Conformal irradiation of concave-shaped PTVs in the treatment of prostate cancer by simple 1D intensity-modulated beams

A novel method for predicting late genitourinary toxicity after prostate radiation therapy and the need for age-based risk-adapted dose constraints

BACKGROUND

There are no well-established normal tissue sparing dose-volume histogram (DVH) criteria that limit the risk of urinary toxicity from prostate radiation therapy (RT). The aim of this study was to determine which criteria predict late toxicity among various DVH parameters when contouring the entire solid bladder and its contents versus the bladder wall. The area under the histogram curve (AUHC) was also analyzed.

METHODS AND MATERIALS

From 1993 to 2000, 503 men with prostate cancer received 3-dimensional conformal RT (median follow-up time, 71 months). The whole bladder and the bladder wall were contoured in all patients. The primary endpoint was grade ≥2 genitourinary (GU) toxicity occurring ≥3 months after completion of RT. Cox regressions of time to grade ≥2 toxicity were estimated separately for the entire bladder and bladder wall. Concordance probability estimates (CPE) assessed model discriminative ability. Before training the models, an external random test group of 100 men was set aside for testing. Separate analyses were performed based on the mean age (≤ 68 vs >68 years).

RESULTS

Age, pretreatment urinary symptoms, mean dose (entire bladder and bladder wall), and AUHC (entire bladder and bladder wall) were significant (P<.05) in multivariable analysis. Overall, bladder wall CPE values were higher than solid bladder values. The AUHC for bladder wall provided the greatest discrimination for late bladder toxicity when compared with alternative DVH points, with CPE values of 0.68 for age ≤68 years and 0.81 for age >68 years.

CONCLUSION

The AUHC method based on bladder wall volumes was superior for predicting late GU toxicity. Age >68 years was associated with late grade ≥2 GU toxicity, which suggests that risk-adapted dose constraints based on age should be explored.

A comparison of forward and inverse planned conformal, multi segment and intensity modulated radiotherapy for the treatment of prostate and pelvic nodes

BACKGROUND AND PURPOSE

Full inverse planned intensity modulated radiotherapy (IMRT) may be indicated to treat concave targets like prostate and pelvic nodes, because concave dose distributions cannot be generated with conformal radiotherapy (CRT). We investigated whether this concave dose distribution can be produced using simplified forward planned multi segment radiotherapy (MSRT).

PATIENTS AND METHODS

CRT, MSRT and IMRT dose distributions were calculated and compared for five patients treated in our current IMRT prostate and pelvic node dose escalation trial. The same beam arrangement was used for CRT, MSRT and IMRT, increasing the number of segments. The MSRT concave dose distribution was realised regarding left and right pelvic nodes as two separate targets. The IMRT dose distribution had been used to treat the patients using a step and shoot delivery.

RESULTS

Contrary to CRT, forward planned MSRT concave dose distributions had improved target coverage at lower or equivalent bowel doses than inverse planned IMRT. The five MSRT beams had a maximum of three segments per beam. Both lateral beams had two segments to deliver the two dose levels to prostate and nodes. The posterior field needed a third segment to avoid using a central block. The two anterior oblique beams needed a third segment to account for the different beam weighting because the nodes were irradiated partially using four and partially using five beams. Inverse planned IMRT used up to 15 segments in any one beam, with an average of 11.4 per beam.

CONCLUSIONS

Concave dose distributions for prostate and pelvic node treatment were generated using forward planned multi segment techniques. The plans met clinical constraints used in our IMRT protocol. MSRT presented a significant advantage over both CRT and IMRT.

