Evaluation of OAR dose sparing and plan robustness of beam-specific PTV in lung cancer IMRT treatment

Optimizing 3DCT image registration for interfractional changes in carbon-ion prostate radiotherapy

To perform setup procedures including both positional and dosimetric information, we developed a CT-CT rigid image registration algorithm utilizing water equivalent pathlength (WEPL)-based image registration and compared the resulting dose distribution with those of two other algorithms, intensity-based image registration and target-based image registration, in prostate cancer radiotherapy using the carbon-ion pencil beam scanning technique. We used the data of the carbon ion therapy planning CT and the four-weekly treatment CTs of 19 prostate cancer cases. Three CT-CT registration algorithms were used to register the treatment CTs to the planning CT. Intensity-based image registration uses CT voxel intensity information. Target-based image registration uses target position on the treatment CTs to register it to that on the planning CT. WEPL-based image registration registers the treatment CTs to the planning CT using WEPL values. Initial dose distributions were calculated using the planning CT with the lateral beam angles. The treatment plan parameters were optimized to administer the prescribed dose to the PTV on the planning CT. Weekly dose distributions using the three different algorithms were calculated by applying the treatment plan parameters to the weekly CT data. Dosimetry, including the dose received by 95% of the clinical target volume (CTV-D95), rectal volumes receiving > 20 Gy (RBE) (V20), > 30 Gy (RBE) (V30), and > 40 Gy (RBE) (V40), were calculated. Statistical significance was assessed using the Wilcoxon signed-rank test. Interfractional CTV displacement over all patients was 6.0 ± 2.7 mm (19.3 mm maximum standard amount). WEPL differences between the planning CT and the treatment CT were 1.2 ± 0.6 mm-H₂O (< 3.9 mm-H₂O), 1.7 ± 0.9 mm-H₂O (< 5.7 mm-H₂O) and 1.5 ± 0.7 mm-H₂O (< 3.6 mm-H₂O maxima) with the intensity-based image registration, target-based image registration, and WEPL-based image registration, respectively. For CTV coverage, the D95 values on the planning CT were > 95% of the prescribed dose in all cases. The mean CTV-D95 values were 95.8 ± 11.5% and 98.8 ± 1.7% with the intensity-based image registration and target-based image registration, respectively. The WEPL-based image registration was CTV-D95 to 99.0 ± 0.4% and rectal Dmax to 51.9 ± 1.9 Gy (RBE) compared to 49.4 ± 9.1 Gy (RBE) with intensity-based image registration and 52.2 ± 1.8 Gy (RBE) with target-based image registration. The WEPL-based image registration algorithm improved the target coverage from the other algorithms and reduced rectal dose from the target-based image registration, even though the magnitude of the interfractional variation was increased.

Comparison of dosimetries of carbon-ion pencil beam scanning, proton pencil beam scanning and volumetric modulated arc therapy for locally recurrent rectal cancer

We compared the dose distributions of carbon-ion pencil beam scanning (C-PBS), proton pencil beam scanning (P-PBS) and Volumetric Modulated Arc Therapy (VMAT) for locally recurrent rectal cancer. The C-PBS treatment planning computed tomography (CT) data sets of 10 locally recurrent rectal cancer cases were randomly selected. Three treatment plans were created using identical prescribed doses. The beam angles for C-PBS and P-PBS were identical. Dosimetry, including the dose received by 95% of the planning target volume (PTV) (D95%), dose to the 2 cc receiving the maximum dose (D2cc), organ at risk (OAR) volume receiving > 15Gy (V15) and > 30Gy (V30), was evaluated. Statistical significance was assessed using the Wilcoxon signed-rank test. Mean PTV-D95% values were > 95% of the volume for P-PBS and C-PBS, whereas that for VMAT was 94.3%. However, PTV-D95% values in P-PBS and VMAT were < 95% in five and two cases, respectively, due to the OAR dose reduction. V30 and V15 to the rectum/intestine for C-PBS (V30 = 4.2 ± 3.2 cc, V15 = 13.8 ± 10.6 cc) and P-PBS (V30 = 7.3 ± 5.6 cc, V15 = 21.3 ± 13.5 cc) were significantly lower than those for VMAT (V30 = 17.1 ± 10.6 cc, V15 = 55.2 ± 28.6 cc). Bladder-V30 values with P-PBS/C-PBS (3.9 ± 4.8 Gy(RBE)/3.0 ± 4.0 Gy(RBE)) were significantly lower than those with VMAT (7.9 ± 8.1 Gy). C-PBS provided superior dose conformation and lower OAR doses compared with P-PBS and VMAT. C-PBS may be the best choice for cases in which VMAT and P-PBS cannot satisfy dose constraints. C-PBS could be another choice for cases in which VMAT and P-PBS cannot satisfy dose constraints, thereby avoiding surgical resection.

The status of medical physics in radiotherapy in China

PURPOSE

To present an overview of the status of medical physics in radiotherapy in China, including facilities and devices, occupation, education, research, etc. MATERIALS AND METHODS: The information about medical physics in clinics was obtained from the 9-th nationwide survey conducted by the China Society for Radiation Oncology in 2019. The data of medical physics in education and research was collected from the publications of the official and professional organizations.

RESULTS

By 2019, there were 1463 hospitals or institutes registered to practice radiotherapy and the number of accelerators per million population was 1.5. There were 4172 medical physicists working in clinics of radiation oncology. The ratio between the numbers of radiation oncologists and medical physicists is 3.51. Approximately, 95% of medical physicists have an undergraduate or graduate degrees in nuclear physics and biomedical engineering. 86% of medical physicists have certificates issued by the Chinese Society of Medical Physics. There has been a fast growth of publications by authors from mainland of China in the top international medical physics and radiotherapy journals since 2018.

CONCLUSIONS

Demand for medical physicists in radiotherapy increased quickly in the past decade. The distribution of radiotherapy facilities in China became more balanced. High quality continuing education and training programs for medical physicists are deficient in most areas. The role of medical physicists in the clinic has not been clearly defined and their contributions have not been fully recognized by the community.