ESTRO 2022 - Abstract Book

S842

Abstract book

ESTRO 2022

Purpose or Objective To reduce liver and lung dose during right breast irradiation while maintaining optimal dose to the target volume. This dose reduction has the potential to decrease acute side effects and long-term toxicity. Materials and Methods Sixteen patients treated with radiation therapy for localized carcinoma of the right breast were included. Six patients were evaluated retrospectively and 10 prospectively. For the planning CT, each patient was immobilised on an indexed board with the arms placed above the head. CT scans were acquired in free-breathing (FB) as well as with deep inspiration breath hold (DIBH). Both scans were acquired with the same length. Planning target volumes (PTV's) were created with a 5mm margin from the respective clinical target volumes (CTV's) on both CT datasets. The liver was outlined as scanned. Dose metrics evaluated were as follows: differences in PTV coverage, dose to the liver (max, mean, V90%, V50%, V30%), dose to lung (mean, V20Gy, relative electron density) and dose to heart (max). The p-values were calculated using Wilcoxon signed- rank tests. A p-value was significant < 0.05. Results Differences in PTV coverage between plans under FB and DIBH were less than 2%. Maximum liver dose was significantly less using DIBH: 40.3Gy versus FB 17.5 Gy (p < 0.001). Dose metrics are summarized in Table 1. The volume of the liver receiving 10% of the dose was significantly less using DIBH with 72.2ccm versus 1.88ccm under FB (p < 0.001). The absolute volume receiving 20Gy in the right lung was larger using DIBH: 291ccm versus 230ccm under FB (p < 0.001), and the relative lung volume was less with DIBH: 11.5% versus 14% in FB (p = 0.007). The relative electron density was less with DIBH: 0.59 versus 0.62 with FB, (p < 0.001). This additionally suggests that the lung tissue receives less dose due to lesser dense lung tissue when using DIBH.

Table 1 - Dose metrics for liver and right lung

(DIBH-FB) (%) Rel. Diff.

(DIBH-FB) Abs. Diff

FB

DIBH

p-value

Liver - Max Dose (Gy)

40.3 17.54 -48.01 72.22 1.88 -96.22 230.45 291.35 34.64 14.02 11.5 -15.8

-19.25 -54.65 76.75

<0.001 <0.001 <0.001

Liver - V10% Isodose (ccm) Right Lung - V20Gy (ccm) Right Lung - V20Gy (%)

-1.95 -0.03

0.007

Right Lung V20Gy - relative electron density 0.62 0.59 -4.87

<0.001

Conclusion Radiation of the right breast using DIBH spares liver and lung tissue significantly and thus carries the potential of a new best practice for localized right sided breast cancer.

OC-0950 Dosimetric comparisons of wide tangent and VMAT techniques in breast IMC radiotherapy

J. Varsani 1 , Y. Tsang 2 , S. Nunes 3 , S. Ahmed 3 , V. Newton 3 , C. Westbury 4

1 Mount Vernon Cancer Centre, Radiotherapy Physics, Mount Vernon Cancer Centre, United Kingdom; 2 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom; 3 Mount Vernon Cancer Centre, Radiotherapy Physics, Northwood, United Kingdom; 4 Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom Purpose or Objective In the United Kingdom (UK), the inclusion of internal mammary chain (IMC) radiotherapy (RT) has been adopted within national guidelines since the publication of clinical trials which show benefit. Currently in our institution a 3arcs volumetric modulated arc therapy (VMAT) technique is offered to patients where a wide tangents (WT) technique does not meet defined dosimetric constraints. However, the beam on time is considerably longer, increasing the number of breath holds a patient has to undergo. This study aims to investigate dosimetric differences in terms of target volume coverage and organs at risk (OAR) sparing between 2arcs and 3arcs VMAT versus WT. Materials and Methods Planning computed tomography (CT) scans of 20 patients previously treated with whole breast/chestwall RT at our institution were included. For all datasets, a single observer (a breast consultant clinical oncologist) delineated breast/chestwall clinical target volumes (CTV) and axillary levels 1-4 and IMC nodal CTV on the planning CT with a planning target volume (PTV) margin of 5mm; ipsilateral/contralateral lungs, heart, contralateral breast/chestwall, spinal cord, thyroid and oesophagus were contoured using ESTRO guidelines. For each patient, 3 RT planning techniques: WT, 2arcs and 3arcs VMAT were planned by a single radiation therapist with the prescription dose of 40Gy in 15 fractions following the departmental protocol. PTV coverage and OAR doses were measured for each technique. Kruskai-Wallis ANOVA was performed to detect statistically significant differences in dosimetric parameters between techniques followed by Bonferroni-type multiple comparisons with adjusted P-values ≤ 0.05 indicating statistical significance Results Within the cohort, there were 10 left sided (6 breast and 4 chestwall) and 10 right sided (8 breast and 2 chestwall) patients resulting 60 plans in total. The median (range) of total monitor units (MU) were 567 (536-592), 781 (667-863) and 773 (647-