Biomedical subjects

Hanne M Kooy

Publications and source records attributed to Hanne M Kooy.

8 recordsLinked to original sources

PubMed2006

Incorporation of the aperture thickness in proton pencil-beam dose calculations.

Field-specific apertures, of sufficient range-absorbing thickness, are used in the majority of proton-therapy treatments today. In current practice, these apertures are modelled as objects of infinitesimal thickness. Such an approximation, however, is not accurate if the aperture edge is close to, or extends over, the beam axis. Practical situations in which this occurs include off-axis patch fields, small apertures, and fields shaped with a multileaf collimator. We develop an extension of the pencil-beam dose model to incorporate the aperture thickness. We derive an exact solution as well as a computationally simpler approximate implementation. The model is validated using measurements of the lateral penumbra. For a set-up with a source size of 2.76 cm, a source-to-axis distance of 227 cm, and a aperture-to-axis distance of 35 cm, the maximum increase in penumbra for a 6 cm thick aperture compared to the thin-aperture model is about 2 mm. The maximum shift in the 95% isodose contour line is larger. The overall effect depends on the aperture thickness, the position of the aperture edge and the intrinsic source size and SAD, but is fairly insensitive to aperture-to-skin distance and depth in patient.

Algorithms↗

PubMed2006

Four-dimensional proton treatment planning for lung tumors.

PURPOSE: In proton radiotherapy, respiration-induced variations in density lead to changes in radiologic path lengths and will possibly result in geometric misses. We compared different treatment planning strategies for lung tumors that compensate for respiratory motion. METHODS AND MATERIALS: Particle-specific treatment planning margins were applied to standard helical computed tomography (CT) scans as well as to "representative" CT scans. Margins were incorporated beam specific laterally by aperture widening and longitudinally by compensator smearing. Furthermore, treatment plans using full time-resolved 4D-computed tomography data were generated. RESULTS: Four-dimensional treatment planning guaranteed target coverage throughout a respiratory cycle. Use of a standard helical CT data set resulted in underdosing the target volume to 36% of the prescribed dose. For CT data representing average target positions, coverage can be expected but not guaranteed. In comparison to this strategy, 4D planning decreased the mean lung dose by up to 16% and the lung volume receiving 20 Gy (prescribed target dose 72 Gy) by up to 15%. CONCLUSION: When the three planning strategies are compared, only 4D proton treatment planning guarantees delivery of the prescribed dose throughout a respiratory cycle. Furthermore, the 4D planning approach results in equal or reduced dose to critical structures; even the ipsilateral lung is spared.

Algorithms↗

PubMed2005

The prediction of output factors for spread-out proton Bragg peak fields in clinical practice.

The reliable prediction of output factors for spread-out proton Bragg peak (SOBP) fields in clinical practice remained unrealized due to a lack of a consistent theoretical framework and the great number of variables introduced by the mechanical devices necessary for the production of such fields. These limitations necessitated an almost exclusive reliance on manual calibration for individual fields and empirical, ad hoc, models. We recently reported on a theoretical framework for the prediction of output factors for such fields. In this work, we describe the implementation of this framework in our clinical practice. In our practice, we use a treatment delivery nozzle that uses a limited, and constant, set of mechanical devices to produce SOBP fields over the full extent of clinical penetration depths, or ranges, and modulation widths. This use of a limited set of mechanical devices allows us to unfold the physical effects that affect the output factor. We describe these effects and their incorporation into the theoretical framework. We describe the calibration and protocol for SOBP fields, the effects of apertures and range-compensators and the use of output factors in the treatment planning process.

Humans↗

PubMed2005

Intra- and interfractional patient motion for a variety of immobilization devices.

The magnitude of inter- and intrafractional patient motion has been assessed for a broad set of immobilization devices. Data was analyzed for the three ordinal directions--left-right (x), sup-inf (y), and ant-post (z)--and the combined spatial displacement. We have defined "rigid" and "non-rigid" immobilization devices depending on whether they could be rigidly and reproducibly connected to the treatment couch or not. The mean spatial displacement for intrafractional motion for rigid devices is 1.3 mm compared to 1.9 mm for nonrigid devices. The modified Gill-Thomas-Cosman frame performed best at controlling intrafractional patient motion, with a 95% probability of observing a three-dimensional (3D) vector length of motion (v95) of less than 1.8 mm, but could not be evaluated for interfractional motion. All other rigid and nonrigid immobilization devices had a v95 of more than 3 mm for intrafractional patient motion. Interfractional patient motion was only evaluated for the rigid devices. The mean total interfractional displacement was at least 3.0 mm for these devices while v95 was at least 6.0 mm.

Algorithms↗

PubMed2005

Target volume dose considerations in proton beam treatment planning for lung tumors.

