Scanned proton radiotherapy for mobile targets-the effectiveness of re-scanning in the context of different treatment planning approaches and for different motion characteristics

The most advanced delivery technique for proton radiotherapy is active spot scanning. So far, predominantly static targets have been treated with active spot scanning, since mobile targets in combination with dynamic treatment delivery can lead to interplay effects, causing inhomogeneous dose distributions. One way to mitigate motion effects is re-scanning. In this study we investigate the effectiveness of re-scanning in relation to different plan parameters (number of fields, field directions, number of re-scans) as well as in respect to different motion parameters (motion amplitude, motion starting phase). A systematic study was performed for three liver patients, for which ten different plans have been calculated, respectively. The treatment plans were evaluated for three different scenarios (static, motion/single-scan-delivery, motion/re-scanned-delivery). The choice of motion parameters was based on an evaluation of the 4D CT data sets of the three patients. It is shown that the effect of motion/re-scanning per fraction is largest the fewer fields per plan are used and the more the field direction differs from the main motion direction. For amplitudes up to 6 mm, re-scanning may not be required if multiple fields are used, since only dose blurring effects appear that cannot be compensated by re-scanning. For larger motion amplitudes two planning strategies are proposed.

Second cancers in children treated with modern radiotherapy techniques

BACKGROUND AND PURPOSE

The scattered radiation from the treatment volume might be more significant for children than for adults and, as a consequence, modern radiotherapy treatment techniques such as IMRT and passive proton therapy could potentially increase the number of secondary cancers. In this report, secondary cancer risk resulting from new treatment technologies was estimated for an adult prostate patient and a child.

MATERIAL AND METHODS

The organ equivalent dose (OED) concept with a linear-exponential, a plateau and a linear dose-response curve was applied to dose distributions of an adult prostate patient and a child with a rhabdomyosarcoma of the prostate. Conformal radiotherapy, IMRT with 6MV photons and proton therapy were planned. OED (cancer risk) was estimated for the whole body, the rectum and the bladder. In addition, relative cumulative risk was calculated.

RESULTS

Secondary cancer risk in the adult is not more than 15% it increased when IMRT or passive proton therapy was compared to conventional treatment planning. In the child, risk remains practically constant or was even reduced for proton therapy. The cumulative risk in the child relative to that in the adult can be as large as 10-15.

CONCLUSIONS

By a comparison between an adult patient and a child treated for a disease of the prostate, it was shown that modern radiotherapy techniques such as IMRT and proton therapy (active and passive) do not increase the risk for secondary cancers.

The Impact of Dose Escalation on Secondary Cancer Risk After Radiotherapy of Prostate Cancer

PURPOSE

To estimate secondary cancer risk due to dose escalation in patients treated for prostatic carcinoma with three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated RT (IMRT), and spot-scanned proton RT.

METHODS AND MATERIALS

The organ equivalent dose (OED) concept with a linear-exponential, a plateau, and a linear dose-response curve was applied to dose distributions of 23 patients who received RT of prostate cancer. Conformal RT was used in 7 patients, 8 patients received IMRT with 6- and 15-MV photons, and 8 patients were treated with spot-scanned protons. We applied target doses ranging from 70 Gy to 100 Gy. Cancer risk was estimated as a function of target dose and tumor control probability.

RESULTS

At a 100-Gy target dose the secondary cancer risk relative to the 3D treatment plan at 70 Gy was +18.4% (15.0% for a plateau model, 22.3% for a linear model) for the 6-MV IMRT plan, +25.3% (17.0%, 14.1%) for the 15-MV IMRT plan, and -40.7% (-41.3%, -40.0%) for the spot-scanned protons. The increasing risk of developing a radiation-associated malignancy after RT with increasing dose was balanced by the enhanced cure rates at a larger dose.

CONCLUSIONS

Cancer risk after dose escalation for prostate RT is expected to be equal to or lower than for conventional 3D treatment at 70 Gy, independent of treatment modality or dose-response model. Spot-scanned protons are the treatment of choice for dose escalation because this therapy can halve the risk of secondary cancers.