A dose-volume-based tool for evaluating and ranking IMRT treatment plans

External beam radiotherapy is commonly used for patients with cancer. While tumor shrinkage and palliation are frequently achieved, local control and cure remain elusive for many cancers. With regard to local control, the fundamental problem is that radiotherapy‐induced normal tissue injury limits the dose that can be delivered to the tumor. While intensity‐modulated radiation therapy (IMRT) allows for the delivery of higher tumor doses and the sparing of proximal critical structures, multiple competing plans can be generated based on dosimetric and/or biological constraints that need to be considered/compared. In this work, an IMRT treatment plan evaluation and ranking tool, based on dosimetric criteria, is presented. The treatment plan with the highest uncomplicated target conformity index (TCI+) is ranked at the top. The TCI+ is a dose‐volume‐based index that considers both a target conformity index (TCI) and a normal tissue‐sparing index (NTSI). TCI+ is designed to assist in the process of judging the merit of a clinical treatment plan. To demonstrate the utility of this tool, several competing lung and prostate IMRT treatment plans are compared. Results show that the plan with the highest TCI+ values accomplished the competing goals of tumor coverage and critical structures sparing best, among rival treatment plans for both treatment sites. The study demonstrates, first, that dose‐volume‐based indices, which summarize complex dose distributions through a single index, can be used to automatically select the optimal plan among competing plans, and second, that this dose‐volume‐based index may be appropriate for ranking IMRT dose distributions.

PACS numbers: 87.53.‐j, 87.53.Tf

Incorporating model parameter uncertainty into inverse treatment planning

Radiobiological treatment planning depends not only on the accuracy of the models describing the dose-response relation of different tumors and normal tissues but also on the accuracy of tissue specific radiobiological parameters in these models. Whereas the general formalism remains the same, different sets of model parameters lead to different solutions and thus critically determine the final plan. Here we describe an inverse planning formalism with inclusion of model parameter uncertainties. This is made possible by using a statistical analysis-based frameset developed by our group. In this formalism, the uncertainties of model parameters, such as the parameter a that describes tissue-specific effect in the equivalent uniform dose (EUD) model, are expressed by probability density function and are included in the dose optimization process. We found that the final solution strongly depends on distribution functions of the model parameters. Considering that currently available models for computing biological effects of radiation are simplistic, and the clinical data used to derive the models are sparse and of questionable quality, the proposed technique provides us with an effective tool to minimize the effect caused by the uncertainties in a statistical sense. With the incorporation of the uncertainties, the technique has potential for us to maximally utilize the available radiobiology knowledge for better IMRT treatment.

Comparison between manual and automatic segment generation in step-and-shoot IMRT of prostate cancer

PURPOSE

To compare two methods to generate treatment plans for intensity-modulated radiotherapy (IMRT) of prostate cancer, delivered in a step-and-shoot mode. The first method uses fluence optimization (inverse planning) followed by conversion of the fluence weight map into a limited number of segments. In the second method, segments are manually assigned using a class solution (forward planning), followed by computer optimization of the segment weights.

METHODS

Treatment plans for IMRT, utilizing a simultaneous integrated boost, were created. Plans comprise a five-field technique to deliver 78 Gy to the prostate plus seminal vesicles. Five patients were evaluated. Optimization objectives of both planning approaches concerned dose coverage of the target volumes and the dose distribution in the rectal wall. The two methods were evaluated by comparing dose distributions, the complexity of the resulting plan and the time expenditure to generate and to deliver the plan.

RESULTS

For both planning approaches 99% of the target volumes received 95% of the prescribed dose, which complies with our planning objectives. Inverse planning resulted in more conformal dose distributions than forward planning (conformity index: 1.37 versus 1.51). Inverse planning reduced the dose to the rectal wall compared to a manually designed plan, albeit to a small extent. The theoretical probability of severe rectal proctitis and/or stenosis was reduced on average by 1.9% with inverse planning. Maximal sparing of the rectal wall was achieved with inverse planning for a patient whose target volume was partly wrapped around the rectum. The number of segments generated with inverse planning ranged between 33 and 52, and between 9 and 13 segments for manually created segments.

CONCLUSION

Dose coverage of the planning target volumes is adequate for both approaches of planning. Inverse planning results in slightly better dose distributions with respect to the rectal wall compared to manual planning, at the cost of an increase of the number of segments by a factor of 3.

Intensity-Modulated Radiation Therapy for Prostate Cancer

Prostate cancer is among the most common solid malignancies. A number of treatment alternatives exist for localized prostate cancer, including observation, prostatectomy, brachytherapy, and external-beam radiation therapy (EBRT). External-beam radiation therapy has changed dramatically during the past several years. Older techniques paved the way for 3-dimensional conformal radiation therapy (CRT), which in turn facilitated the introduction of intensity-modulated radiation therapy (IMRT). The prostate has served as a model disease site for the implementation of IMRT. As indicated by a growing body of experience, IMRT for prostate cancer represents a major technologic and clinical advance for radiation therapy. In this article, a review is provided of the evolution of EBRT leading to IMRT, the unique features making the prostate an ideal disease site for employing IMRT, the details of the clinical implementation of prostate IMRT and supporting technologic advancements, and the currently reported clinical outcomes of IMRT in prostate cancer. In addition, future directions of prostate IMRT, both technologic and clinical, are discussed.

Inverse and forward optimization of one- and two-dimensional intensity-modulated radiation therapy-based treatment of concave-shaped planning target volumes: the case of prostate cancer

BACKGROUND

Intensity-modulated radiation therapy (IMRT) was suggested as a suitable technique to protect the rectal wall, while maintaining a satisfactory planning target volume (PTV) irradiation in the case of high-dose radiotherapy of prostate cancer. However, up to now, few investigations tried to estimate the expected benefit with respect to conventional three-dimensional (3D) conformal radiotherapy (CRT).

PURPOSE

Estimating the expected clinical gain coming from both 1D and 2D IMRT against 3DCRT, in the case of prostate cancer by mean of radiobiological models. In order to enhance the impact of IMRT, the case of concave-shaped PTV including prostate and seminal vesicles (P+SV) was considered.

MATERIALS AND METHODS

Five patients with concave-shaped PTV including P+SV were selected. Two different sets of constraints were applied during planning: in the first one a quite large inhomogeneity of the dose distribution within the PTV was accepted (set (a)); in the other set (set (b)) a greater homogeneity was required. Tumor control probability (TCP) and normal tissue control probability (NTCP) indices were calculated through the Webb-Nahum and the Lyman-Kutcher models, respectively. Considering a dose interval from 64.8 to 100.8 Gy, the value giving a 5% NTCP for the rectum was found (D(NTCP(rectum)=5%)) using two different methods, and the corresponding TCP(NTCP(rectum)=5%) and NTCP(NTCP(rectum)=5%) for the other critical structures were derived. With the first method, the inverse optimization of the plans was performed just at a fixed 75.6 Gy ICRU dose; with the second method (applied to 2/5 patients) inverse treatment plannings were re-optimized at many dose levels (from 64.8 to 108 Gy with 3.6 Gy intervals). In this case, three different values of alpha/beta (10, 3, 1.5)were used for TCP calculation. The 3DCRT plan consisted of a 3-fields technique; in the IMRT plans, five equi-spaced beams were applied. The Helios Inverse Planning software from Varian was used for both the 2D IMRT and the 1D IMRT inverse optimization, the last one being performed fixing only one available pair of leaves for modulation. A previously proposed forward 1D IMRT 'class solution' technique was also considered, keeping the same irradiation geometry of the inversely optimized IMRT techniques.

RESULTS

With the first method, the average gains in TCP(NTCP(rectum)=5%) of the 2D IMRT technique, with respect 3DCRT, were 10.3 and 7.8%, depending on the choice of the DVHs constraints during the inverse optimization procedure (set (a) and set (b), respectively). The average gain (DeltaTCP(NTCP(rectum)=5%)) coming from the inverse 1D IMRT optimization was 5.0%, when fixing the set (b) DVHs constraints. Concerning the forward 1D IMRT optimization, the average gain in TCP(NTCP(rectum)=5%) was 4.5%. The gain was found to be correlated with the degree of overlapping between rectum and PTV. When comparing 2D IMRT and 1D IMRT, in the case of the more realistic set (b) constraints, DeltaTCP(NTCP(rectum)=5%) was always less than 3%, excepting one patient with a very large overlap region. Basing our choice on this result, the second method was applied to this patient and one of the remaining. Through the inverse re-optimization of the treatment plans at each dose level, the gain in TCP(NTCP(rectum)=5%) of the inverse 2D technique was significantly higher than the ones obtained by applying the first method (concerning the two patients: +6.1% and +2.4%), while no significant benefit was found for inverse 1D. The impact of changing the alpha/beta ratio was less evident in the patient with the lower gain in TCP(NTCP(rectum)=5%).