We performed a treatment planning study in order to gather basic insight in the effect of setup errors and breathing motion on the cumulative proton dose to a lung tumor. We used a simplified geometry that simulates a 50 mm diameter gross tumor volume (GTV) located centrally inside lung tissue. The GTV was expanded with a uniform 5 mm margin into a clinical target volume (CTV) and into a variety of planning target volume (PTV's). Proton beam apertures were designed to conform the prescribed dose laterally to the PTV while the range compensator was designed to provide distal coverage of the CTV. Different smearing distances were applied to the range compensators, and the cumulative dose in the CTV was evaluated for different combinations of breathing motion and systematic setup errors. Evaluation parameters were the dose to 99% of the CTV (D99) and the equivalent uniform dose (EUD), with a surviving fraction at 2 Gy of SF2 = 0.5. For a single proton field designed to a 15 mm expansion of the CTV and without smearing applied to the range compensator, D99 of the CTV reduced from 96% for no tumor displacement to 41% and 13% for systematic setup errors of 5 and 10 mm, respectively. For a representative clinical combination, of 5 mm systematic error and 10 mm breathing amplitude, the EUD of the CTV was about 40 Gy (prescribed dose 70 Gy) regardless the CTV to PTV margin, and without smearing. Smearing the range compensator increases the dose to the CTV substantially with a lateral margin and smearing distance of 7.5 mm providing ample tumor coverage. In this latter case, D99 of the target volume increased to 87% for a single field treatment plan. Smearing does, however, lead to an increase in dose to normal tissues distal to the clinical target volume. Next to countering geometric mismatches due to patient setup, smearing can also be used to counter the detrimental effects of breathing motion on the dose to the clinical target volume. We show that the lateral margin and smearing distance can be substantially smaller than the maximum tumor displacement due to setup errors and patient breathing, as measured by the D99 and the EUD.

Biophysical Phenomena↗

PubMed2005

Stereotactic radiotherapy for vestibular schwannomas: favorable outcome with minimal toxicity.

OBJECTIVE: To determine the outcome and toxicity in patients with vestibular schwannomas treated with conventionally fractionated stereotactic radiotherapy (SRT) and to identify prognostic factors that are predictive of outcome. METHODS: Between 1992 and 2001, 70 patients with vestibular schwannomas were treated with linear accelerator-based SRT in our institutions. Eleven patients had neurofibromatosis Type II (NF2). The median age was 53 years (range, 17-82 yrs). The median tumor volume was 2.4 cm3 (range, 0.05-21.1 cm3). The indications for SRT were distributed as follows: 47% newly diagnosed, 31% progressive tumors after watchful waiting, 3% adjuvant postoperative radiation, and 19% recurrent tumors after surgical resection. The median dose was 54 Gy in 1.8 Gy per fraction, prescribed to 95% of the isodose line. Relocatable stereotactic frames were used for daily treatments. The median follow-up was 45.3 months. RESULTS: Tumor recurrence was defined as progressive enlargement of tumor on follow-up magnetic resonance imaging studies. One patient had a tumor recurrence at 38 months after SRT. The actuarial tumor control rates were 100 and 98% at 3 and 5 years, respectively. Three patients with a median tumor volume of 16.2 cm3 required surgical resection for persistent or increasing symptoms at a median of 37 months. The actuarial freedom from resection rates were 98 and 92% at 3 and 5 years, respectively. In multivariate analysis, tumor volume at time of treatment was predictive for neurosurgical intervention (surgical resection or shunt placement) after SRT (P = 0.001). The 3- and 5-year actuarial rates of freedom from any neurosurgical intervention were 100 and 97% for patients with tumor volume less than 8 cm3 and 74 and 47% respectively for patients with tumor of at least 8 cm3 (P < 0.0001). The 3-year actuarial rates of facial and trigeminal nerve preservation were 99 and 96%, respectively. Surgery before SRT was predictive of posttreatment trigeminal neuropathy. The 3-year actuarial rates of freedom from trigeminal neuropathy were 86 and 98% for patients with and without previous resection, respectively (P = 0.04). There was no difference in tumor control and cranial nerve function preservation rates seen in NF2 patients compared with non-NF2 patients. No second primary cancer or malignant transformation was observed. CONCLUSION: SRT in the conventionally fractionated approach results in a very favorable outcome with minimal toxicity, with results comparable to those of the best of the radiosurgery series. Patients with large tumors are more likely to undergo neurosurgical interventions after SRT. Patients who have undergone previous surgery are at increased risk of developing trigeminal neuropathy.

Adolescent↗

PubMed2003

Monitor unit calculations for range-modulated spread-out Bragg peak fields.

We derive, from first principles, a model to predict the output factors for spread-out Bragg peak proton fields (SOBP). The model is based on the simple observation that the output factor is the ratio of SOBP plateau dose to the dose measured in the ionization reference chamber. The latter, in turn, equates to the entrance dose of the SOBP corrected for inverse square. We use a theoretical derivation of this ratio to establish the relationship between the output factor and the distal range and modulation width of the SOBP. In addition, the theoretical derivation reduces the dependence on the distal range and modulation width into a single factor r = (R - M)/M. We compare the theoretical derivation against measurements obtained at the Northeast Proton Therapy Facility for output factors for clinical fields. The agreement between measurements and prediction is 2.9%.

Calibration↗

PubMed2002

Time trends in organ position and volume in patients receiving prostate three-dimensional conformal radiotherapy.

Using multiple computed tomography (CT) scans, 50 patients undergoing prostate radiotherapy were tested for clinically significant time trends in the target and surrounding critical structures. Significant trends were observed toward increasing bladder volume and increasing bowel-to-planning target volume separation; however, no trends were observed in the prostate, seminal vesicles, or rectum. The subset of patients undergoing hormone therapy was also tested and did not independently exhibit any significant time trends.

Androgen Antagonists↗