Impact of margin on tumour and normal tissue dosimetry in prostate cancer patients treated with IMRT using an endorectal balloon for prostate immobilization

Impact of rectal balloon-filling materials on the dosimetry of prostate and organs at risk in photon beam therapy

The use of rectal balloon in radiotherapy of prostate cancer is shown to be effective in reducing prostate motion and minimizing rectal volume, thus reducing rectal toxicity. Air‐filled rectal balloon has been used most commonly, but creates dose perturbation at the air‐tissue interface. In this study, we evaluate the effects of rectal balloon‐filling materials on the dose distribution to the target and organs at risk. The dosimetric impact of rectal balloon filling was studied in detail for a typical prostate patient, and the general effect of the balloon filling was investigated from a study of ten prostate patients covering a wide range of anterior–posterior and left–right separations, as well as rectal and bladder volumes. Hounsfield units (HU) of the rectal balloon filling was changed from −1000 HU to 1000 HU at an interval of 250 HU, and the corresponding changes in the relative electron density (RED) was calculated. For each of the HU of the rectal balloon filling, a seven‐field IMRT plan was generated with 6 MV and 15 MV photon beams, respectively. Dosimetric evaluation was performed with the AAA algorithm for inhomogeneity corrections. A detailed study of the rectal balloon filling shows that the GTV, PTV, rectal, and bladder mean dose decreased with increasing values of RED in the rectal balloon. There is significant underdosage in the target volume at the rectum–prostate interface with an air‐filled balloon as compared to that with a water‐filled balloon for both 6 MV and 15 MV beams. While the dosimetric effect of the rectal balloon filling is reduced when averaged over ten patients, generally an air‐filled balloon results in lower minimum dose and lower mean dose in the overlap region (and possibly the PTV) compared to those produced by water‐filled or contrast‐filled balloons. Dose inhomogeneity in the target volume is increased with an air‐filled rectal balloon. Thus a water‐filled or contrast‐filled rectal balloon is preferred to an air‐filled rectal balloon in EBRT of prostate treatment.

PACS numbers: 87.55.D‐, 87.55.de, 87.55.dk, 87.55.Gh, 87.55.kd

Assessment of intrafractional movement and internal motion in radiotherapy of rectal cancer using megavoltage computed tomography

PURPOSE

The aim of this study was to provide estimates of setup and internal margins of patients treated for rectal carcinoma using helical tomotherapy and to assess possible margin adaptations. Using helical tomotherapy, highly conformal dose distributions can be created, and the integrated megavoltage computed tomography (MVCT) modality allows very precise daily patient positioning. In clinical protocols, however, margins originating from traditional setup procedures are still being applied. This work investigates whether this modality can aid in redefining treatment margins.

METHODS AND MATERIALS

Ten patients who were treated with tomotherapy underwent MVCT scanning before and after 10 treatments. Using automatic registration the necessary setup margin was investigated by means of bony landmarks. Internal margins were assessed by delineating and describing the mesorectal movement.

RESULTS

Based on bony landmarks, movement of patients during treatments was limited to 2.45 mm, 1.99 mm, and 1.09 mm in the lateral, longitudinal, and vertical direction, respectively. Systematic errors were limited to <1 mm. Measured movement of the mesorectal space was -1.6 mm (+/- 4.2 mm) and 0.1 mm (+/- 4.0 mm) for left and right lateral direction. In the antero-posterior direction, mean shifts were -2 mm (+/- 6.8 mm) and -0.4 mm (+/- 3.8 mm). Mean shifts in the cranio-caudal direction were respectively -3.2 mm (+/- 5.6 mm) and -3.2 mm (+/- 6.8 mm).

CONCLUSIONS

The use of the integrated MVCT on the tomotherapy system can minimize the setup margin for rectal cancer, and can also be used to adequately describe the internal margin allowing for direct treatment margin adaptation.

Thirty year celebration of journal publications on radiation oncology medical physics

The Australasian Physical & Engineering Sciences in Medicine Journal (APESM) is an avenue for the profession to report scientific work in medicine; provide a facility for the publication of current work, new research and new techniques developed or reviewed; report on professional news from elsewhere and; publish the Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) policies and protocols. The journal is a vital instrument within the ACPSEM organisation with a worldwide circulation. This review of APESM on medical physics in radiation oncology is meant to be a progress summary of work in that specialty. Even so, it has become a lengthy appraisal due to the many years involved. In considering publications related to medical physics in radiation oncology, this review has shown the progression of the College journal to an international journal. There is an increase in the number of papers contributed from Asia and other countries world wide for this discipline. Growth in the number of contributions should continue to rise. In order to provide some appreciation of where the present medical physics activity arose from, this article commences its discussion in 1959 and progresses towards the present, describing along the way, from radiation oncology papers published in APESM, the use of linear accelerators, brachytherapy, the medical physics workforce, the formation of the ACPSEM, and the more modern developments in radiotherapy such as 3-D treatment planning and IMRT.