CONCLUSIONS

The expected benefit due to IMRT with respect to 3DCRT seems to be relevant when the overlap between PTV and rectum is high. Moreover, the difference between the inverse 2D and the simpler inverse or forward 1D IMRT techniques resulted in being relatively modest, with the exception of one patient, having a very large overlap between rectum and PTV. Optimizing the inverse planning at each dose level to find TCP(NTCP(rectum)=5%)e level to find TCP(NTCP(rectum)=5%) can improve the performances of inverse 2D IMRT, against a significant increase of the time for planning. These results suggest the importance of selecting the patients that could have significant benefit from the application of IMRT.

Influence of dose calculation model on treatment plan evaluation in conformal radiotherapy: a three-case study

In modern conformal radiotherapy (CRT), we attempt to increase its therapeutic ratio, thus improving the survival chances and/or quality of life for patients. It is common to acknowledge that poor local tumor control or increased normal tissue complications may arise from inaccurate targeting of the tumor, failure to conform the high-dose distribution to the target volume, and inaccurately delivered radiation doses. A further cause for concern is the influence that errors or inaccuracies in the dose calculation may have on the management of radiation therapy. Such errors arise from inherent limitations in the calculation algorithm used, which are more significant in some anatomical sites than others. Furthermore, an estimate of the therapeutic ratio is given by the ratio of tumor control probability (TCP) and normal tissue complication probability (NTCP). The effectiveness of these predictive indicators also depends on the accuracy of the calculated dose distributions in the target and surrounding normal structures. In this work, we compared CRT dose distributions of plans for the treatment of prostate, head-and-neck, and lung tumors using the measurement-based Clarkson and model-based Superposition dose calculation algorithms. Dose-volume histograms (DVHs) for the planning target volume (PTV) and sensitive structures, as well as NTCP and TCP, were compared. Dose distributions, observed in the lung and head-and-neck plans, vary significantly with respect to dose conformity as a function of algorithm used. Differences in the calculated maximum dose of up to 14% were observed in the PTV and sensitive structures for the lung and head-and-neck Clarkson-based plans, respectively, compared to the Superposition-based plans. Furthermore, a difference in the biological outcomes of up to 14% in the NTCP and 4% in the TCP was noticed. The CRT plans show the importance of accurate modeling of the effect of tissue inhomogeneities on dose distributions in the target and critical structures for lung and head-and-neck treatments.

A dose-volume histogram analysis of the seminal vesicles in men treated with conformal radiotherapy to 'prostate alone'

BACKGROUND AND PURPOSE

There is no consensus on whether the seminal vesicles should be included in the clinical target volume (CTV) for radiotherapy of localized prostate cancer. To inform the debate, we have undertaken a dose-volume histogram (DVH) analysis of the seminal vesicles in patients treated with escalated dose conformal radiation to 'prostate alone'.

MATERIAL AND METHODS

Twenty-five consecutive patients receiving conformal radiation to the prostate, to a dose of 75.6 Gy in 42 daily fractions, were studied. The CTV was defined as the prostate only, and the planning target volume (PTV) was defined by a 10 mm margin, except posteriorly where the margin was 7 mm. DVHs were calculated for the entire seminal vesicles, and for 6 mm segments through the seminal vesicles.

RESULTS

Incorporating a correction for organ motion, the D90 (minimum dose received by 90% of the volume of interest) for the most inferior 6 mm volume of the seminal vesicles (SV1) ranged from 25 to 70 Gy, and the percentage volume of SV1 receiving 50 Gy ranged from 47-100%. Using a D90 of 50 Gy as a cut-off, eight of the 25 patients had unacceptably low-dose coverage of SV1.

CONCLUSIONS

Escalated dose conformal radiation to the 'prostate alone' does not ensure adequate dose coverage of even the most inferior 6 mm of the seminal vesicles. We consider such treatment acceptable in patients at low risk of seminal vesicle involvement (T1/2ab, Gleason < or = 7, PSA < 10 ng/ml). In higher risk patients, if it is deemed necessary to treat the possibility of sub-clinical seminal vesicle involvement, this should be reflected in the definition of the CTV.

