics, Minsk, Belarus Purpose or Objective The purpose of this work is to develop a method of irradiation for anal cancer patient’s radiotherapy, that can reduce dose, delivered to the bladder, and perform a comparative analysis of dosimetry parameters (D bl_mean , IH, IC) and mean fraction time for the developed method and standard dosimetry planning methods for the anal canal cancer. The duration of the radiotherapy fraction should not exceed 10 minutes. Material and Methods To implement the irradiation in accordance with the established priorities, the VMAT technique was chosen. Irradiation of the anal canal cancer according to the developed method (VMATnew) is performed as follows: •The first rotation of the gantry is a full arc that is performed counterclockwise with the rotation of the treatment table by 5 degrees IEC •The second rotation of the gantry is a full arc that is performed clockwise with the rotation of the treatment table by degrees IEC •The third gantry rotation is a partial arc (140-220 degrees) that is performed with the treatment table position at 0 degrees IEC The planned equivalent dose is 40Gy. For 29 patients, radiation treatment planning was carried out for each of the three standard radiotherapy techniques: 3D CRT, IMRT and VMAT. Also, the dose distribution was calculated for the newly developed VMAT new . The received irradiation plans were evaluated according to the following criteria: •homogeneity index: HI=(D 2% -D 98% )/D 50% ; • conformity index:CI=V 95%/ V PTV ; • average dose received by the bladder (D bl_mean ). For each of the evaluated methods, the mean duration of the treatment session was determined (t mean ). Results Figure 1 shows the coverage of the irradiated target. Table 1 presents the results of the dosimetric parameters analysis for each of the evaluated methods:

Figure 1. Dose distribution in the transversal plane at the isocenter

Table 1. Evaluated dosimetric parameters

Conclusion A new method of irradiation was developed using the VMAT technique for the anal cancer patients, which reduces the dose delivered to the bladder to an average of 25 Gy. The average radiotherapy fraction time taken to deliver a VMATnew treatment plan is 5 minutes. Using the developed VMAT new method and the IMRT technique for the treatment of anal canal cancer, it is possible to achieve a 3Gy reduction in dose, delivered to the bladder in comparision to the standard rotational irradiation of VMAT and 13Gy compared to using 3D CRT. With equal doses to the bladder for VMAT new and IMRT, the average time of the radiotherapy session using the developed VMAT new method is 3 times less. Using the developed VMAT new method allows to increase the homogeneity index of the radiation plan by 16% in comparison with the standard VMAT technique (2 arcs), 20% with IMRT and 33% with 3D CRT. EP-1925 Dosimetric implication of planning organ at risk volumes in stereotactic body radiotherapy of spine S.H. Jeon 1 , J.H. Kim 1 , S. Son 1 , S.Y. Park 1 , J.I. Kim 1 , J.M. Park 1 1 Seoul National University Hospital, Radiation Oncology, SEOUL, Korea Republic of Purpose or Objective To investigate the impact of planning organ at risk volumes (PRV) of neural organ at risk (OAR), i.e. spinal cord (SC) and cauda equina (CE), on the safety and efficacy in stereotactic body radiotherapy (SBRT) of spine. Material and Methods Pairs of SBRT plans were generated using either no PRV (OAR-plans) or 1mm PRV margin (PRV-plans) for 22 spinal lesions. An 1 mm planning target volume (PTV) margin was added to clinical target volume (CTV). All plans were generated using a volumetric-modulated arc therapy technique. A single dose of 18Gy was prescribed to cover ≤95% of PTV and dose constraints from RTOG 0631 trial were used. Three types of uncertainties, i.e. pre- treatment residual error, post-treatment residual error, and intrafraction motion, were acquired by matching of pre-/post-treatment cone-beam computed tomography to simulation computed tomography, and used to generate