Peripheral doses from noncoplanar IMRT for pediatric radiation therapy

OUT-OF-FIELD DOSE MEASUREMENTS FOR 3D CONFORMAL AND INTENSITY MODULATED RADIOTHERAPY OF A PAEDIATRIC BRAIN TUMOUR

The purpose of this study was to measure out-of-field organ doses in clinical conditions in anthropomorphic paediatric phantoms which received a simulated treatment of a brain tumour with intensity modulated radiotherapy (IMRT) and 3D conformal radiotherapy (3D CRT). Organ doses measured with radiophotoluminescent and thermoluminescent dosemeters were on average 1.6 and 3.0 times higher for the 5 y-old than for the 10 y-old phantom for IMRT and 3D CRT, respectively. A larger 5-y to 10-y organ dose ratio for 3D CRT can be explained because the use of a mechanical wedge for the 5-y-old 3D CRT phantom treatment increased out-of-field doses. Due to different configurations of the radiation fields, for both phantoms, the IMRT technique resulted in a higher non-target brain dose and higher eye doses but lower thyroid doses compared to 3D CRT. For 3D CRT (which used a non-coplanar field configuration), eye doses were 3-6% and for IMRT (which used a coplanar field configuration) 27-30% of the treatment dose, respectively. For thyroid and more distant organs, doses were less than 1% of the treatment dose. Comparison of measured doses and doses calculated by the treatment planning system (TPS) showed that the TPS underestimated out-of-field doses both for IMRT and 3D CRT.

Coplanar VMAT vs. noncoplanar VMAT in the treatment of sinonasal cancer

BACKGROUND

Previous studies showed that noncoplanar intensity-modulated radiotherapy (NC-IMRT) for sinonasal cancer is superior to coplanar intensity-modulated radiotherapy (IMRT). Volumetric-modulated arc therapy (VMAT) is a newly introduced treatment modality, and the performance of noncoplanar VMAT for sinonasal cancer has not been well described to date.

PURPOSE

To compare the dosimetry difference of noncoplanar VMAT (NC-VMAT), coplanar VMAT (co-VMAT), and NC-IMRT for sinonasal cancer.

PATIENTS AND METHODS

Ten postoperative patients with sinonasal cancer were randomly selected for planning with NC-VMAT, co-VMAT, and NC-IMRT. Two planning target volumes (PTVs) were contoured representing high-risk and low-risk regions set to receive a median absorbed dose (D50%) of 68 Gy and 59 Gy, respectively. The homogeneity index (HI), conformity index (CI), dose-volume histograms (DVHs), and delivery efficiency were all evaluated.

RESULTS

Both NC-VMAT and co-VMAT showed superior dose homogeneity and conformity in PTVs compared with NC-IMRT. There was no significant difference between NC-VMAT and co-VMAT in PTV coverage. Both VMAT plans provided a better protection for organs at risk (OARs) than NC-IMRT plans, and NC-VMAT showed a small improvement over co-VMAT in sparing of OARs. For peripheral doses, the doses to breast, thyroid, and larynx in the NC-IMRT plans were significantly higher than those in both VMAT plans. Compared to NC-VMAT, co-VMAT significantly reduced peripheral doses. NC-VMAT and co-VMAT reduced the average delivery time by 63.2 and 64.2%, respectively, in comparison with NC-IMRT. No differences in delivery efficiency were observed between the two VMAT plans.

CONCLUSION

Compared to NC-VMAT, co-VMAT showed similar PTV coverage and comparable OAR sparing but significantly reduced peripheral doses and positioning uncertainty. We propose to give priority to coplanar VMAT in the treatment of sinonasal cancer.

Peripheral dose measurements with diode and thermoluminescence dosimeters for intensity modulated radiotherapy delivered with conventional and un-conventional linear accelerator

The objective of this paper was to measure the peripheral dose (PD) with diode and thermoluminescence dosimeter (TLD) for intensity modulated radiotherapy (IMRT) with linear accelerator (conventional LINAC), and tomotherapy (novel LINAC). Ten patients each were selected from Trilogy dual-energy and from Hi-Art II tomotherapy. Two diodes were kept at 20 and 25 cm from treatment field edge. TLDs (LiF:MgTi) were also kept at same distance. TLDs were also kept at 5, 10, and 15 cm from field edge. The TLDs were read with REXON reader. The readings at the respective distance were recorded for both diode and TLD. The PD was estimated by taking the ratio of measured dose at the particular distance to the prescription dose. PD was then compared with diode and TLD for LINAC and tomotherapy. Mean PD for LINAC with TLD and diode was 2.52 cGy (SD 0.69), 2.07 cGy (SD 0.88) at 20 cm, respectively, while at 25 cm, it was 1.94 cGy (SD 0.58) and 1.5 cGy (SD 0.75), respectively. Mean PD for tomotherapy with TLD and diode was 1.681 cGy SD 0.53) and 1.58 (SD 0.44) at 20 cm, respectively. The PD was 1.24 cGy (SD 0.42) and 1.088 cGy (SD 0.35) at 25 cm, respectively, for tomotherapy. Overall, PD from tomotherapy was found lower than LINAC by the factor of 1.2-1.5. PD measurement is essential to find out the potential of secondary cancer. PD for both (conventional LINAC) and novel LINACs (tomotherapy) were measured and compared with each other. The comparison of the values for PD presented in this work and those published in the literature is difficult because of the different experimental conditions. The diode and TLD readings were reproducible and both the detector readings were comparable.

Noncoplanar intensity-modulated radiation therapy for young female patients with mediastinal lymphoma

The purpose of this study is to apply noncoplanar intensity-modulated radiation therapy (Nonco_IMRT) to young female patients with mediastinal lymphoma. Nonco_IMRT was evaluated through a planning comparison study with coplanar IMRT (Co_IMRT) and conventional anteroposterior and posteroanterior fields (AP-PA) plans. Co_IMRT was performed with five equally spaced beams starting from a gantry angle of 216°. Nonco_IMRT used two noncoplanar beams in the sagittal plane to replace the Co_IMRT beams that directly irradiated the breasts. Nineteen young female patients were enrolled in the retrospective study. Dose coverage of the planning target volume (PTV) and the dose delivered to organs at risk (OARs) were analyzed. For all patients, PTV coverage and heart V30 were similar between the two IMRT techniques (p > 0.05). Compared to Co_IMRT, the mean dose delivered and regions receiving a low radiation dose were significantly reduced for bilateral breasts and lungs in Nonco_IMRT (p < 0.05). Breast V5 and lung V5 were relatively reduced by 21% and 12%, respectively. Compared with the conventional AP-PA plan, Nonco_IMRT had better PTV coverage and OARs sparing, except for being larger in V5 to breast and lung. In IMRT for young female patients with mediastinal lymphoma, using of Nonco_IMRT significantly reduces the radiation dose to the breasts and lungs compared with Co_IMRT, and consequently reduces the risk of breast second cancer and pulmonary toxicity. Besides young female patients, Nonco_IMRT can also benefit other mediastinal lymphoma patients.