Plan space: representation of treatment plans in multidimensional space

PURPOSE

Treatment planning balances the need to provide adequate radiation coverage of the target with the need to reduce against the risk of overdosing normal tissues. An acceptable plan fulfills minimum dose-volume criteria for irradiation of tumor and normal tissues. However, multiple plans can satisfy these minimum criteria, and some plans provide for better protection of normal tissue than others. Here, we present a method to help the planner compare plans and decide whether a particular plan is the "best" plan on the basis of a set of certain dose-volume conditions.

METHODS AND MATERIALS

Treatment plans are represented as points in multidimensional space. One dimension is assigned to the target and one to normal tissues of each anatomic structure under consideration. Minimum target dose is used as the target axis coordinate and the percentage volume of each normal structure receiving more than a specified dose as each normal-tissue coordinate. Images of plan space are developed for model phantom anatomy as well as for two clinical cases in the thorax and abdomen.

RESULTS

When a sufficient number of plans have been plotted, a feasibility boundary becomes evident. This hypersurface in plan space represents the limit of the given treatment technique. By using a plan on this boundary, the benefits of a given treatment modality are maximized, providing assurance that the selected plan is the "best" plan. The beam angles and relative weights of a plan can be changed to alter its position in plan space, allowing improvements in an existing plan. Frequently, plans with normal-tissue dose distributions superior to the minimum acceptable criteria can be selected. The benefits of using plan-space images have been demonstrated at sites in the thorax and abdomen.

CONCLUSION

Instead of defining dose-volume criteria at the outset, it is possible to select the best achievable plans by first evaluating the space of possible plans for a particular patient's unique anatomy and then choosing the plan with the optimum dose-volume characteristics. No attempt is made to arrive at a single plan score so that explicit judgments about the relative worth of individual structures are avoided. Visualization of plan-space images allows physicians to make choices based on their assessment of the relative significance of irradiation of each normal structure.

Partial boosting of prostate tumours

BACKGROUND AND PURPOSE

In this planning study we propose a class solution for partial boosting of prostate tumours. Treatment margins and rectum dose are similar to that of the conventional treatment and are supposed to have no direct link to the level of dose escalation. We also study the robustness of our class solution in the presence of geometrical deviations.

METHODS AND MATERIALS

To study the specifications of the class solution ten patients with histologically confirmed prostate cancer were replanned. Besides a conventional plan for each patient, different partial boost plans were produced with an inverse treatment-planning tool. We also simulated treatment geometrical deviations to estimate their effect on partial boost plans.

RESULTS

In our class solution we use three contours in our inverse treatment planning, which are based on the classical CTV. A three beam arrangement appeared to produce a dose distribution, which is comparable to that of a five or seven beam geometry. Comparison of partial boost plans and conventional plans indicated that all conditions for a partial boost plan could be satisfied with the proposed class solution. Simulation of treatment geometrical deviations showed that large random deviations have a minor effect on the overall dose distributions, while systematic deviations may decrease the boost dose and increase the rectal dose.

CONCLUSIONS

We presented a class solution for partial boosting of prostate tumours in which the level of dose escalation is dealt with separately from the margin size and the nominal rectum dose. The framework put forward in this study allows practical introduction of intensity modulated radiotherapy in routine clinical practice using current standards of imaging and position verification.

Immobilization devices for intensity-modulated radiation therapy (IMRT)

Three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT) plans show radiation dose distribution that is highly conformal to the target volume. The successful clinical implementation of these radiotherapy modalities requires precise positioning of the target to avoid a geographical miss. Effective reduction in target positional inaccuracies can be achieved with the proper use of immobilization devices. This paper reviews some of the immobilization devices that have been used and/or have the potential of being used for IMRT. The immobilization devices being reviewed include stereotactic frame, Talon system, thermoplastic molds, Alpha Cradles, and Vac-Lok system. The implementation of these devices at various anatomical sites is discussed.