PO-0962: CBCT dose calculation in head and neck adaptive radiotherapy: accuracy assessment of four methods
Year: 2018
Type: conference-abstract
Abstract: Purpose or ObjectiveDelivered radiation dose (DA) to critical OARs in patients undergoing curative radiotherapy for squamous cell carcinoma of the head & neck (HNSCC) can differ from planned dose (DP), potentially increasing long-term morbidity risk.This study aims to quantify these differences, and examines their relationship to anatomical changes during treatment.Material and Methods 87 HNSCC patients were selected (72 oropharynx, 8 larynx, 5 oral cavity, 2 hypopharynx).All were treated with IMRT and daily MVCT-IG on TomoTherapy HiArt units, with a 3-dose level technique (65Gy/30#/6wks).All patients were weighed before fraction 1 and in the final treatment week.Patient separation (lateral neck dimension -LND -the transverse skin-to-skin distance at the level of the C1 vertebra) was measured on day 1 & 30 MVCTs, and differences noted (ΔLND).An automated function within Prosoma segmentation software defined the skin surface at the same slice on the same images (days 1 & 30); the slice surface area of this contour (SSA) was measured, and the change recorded (ΔSSA).Doses to salivary glands (parotid -PG, submandibular -SMG, ipsilateral & contralateral to primary tumour), and superior pharyngeal constrictor muscles (SPCs) were assessed.All structures were re-contoured on kVCT planning scans by one observer.An open-source deformable image registration algorithm (Elastix) was used to propagate kVCT contours to daily MVCTs.Daily IG scans, TomoTherapy sinogram data and an in-house raytracing dose calculation algorithm (CheckTomo) were used to compute daily dose cubes.Individual voxel histories were created and DA DVHs produced.DA data for individual structures was only reported where the full volume was included on every IG scan.To optimise comparison, DP was recomputed on the kVCT with CheckTomo.Mean dose was used for comparison.Differences were assessed with paired t-tests.Relationships between anatomical metrics and dose differences to each organ were assessed with univariate linear regression. ResultsMean weight loss was 6.7kg (7.9%).Mean reductions in LND and SSA were 12.8mm (8.3%) and 11.7cm2 (5.1%).Delivered mean dose was higher than planned mean dose for all 5 structures (Table 1, Figure 1).The biggest observed dose difference was for the contralateral PG (7.6% increase); the greatest individual difference was 6.3Gy (DP 23.3, DA 29.6).No relationship between anatomical changes and dose differences was seen for the contralateral PG, either SMG, or the SPCs.A borderline significant relationship between greater reduction in LND and higher DA was observed for ipsilateral PG.
Source: Radiotherapy and Oncology
Language: English
Subfield: Radiation
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $3,770
OpenAlex ID: W2809259917