Technical Note: First report on an in vivo range probing quality control procedure for scanned proton beam therapy in head and neck cancer patients

Abstract

Purpose The capability of proton therapy to provide highly conformal dose distributions is impaired by range uncertainties. The aim of this work is to apply range probing (RP), a form of a proton radiography‐based quality control (QC) procedure for range accuracy assessment in head and neck cancer (HNC) patients in a clinical setting. Methods and Materials This study included seven HNC patients. RP acquisition was performed using a multi‐layer ionization chamber (MLIC). Per patient, two RP frames were acquired within the first two weeks of treatment, on days when a repeated CT scan was obtained. Per RP frame, integral depth dose (IDD) curves of 81 spots around the treatment isocenter were acquired. Range errors are determined as a discrepancy between calculated IDDs in the treatment planning system and measured residual ranges by the MLIC. Range errors are presented relative to the water equivalent path length of individual proton spots. In addition to reporting results for complete measurement frames, an analysis, excluding range error contributions due to anatomical changes, is presented. Results Discrepancies between measured and calculated ranges are smaller when performing RP calculations on the day‐specific patient anatomy rather than the planning CT. The patient‐specific range evaluation shows an agreement between calculated and measured ranges for spots in anatomically consistent areas within 3% (1.5 standard deviation). Conclusions The results of an RP‐based QC procedure implemented in the clinical practice for HNC patients have been demonstrated. The agreement of measured and simulated proton ranges confirms the 3% uncertainty margin for robust optimization. Anatomical variations show a predominant effect on range accuracy, motivating efforts towards the implementation of adaptive radiotherapy.

Range probing as a quality control tool for CBCT‐based synthetic CTs: In vivo application for head and neck cancer patients

“…Retrospective RP QC measurements from seven HNC patients were retrieved, together with CBCT and repeat CT (rCT) images, acquired on the same days as the RP 26 . RP QC measurements were performed in our clinic as an in vivo QC check for HNC patients treated with proton therapy 26 .…”

“…Retrospective RP QC measurements from seven HNC patients were retrieved, together with CBCT and repeat CT (rCT) images, acquired on the same days as the RP 26 . RP QC measurements were performed in our clinic as an in vivo QC check for HNC patients treated with proton therapy 26 . RP fields in two different fractions (referred as sessions 1 and 2) were collected for each patient (numbered from 1 to 7), resulting in a dataset of 14 RP fields with their corresponding rCT and CBCT.…”

“…Range probing (RP) has been suggested as a QC tool for in vivo proton range verification [20][21][22][23][24][25][26] .…”

“…The appearance of this effect, however, demonstrates the importance of QC tools for sCTs and the capability of RP measurements to detect small range errors. With the proposed RP QC procedure, residual range errors can be obtained with an accuracy of 0.5 mm 20,26,29,31,32 . For the current dataset, Meijers et al estimated an overall uncertainty for the RRE of 1%, taking into account energy fluctuations, interfractional motion, residual setup errors, rigid registration of the planning CT and rCT, and anatomical inconsistencies between acquisitions in the treatment room and in the CT imaging room 26 .…”

“…Meijers et al . acquired RP measurements in HNC patients and evaluated the discrepancies between measured and simulated depth dose profiles, demonstrating the feasibility to employ RP‐based QC in clinical practice 26 …”