ESTRO 37 Abstract book

S1047

ESTRO 37

Finally, a PTV margin is added to the CTV and is computed from the statistics linked to geometric errors (patient setup, inter- and intra fraction motion). The PTV margin is also large enough in order to encompass the CTV with a sufficient amount of systematic geometric errors, typically 90-95%, as determined by the physics team. Although such approach has considerable pragmatic merits, it is fundamentally statistically biased because margin expansions for microscopic infiltration probability and systematic geometric errors should not be added linearly but in quadrature, as they can both be understood as a geometric probability of systematically missing tumor cells. We have implemented here a novel approach for computing the PTV directly from the GTV based on a more statistically sound methodology that, for a given confidence interval, mitigates excess planning target volumes compared to the conventional approach. Material and Methods We have implemented in MATLAB an algorithm that directly expands the GTV to the PTV, for the purposes of this study called the PTVc (PTV Combinatorial). The main inputs are the probability of microscopic infiltration, derived from generic distributions or published data, and the standard deviations for geometric systematic errors. For every point at the surface of the GTV, the probability of microscopic infiltration is corrected for anatomical barriers using editions of the clinician. The latter step could be automatized eventually. A local margin is then defined according to a probability threshold defined for the combined probability distribution. The volume is further dilated for geometric random errors leading to the final PTVc. It is important to mention that our methodology only applies to a conventional GTV-CTV-PTV expansion scheme with a CTV margin linked to a probability of microscopic infiltration; elective CTVs are not considered. We have tested our methodology for 5 lung, 3 H&N and 1 glioblastoma patients. Results The PTVc equals the conventional PTV for parts of the volumes where the physician has estimated that infiltration was impossible, as illustrated on the CT image. Volume mitigation ranges from 8 to 34% (see table).

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: