Dosimetric effects of anatomical changes during fractionated photon radiation therapy in pancreatic cancer patients

Abstract

Pancreatic tumors show large interfractional position variation. In addition, changes in gastrointestinal gas volumes and body contour take place over the course of radiation therapy. We aimed to quantify the effect of these anatomical changes on target dose coverage, for the clinically used fiducial marker‐based patient position verification and, for comparison, also for simulated bony anatomy‐based position verification. Nine consecutive patients were included in this retrospective study. To enable fraction dose calculations on cone‐beam CT (CBCT), the planning CT was deformably registered to each CBCT (13–15 per patient); gas volumes visible on CBCT were copied to the deformed CT. Fraction doses were calculated for the clinically used 10 MV VMAT treatment plan (with for the planning target volume (PTV): D98% = 95%), according to fiducial marker‐based and bony anatomy‐based image registrations. Dose distributions were rigidly summed to yield the accumulated dose. To evaluate target dose coverage, we defined an iCTV +5 mm volume, i.e., the internal clinical target volume (iCTV) expanded with a 5 mm margin to account for remaining uncertainties including delineation uncertainties. We analyzed D98%, Dmean, and D2% for iCTV +5 mm and PTV (i.e., iCTV plus 10 mm margin). We found that for fiducial marker‐based registration, differences between fraction doses and planned dose were minimal. For bony anatomy‐based registration, fraction doses differed considerably, resulting in large differences between planned and accumulated dose for some patients, up to a decrease in D98% of the iCTV +5 mm from 95.9% to 85.8%. Our study shows that fractionated photon irradiation of pancreatic tumors is robust against variations in body contour and gastrointestinal gas, with dose coverage only mildly affected. However, as a result of interfractional tumor position variations, target dose coverage can severely decline when using bony anatomy for patient position verification. Therefore, the use of intratumoral fiducial marker‐based daily position verification is essential in pancreatic cancer patients.

Impact of interfractional anatomical variation and setup correction methods on interfractional dose variation in IMPT and VMAT plans for pancreatic cancer patients: A planning study

“…With respect to the setup correction method, it has been suggested that marker-matching and OM are far superior to skin-markingmatching methods and superior even to BM in photon therapy. 10 Likewise, in carbon ion therapy (CIRT), OM and marker-matching are reportedly superior to BM for the lung and liver. 11,12 Meanwhile, in CIRT for pancreatic cancer, a small effect of the setup correction method on dose distribution was demonstrated 13 ; this is because the dose distribution in particle therapy, compared with photon therapy, is easily distorted by interfractional anatomical variations such as changes in physique and gastrointestinal gas in abdominal therapy.…”

Effect of Changing Phantom Thickness on Helical Radiotherapy Plan: Dosimetric Analysis

“…Weight changes during RT are often seen and also cause the body contour to change. There are very few studies on the dosimetric effect that may occur due to the change of the body contour [29,30]. In this study, it is observed that in the case of a decrease in bolus thickness, HD_ROI and LW_ROI's of the phantom was receiving a higher dose.…”

Density override in treatment planning to mitigate the dosimetric effect induced by gastrointestinal gas in esophageal cancer radiation therapy

“…For pancreatic cancer, the variable gas volume can highly affect the target coverage in proton or carbon-ion therapy and the beam angle should be optimized [10,11,13]. For (photon) RT, the use of intratumoral markers as the surrogate of the pancreatic tumor in setup verification was found robust against the anatomical changes and gastrointestinal gas [12]. Although bony anatomy-based setup verification was used in the present study, no interfractional tumor position variation was simulated.…”

“…There were only a few studies on the dosimetric effect induced by gas changes and its mitigation approach in RT of upper and middle gastrointestinal cancers [10][11][12][13]. For esophageal cancer, the focus was on dosimetric impact of gastric filling [14,15].…”