Abstract
The purpose of this study was to assess treatment margins in free‐breathing irradiation of pancreatic cancer after bone alignment, and evaluate their impact on conformal radiotherapy. Fifteen patients with adenocarcinoma of the head of the pancreas underwent implantation of single fiducial marker. Intrafraction uncertainties were assessed on simulation four‐dimensional computed tomography (4D CT) by calculating maximal intrafraction fiducial excursion (MIFE). In the first ten patients, after bony alignment, the position of the fiducial was identified on weekly acquired megavolt cone‐beam CT (MV‐CBCT). The interfraction residual uncertainties were estimated by measuring the fiducial displacements with respect to the position in the first session. Patient mean (pM) and patient standard deviation (pSD) of fiducial displacement, mean (μM) and standard deviation (μSD) of pM, and root‐mean‐square of pSD (σres) were calculated. In the other five patients, MIFE was added to the residual component to obtain personalized margin. In these patients, conformal kidney sparing (CONKISS) irradiation was planned prescribing 54/45 Gy to PTV1/PTV2. The organ‐at‐risk limits were set according to current NCCN recommendation. No morbidity related to the fiducial marker implantation was recorded. In the first ten patients, along right–left, anterior–posterior, and inferior–superior directions, MIFE was variable (mean±std=0.24±0.13cm,0.31±0.14cm,0.83±0.35cm, respectively) and was at most 0.51, 0.53, and 1.56 cm, respectively. Along the same directions, μM were 0.09,−0.05,−0.05cm,μSD were 0.30, 0.17, 0.33 cm, and σres were 0.35, 0.26, and 0.30 cm, respectively. MIFE was not correlated with pM and pSD. In the five additional patients, it was possible to satisfy recommended dose limits, with the exception of slightly higher doses to small bowel. After bony alignment, the margins for target expansion can be obtained by adding personalized MIFE to the residual interfraction term. Using these margins, conformal free‐breathing irradiation is a reliable option for the treatment of pancreatic cancer.PACS number: 87.55.D‐
Highlights
61 Solla et al.: Free-breathing pancreas irradiation variations, a planning target volume (PTV) is typically added to cover the clinical target volume
Σbone and σbone were assigned a 2 mm value, as the residual error after bone alignment is related to the positioning of the flat panel which was characterized by an accuracy of 2 mm.[12]. To take into account respiratory motion, when the midrespiratory position is known, a linear addition of half peak-to-peak motion amplitude has been suggested.[10,13] half of the maximal intrafraction fiducial excursion (MIFE) was symmetrically added to the fiducial setup margin to allow a direct comparison with the results reported by other studies
Our findings confirmed that in the treatment of pancreatic cancer, the largest motion component was in the IS direction and that personalized anisotropic margin should be used to account for intrafraction respiratory motion in nongated treatment
Summary
61 Solla et al.: Free-breathing pancreas irradiation variations, a planning target volume (PTV) is typically added to cover the clinical target volume. The causes of interfractional changes are daily variations in organ fillings (stomach and bowel), tumor size, patient weight, and treatment-induced tissue changes, whereas intrafractional motion variations may be caused by peristalsis, cardiac motion, and respiration. Intrafractional motion due to respiration can be determined by four-dimensional computed tomography (4D CT) that permits reconstruction of three-dimensional CT scans at various phases of the respiratory cycle. There is substantial residual uncertainty using respiratory gating and patient positioning based purely on bony anatomy. In Jayachandran et al,(6) respiratory-gated portal images showed that bony anatomy matched tumor position in only 20% of the radiation treatments. Interfraction residual errors can be due to bowel gas or daily changes in the position of surrounding normal tissue, daily baseline diaphragm position, or day-to-day variation in the breathing pattern
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