ESTRO 2022 - Abstract Book

S1293

Abstract book

ESTRO 2022

Ten skin cancer patients originally planned with electrons were re-planned using a 6MV flattening filter free (FFF) VMAT beam. A 1 arc and 2 arc plan were generated. Five of the ten patients had scalp/forehead cancers while the other five had a facial cancers including nose, cheek and lip. If required, planning bolus of thickness 0.6cm was added to the skin surface around the PTV to ensure coverage. A PTV+5cm and an additional Other Body structure were created to assess the dose to normal tissue up to 5cm beyond the PTV and all other normal tissue beyond the PTV+5cm structure respectively. Organs at risk (OARs) were contoured and the plans were optimised. The homogeneity index (HI) and the 90%, 95% and 100% dose conformity indices (CI) of the PTV were evaluated. The volume of PTV+5cm and Other Body receiving a low (<5% of prescription), intermediate (between 5 and 50% of prescription) and high (>50% of prescription) dose were analysed. Results Below are tables showing only the significant dosimetric differences between the 1 arc, 2 arc and electron plans. Table 1: Dosimetric comparison of a 1 arc and 2 arc plan with an electron plan.

Table 2: Dosimetric comparison between the 1 arc and 2 arc plan.

Conclusion This study showed that using VMAT in place of electrons provides a greater 95% and 100% PTV conformity for skin cancers of the head and neck but no significant change in the 90% CI. As expected, this comes at the cost of an increased mean dose to the brain and an increased low and intermediate dose to normal tissue beyond the PTV. A 2 arc plan provides better conformity of the PTV than the 1 arc plan, but again is at the cost of an increased intermediate dose to normal tissue. The clinical impact of the low dose bath is not yet widely understood. Additional work may focus on further optimising the VMAT plans to reduce the low dose bath effect and analyse the dose reported to critical OARs.

PO-1513 Radiobiological corrections of dose-volume histograms for treatment gap calculations

K. O'Shea 1 , M. Moore 2 , L. Coleman 2

1 National University of Ireland Galway, Department of Physics, Galway, Ireland; 2 University Hospital Galway, Department of Medical Physics & Clinical Engineering, Galway, Ireland Purpose or Objective Unscheduled treatment interruptions to radiotherapy treatments lead to a decrease in the TCP due to dose lost to ongoing rapid cell repopulation. The dose lost to a treatment gap must be compensated for to prevent an extension onto the patient’s overall treatment time and to re-gain the original TCP. This was the case during the Irish Health Service Executive (HSE) cyberattack of May 2021 which resulted in radiotherapy patients experiencing treatment gaps of up to 12 days. Current treatment gap calculations are performed with a point-dose for the PTV and the OARs, using the prescribed dose to the PTV. This can overestimate the dose to the OARs by assuming all OARs receive the prescription dose. Materials and Methods An in-house tool EQD2VH was created in Python to perform treatment gap calculations using the DVH information provided in the RT Dose DICOM files. The physical dose in each dose bin was converted into EQD2 while accounting for cell repopulation in the PTV. This provided a 2D representation of treatment gap calculations in comparison to the 1D point- dose method. The tool was evaluated using five Category 1 patients (head, neck, and lung cancer patients) who were receiving radiotherapy treatments when the cyberattack occurred. Compensation plans were created for these patients using Royal College of Radiologists (RCR) methodology to evaluate the use of EQD2VH as a clinical aid. Results The 2D representation of treatment gap calculations for the PTV and OARs provided a better representation of each compensation option in comparison to the 1D point-dose calculation method. EQD2VH provided a visual analysis of each plan to each individual structure and was used as a tool to find a balance between the dose to the PTV and the OARs. The DVH statistics provided by EQD2VH were used for comparisons with dose constraints.

Conclusion